Citation
Internal loading in shallow lakes

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Title:
Internal loading in shallow lakes
Added title page title:
Shallow lakes
Creator:
Pollman, Curtis Devin, 1951-
Publication Date:
Language:
English
Physical Description:
vii, 191 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Lakes ( lcsh )
Limnology ( lcsh )
Lake Apopka ( local )
Sediments ( jstor )
Lakes ( jstor )
Phosphorus ( jstor )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1983.
Bibliography:
Includes bibliographical references (leaves 178-190).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Curtis Devin Pollman.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
029290033 ( ALEPH )
ACA4587 ( NOTIS )
09882701 ( OCLC )

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Full Text














INTERNAL LOADING IN SHALLOW LAKES


By

CURTIS DEVIN POLLMAN



























A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1983

















ACKNOWLEDGMENTS


Support from numerous sources assisted me during the course of my

studies. Charles Hendry, Tom Belanger, Carl Miles, and Charles Fellows

all sacrified time and effort in accommodating me during the field

portion of my studies. Further appreciation is extended to Charles

Hendry for introducing me to some of the subtle nuances of the

analytical equipment critical to the success of this research.

I am deeply grateful for the support of my chairman, Dr. P.L.

Brezonik whose continued faith, gentle proddings, and guidance helped

provide the inspiration to complete my research. I also wish to

acknowledge the other members of my graduate committee, Dr. Wayne

C. Huber, Dr. Donald Graetz, Dr. Edward S. Deevey, and Dr. Thomas

L. Crisman, whose critical review and comments helped produce the final

manuscript. Dr. A.J. Mehta and Dr. Gregory M. Powell both provided

invaluable help in approaching the development of a sediment

resuspension model.

Eileen Town, who typed the initial and final manuscripts, deserves

special recognition for her professional competence and perserverance.

Appreciation is also extended to Carla S. Jones and Marlene A. Hobel for

putting up with the neurotic antics of an expectant author and providing

the finishing touches to the manuscript.










I particularly wish to thank my parents, Mr. and Mrs. Ralph

C. Pollman, and my grandmother, Mrs. Harold G. Lengerich, for

spiritually and financially sustaining me during the ebb moments of my

graduate career. My wife Kathleen sacrificed much for the sake of my

work and is especially deserving of recognition. Without her support

and encouragement, this work doubtlessly would have remained

uncompleted. Finally, I wish to dedicate this work to the memory of my

twin brother, Chris, whose recent departure from life instilled in the

author a sense of necessity. His spiritual guidance was prodigious and

is profoundly missed.


















TABLE OF CONTENTS


PAGE


ACKNOWLEDGMENTS .

ABSTRACT ....


CHAPTER


INTRODUCTION. . 1

Forms of Sedimentary Phosphorus. 2
Mechanisms of Nutrient Release 6

Bioturbation .. 6
Gas Ebullition . 8
Sediment Resuspension. ... 10
Molecular Diffusion. ... 13

Summary. . .. 15


SITE DESCRIPTION AND METHODOLOGY .


. 17


Physical Description and Limnology
Physical Description and Limnology
Materials and Methods. .


of Lake
of Lake
. .


Okeechobee.
Apopka. .


Physical and Chemical Characterization .
Sorption Experiments .

PHYSICAL-CHEMICAL SEDIMENT CHARACTERISTICS .

Physical Characteristics .
Chemical Characteristics .

RELEASE STUDIES .

Turbulence Study .

Fresh Sediment .
Partially Dessicated Sediment. .

In Situ Studies. .


. 57

. 71













PAGE


NUTRIENT RELEASE MODEL ... .92

Development of a Langmuir Sorption Model ...... .97
Application of Langmuir Model to Lake Apopka and
Lake Okeechobee Sediments. ... 108

Equilibrium Phosphorus Concentration (EPC) ... .118
Desorption . ... .. .123
Effect of pH on P Sorption by Lake Okeechobee
and Lake Apopka Sediments. ... 127

Development of a Sediment Resuspension Phosphorus
Release Model for Lake Okeechobee and Lake Apopka. 131


Nutrient Release Submodel. .
Sediment Resuspension Model. .
Calculation of Wind-Induced Stress at the
Sediment-Water Interface .
Determination of Critical Shear Stress .
Calculation of Sediment Resuspension Rates .


Sediment Dispersion Model. .
Application of the Integrated Model to
and Lake Okeechobee. .


. 131
. 133


Lake Apopka


. 134
. 137
. 142
. 144

. 150


DISCUSSION AND SUMMARY ... .161


Discussion of Internal Loading .
Summary .


REFERENCES . .

BIOGRAPHICAL SKETCH. .


. 178

. 191


. 161
. 174


1 I I

















Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



INTERNAL LOADING IN SHALLOW LAKES


By

Curtis Devin Pollman

April 1983


Chairman: Patrick L. Brezonik

Major Department: Environmental Engineering Sciences


Internal loading of phosphorus may be significant in shallow lakes

where physical and biological processes can accelerate rates of solute

exchange across the sediment-water interface. Sediments were collected

from Lake Apopka and Lake Okeechobee located in central and south

Florida, respectively, and characterized with respect to physical and

chemical attributes of the sediment that control the dynamics of

phosphorus exchange between sediments and overlying water. A series of

experiments were conducted on bulk sediment to determine rates of

phosphorus exchange across the sediment-water interface as a function

of turbulence and physical state of the sediment.

The shallow nature and broad fetch characteristic of Lake Apopka

(mean depth 1.7 m) and Lake Okeechobee (mean depth 2.8 m) suggest that










sediment resuspension and the concomitant release of sorbed phosphorus

may be important in the cycling of phosphorus. As a result, a model

was developed incorporating physical, chemical, and hydrodynamic

components to predict internal loading of phosphorus in shallow lakes.

Phosphorus releases are predicted on a single event basis in response

to wind-driven turbulent mixing in the water column. The model

synthesizes a wave-hindcasting submodel to determine sediment resus-

pension rates and a phosphorus release submodel derived from Langmuir

sorption theory. Langmuir constants used as model inputs were derived

from adsorption-desorption isotherms conducted at controlled pH.

Under ambient conditions in the water column, both Lake Apopka and

Lake Okeechobee sediments are predicted to release phosphorus upon

resuspension. Results indicate that a moderate wind event (approxi-

mately 8.9 m/s) can more than double mid-lake concentrations of ortho-

phosphorus in Lake Apopka from 20 to 61 ug P/L at pH 8.3. Phosphorus

release from Lake Okeechobee sediments is substantially lower

(approximately 3 to 8 ug P/L). Release is primarily from desorptive

processes; however, in Lake Apopka entrained pore fluid contributes

approximately 30 percent of the net release. The cumulative effect of

cyclonic and convective disturbances during the course of a year

results in internal loading in Lake Apopka easily exceeding external

loading rates. Sediment resuspension is less significant in Lake

Okeechobee, and the nutrient regime appears to be controlled primarily

by external loading.









vii
















CHAPTER I
INTRODUCTION



Within the past two decades, the widespread recognition of lake

degradation as an issue of major environmental concern has spawned

accelerated efforts to better protect and manage lake resources (Born

1979; U.S. EPA 1979). Phosphorus has been implicated as the most

important nutrient limiting phytoplankton growth and production in many

freshwater lakes (Vollenweider 1968; Vallentyne 1970; Likens 1972;

Schindler 1977). Consequently, many lake restoration schemes have

focused on curbing external or cultural input of phosphorus to

lacustrine systems.

In the decade following the introduction of Vollenweider's (1968)

now classic paper, great emphasis was placed by limnologists on modeling

one factor that influences productivity--external nutrient loading. In

spite of the emphasis given this narrow focal point, interest also has

been sustained in observing the contribution of phosphorus from lake

sediments to the overlying water--"internal loading" (cf. Golterman

1977). Much of this work was inspired by early studies such as those

conducted by Mortimer (1941, 1942) on Esthwaite Water in the English

Lake District. More recent work by Fee (1979) has rekindled speculation

and debate on the importance of lake morphometry to lake trophic state

and, in particular, the area of sediment in direct contact with the

trophogenic zone.











Lacustrine sediments may consist of up to 0.75 percent (by weight)

phosphorus (Jones and Bowser 1978). This nutrient pool represents a

potential source of supply to the overlying water column approximating

8 g P/m2 for each centimeter of the sediment column actively engaged

in exchange across the mud-water interface. In view of the work of

Vollenweider (1968, 1975, 1976) and others in defining critical areal

nutrient loading rates, it becomes quite evident that the availability

of sedimentary phosphorus for exchange and the factors that mediate the

transfer of the available pool, both abiotic and biotic, may have a

profound influence on the nutrient regime of a lake. The following

section is thus devoted to a discussion of the chemical nature of

sedimentary phosphorus and its availability for exchange and biological

uptake; factors that control the rate of exchange across the sediment-

water interface are discussed in subsequent sections.



Forms of Sedimentary Phosphorus

A variety of analytical techniques has been used to fractionate

sedimentary P with the ultimate objective of correlating a defined

fraction with biological response (i.e., nutrient uptake). Early

fractionation schemes borrowed heavily from the pioneering work in soils

by Chang anc Jackson (1957). Perhaps the most definitive work on

lacustrine sediments to supersede the efforts of Chang and Jackson

(1957) in soils was that of Williams et al. (1971). Williams et al.

(1971) operationally defined four distinct species of sedimentary P:

(1) inorganic P present as orthophosphate ion sorbed on surfaces of P

retaining minerals (nonoccluded P); (2) inorganic P present as a


I










coprecipitate or minor component of an amorphous phase (occluded P);

(3) orthophosphate P present in discrete phosphatic minerals such as

apatite Cal0(PO4)6X2, where X = OH-, F-, or 1/2 C03-2; and (4) P present

as an organic ester or directly bonded to carbon atoms (organic P). The

chemical nature of sedimentary P has been the subject of rather

intensive reviews by both Jones and Bowser (1978) and Armstrong (1979).

In more recent work by Williams et al. (1976), the categorization

of sedimentary P has been reduced to three major types: (1) organic

phosphorus, which encompasses all P associated with organic material via

C-O-P or C-P bonds; (2) apatite P comprised of orthophosphorus present

within the crystalline lattice of apatite grains; and (3) nonapatite

inorganic phosphorus (NAI-P) which consists of the remaining ortho-

phosphorus fraction including that dissolved in the interstitial

solution. This latter category constitutes the most labile fraction and

is generally associated with exchangeability. In the development of

these categories, Williams et al. (1976) assumed that nonorthophosphatic

forms of inorganic phosphorus, e.g., polyphosphate ions (including

pyrophosphate) are present in sediments only in negligible quantities.

Isotopic exchange studies conducted by Li et al. (1972) using P-32

as a tracer provide direct evidence that a substantial fraction of the

total inorganic pool in sediments is exchangeable. Exchangeable P

comprised 19 to 43 percent of the total inorganic content of sediments

derived from four Wisconsin lakes. Total exchangeable P was found to be

relatively insensitive to changes in redox status; this was interpreted

by Li et al. (1972) as consistent with the retention of inorganic P in

sediments by an iron-rich gel complex (Williams et al. 1971;










Shukla et al. 1971). Li et al. (1972) hypothesize that although the

equilibrium position between sorbed and free inorganic P for the iron

gel complex is altered by a shift in redox potential, the total

exchangeable pool is not appreciably increased. Conversely, if

inorganic P existed primarily as a discrete crystalline iron phosphate

such as strengite, total exchangeable P should increase significantly

upon dissolution of the solid phase during anaerobiosis.

The high rate of exchangeability observed by Li et al. (1972)

provides further qualitative evidence of the retention of inorganic P as

a nonoccluded form through sorption to an amorphous iron hydrous oxide

gel. Incorporation of P into a crystalline lattice is anticipated to

yield a reduction of exchangeability primarily because of a reduction in

surface area. Additional experiments conducted by Li et al. (1972) with

sediments equilibrated with solutions spiked with orthophosphorus

demonstrated equivalent degrees of exchange for sorbed and native

inorganic P. The authors suggested that native inorganic P is retained

in sediments by the same mechanism as sorbed P.

Alternative fractionation schemes to define the labile fraction of

sedimentary P available for algal uptake have involved chelating agents,

anion and cation exchange resins (Wildung and Schmidt 1973; Huettl

et al. 1979), and desorption potential (Schaffner and Oglesby 1978)

Golterman (1973) used 0.01 N NTA (nitrilotriacetic acid) to extract P

associated with calcium (Ca) and iron (Fe) constituents in sediments.

Golterman (1977) found essentially no difference between NTA extracted

phosphorus and calculated P uptake by the alga Scenedesmus utilizing the







5


same sediment as its solitary source of P. Grobler and Davies (1979)

found NTA-extractable P to generally be a more applicable measure of

algal available P than total inorganic P. Although both fractions were

correlated with algal uptake, the most consistent relationship was

observed between P availability and NTA extractable P. A non-linear

relationship between algal available P and sediment inorganic P was

demonstrated by Grobler and Davies (1978); the fraction of inorganic P

comprised by algal available P varied with sediment type and was quite

variable, ranging from 4 to 98 percent.

As a means for differentiating between Fe-bound and Ca-bound P,

Golterman (1977) proposed an initial extraction with 0.01 M Ca-NTA.

According to Golterman, complexation of the added Ca by NTA prevents the

solubilization of sedimentary calcium and extraction of Ca-P; conse-

quently only iron-bound P is extracted. Subsequent work by Williams

et al. (1980), however, does not support Golterman's contention that

Ca-NTA selectively extracts Fe- and aluminum (Al)-associated P, leaving

Ca-P intact. Extraction with a neutral, noncomplexing agent (1 M NaCI)

yielded extraction efficiencies comparable with Ca-NTA. Extraction with

Ca-NTA rarely exceeded 10 percent of the NAI-P fraction. Uptake of

sedimentary P by S. quadricauda was closely correlated with the NAI-P

fraction by Williams et al. (1980) and constituted approximately

75 percent of NAI-P; apatite-P was virtually unavailable for algal

growth. These results qualitatively confirm the earlier observations of

Grobler and Davies (1978), who noted that algal available P generally

exceeded NTA-extractable P by a factor of approximately 5 to 6.











Mechanisms of Nutrient Release

As detritus accumulates in surficial deposits, metabolism of

accreting organic material results in the buildup of interstitial

phosphorus concentration to levels that exceed those in the overlying

water by as much as several orders of magnitude.. Release to the

overlying water is accomplished by burrowing and irrigation activities

of benthic organism (i.e., bioturbation), by gas ebullition from

anaerobic decomposition processes, by scouring of the interface by

wind-induced waves, and simply by molecular diffusion across the

concentration gradient that usually exists between sediment pore water

and the overlying water.



Bioturbation

Based on analysis of Pb-210 and Cs-137 profiles in recent sediments

of the Great Lakes, Robbins and Edgington (1975) suggested that post-

depositional movement of material occurred within the surficial 10 cm of

sediment. Robbins and Edgington (1975) postulated that the observed

redistribution of sediment particles was the result of either physical

mixing or bioturbation. Activities of the benthic community, such as

burrowing, irrigation, and feeding, result in fluid and particle

transport near (and across) the sediment-water interface. Bioturbation

increases the influx of water into sediments; this is balanced by an

equal volume of interstitial solution flushed from sediments (Petr

1977). Hargrave and Connolly (1978) observed a great deal of spatial

variability in nutrient and gas fluxes across the sediment-water inter-

face of a subtidal flat and attributed the variability to heterogeneity










in the distribution of benthic macrofauna. Berger and Heath (1968)

suggested that homogenization of the upper sediments is complete when

sediment displacement by benthic infauna is comparable to or exceeds the

sediment accumulation rate. As an extension of this assumption,

Krezoski et al. (1978) used average defecation rates for Tubifex tubifex

to estimate sediment displacement, and they calculated that oligochaete

densities in profundal sediments of Lake Huron were sufficient to

displace the upper 3 to 6 cm. Neame (1977) observed a total phosphorus

flux of approximately 650 ug/m2-d across the surficial 3 cm of

sediment in Castle Lake, California, and attributed this flux to

chironomid activity in the oxidized zone of the sediments. Under anoxic

conditions, the phosphorus flux across the interface decreased by two

orders of magnitude.

Aller (1978) reported that benthic infauna activity affects the

dynamics of the sediment-water exchange process by altering the geometry

and kinetics of molecular diffusion in sediments. In the absence of

benthic activity, Aller (1978) successfully used Berner's (1976) one-

dimensional diffusion model to describe the vertical distribution of

interstitial phosphorus in marine sediments. In the presence of the

highly mobile protobranch, Yoldia limatula, pore-water transport was

characterized by a non-steady-state two-layer model in which an

effective (or biogenic) diffusion coefficient acts in the zone of

feeding and molecular diffusion controls transport in underlying

sediment. The effective biogenic diffusion coefficient for pore-water

transport by Yoldia is approximately 1 x 10-5 cm2/s; this compares

to a tortuosity-corrected molecular diffusion coefficient of










2.4 x 10-6 cm2/s for HP04-2 in undisturbed sediments. Lerman (1979)

indicated that effective diffusion coefficients in the mixed layer are

on the order of 5 to 100 times greater than the molecular diffusion

coefficient.

Several other types of models have been developed to describe the

effects of bioturbation on sediment-water solute exchange. Grundmanis

and Murray (1977) and McCaffrey et al. (1980) suggested that transport

can be simulated by assuming that interstitial water is biogenically

advected between discrete well-mixed reservoirs within the sediment and

overlying water. Aller (1978, 1980) proposed an idealized model which

assumes that specific changes in the average geometry of molecular

diffusion result from the presence of irrigated tube and burrow

structures. Average interstitial concentrations, solute flux, and

apparent one-dimensional diffusion coefficients are influenced by both

the size and spacing of burrows.



Gas Ebullition

The evolution of gas bubbles within anaerobic sediments also may

enhance the flux of dissolved constituents across the sediment-water

interface. Two mechanisms have been proposed by Klump and Martens

(1981) to explain the influence of gas ebullition on exchange. Bubble

tube structures alter the geometry of the sediment-water interface and

increase the surface area available for chemical exchange. If flow

patterns at the interface permit unrestricted import and export of

overlying water within tube structures, the total flux across the

interface is increased. Conversely, restriction of flow may allow











solute concentrations within the tube to reach high concentrations,

establishing a sharp vertical concentration gradient across the

interface. Mixing of the pore water in the tube by rising bubbles can

mix the pore water along the length of the tube, creating a sharp

gradient immediately at the interface. Enhancement of diffusive

transport consequently would result from the increased gradient.

The critical concentration at which bubbles form is dependent on

hydrostatic pressure; thus, resistance to bubble formation increases

with increasing depths (Hutchinson 1957). Gas ebullition generally

occurs in shallow aquatic systems such as marshes and littoral zones,

where surficial sediments are the site of intensive anaerobic metab-

olism. In hypereutrophic Wintergreen Lake, a shallow (T = 3.5 m),

hardwater lake in Michigan, Strayer and Tiedje (1978) measured an

average methane ebullition rate of 21 mmol/m2-d during the period

late May through August. Maximum rates of methane loss by bubble

evolution (35 mmol/m2-d) occurred in late summer. In comparison,

Martens and Klump (1980) measured an average methane ebullition of

16.8 mmol/m2-d from intertidal sediments on the North Carolina

coast between June and October. Ebullition was initiated by the release

of hydrostatic pressure that accompanied low tide. High fluxes of total

dissolved phosphorus were observed; rates up to 120 umol/m2-hr were

observed from these sediments during the summer months. The high rates

were attributed to increased mass transport associated with bubble tubes

maintained by methane gas ebullition.











Sediment Resuspension

A third mechanism that may contribute to internal loading is the

resuspension of sediments in shallow lakes by wind-induced currents

(e.g., Sheng and Lick 1979). Fee (1979) correlated rates of primary

production in unfertilized ELA lakes in western Ontario to the fraction

of epilimnetic surface area in direct contact with bottom sediments.

Fee concluded that epilimnetic nutrient recycling is dominated by

processes occurring at the sediment-water interface and not within the

sediment. Chapra (1982) recently showed that resuspension becomes an

important mechanism in lakes with mean depths less than 9.2 m. Lastein

(1976) showed that approximately 24 percent of the material collected in

sediment traps in Lake Esrom (Sweden) (z = 12.3 m) was from resuspended

sediment, and he hypothesized that wind-induced circulation alone was

sufficient to explain the phenomenon. In Lake Uttran (Sweden), a

shallow (T = 5.7 m), eutrophic lake, Ryding and Forsberg (1977)

correlated aqueous total phosphorus concentrations with the force and

duration of winds blowing lengthwise across the lake, and they suggested

that resuspension played a dominant role in regulating water quality.

In a study of six shallow Danish lakes, Andersen (1974) concluded that

wind-induced mixing of surficial sediments is the most probable

mechanism for the high phosphorus release rates he observed (up to

1.2 g/m2-month). Concomitant laboratory studies indicated that

maximum rates of release from quiescent, undisturbed sediments were of

the order of 0.25 g/m2-month.

Surface waves produce an oscillatory motion which translates in

shallow water to elliptical orbits extending to the sediment-water










interface (Bascom 1964; U.S. Coastal Engineering Research Center 1977).

Near the boundary layer, the elliptical motion is reduced to a simple,

reciprocating horizontal motion, and the maximum horizontal velocity

associated with the oscillating motion is given by


T H
u = (I-I)
m Ts sinh (27 d Ld) (


where Hs = significant wave height;
Ts = significant wave period;
d = depth of the water column;
Ld = wavelength for the particular depth, d; and
um = maximum orbital velocity (Komar and Miller 1973).


When the shear force exceeds the bulk shear strength of the

surficial sediment deposits, sediment resuspension occurs (Alishahi and

Krone 1964; Komar and Miller 1973, 1975; Terwindt 1977; Sternberg 1972).

Scour may be accomplished either by detachment and entrainment of

individual flocs (surface erosion) or removal of aggregated material

(mass erosion) (Lonsdale and Southard 1974; Terwindt 1977). In deposits

where shear strength increases with depth, erosion continues to the

depth where the applied stress is equivalent to the bed strength (Krone

1976). Below the threshold or critical shear velocity, the bed remains

intact and no resuspension occurs. Migniot (1968) demonstrated that for

various types of cohesive sediments, the critical shear velocity may be

expressed as an inverse function of water content. These findings

subsequently were confirmed for marine sediments by Southard et al.

(1971) and Lonsdale and Southard (1974).











The critical shear velocity also is influenced by the mechanical

structure of the sediment, which in turn is related to the solute

characteristics and ionic strength of the interstitial solution and the

eroding solution (Arulananden et al. 1975). For example, critical shear

stress decreases with increasing sodium adsorption ratio (SAR)

(Arulananden et al. 1975):



[Na+]
SAR = [N (1-2)
([Ca2+ + [Mg2])1/2


This observation correlates with the Schulze-Hardy valence rule,

which indicates that the critical coagulation concentration of mono-,

di-, and trivalent ions are in the ratio of z-6, where z is the

charge of the ion (Stumm and Morgan 1981). Increasing the solute

concentration of the pore fluid destabilizes colloidal suspensions

(i.e., agglomerates them and tends to remove them from solution). The

net effect is an increase in bed strength (Arulanandan et al. 1975).

The effects of electrolyte concentration and SAR on critical shear

stress and erosion rates were reviewed in detail by Terwindt (1977).

Mixing or agitation of surficial sediments due to wave action

entrains interstitial (pore) water, resulting in release of solutes such

as nitrogen and phosphorus species to the overlying water (Gahler 19b9;

Lam and Jaquet 1976). For example, resuspension of the upper 1 cm of a

flocculent sediment with a porosity of 80 percent and a pore water

phosphorus content of 1 mg/L would result in a release of 8 mg P/m2.

This is sufficient phosphorus to supply a shallow lake (e.g., 7 = 3 m)

with an additional 2.7 ug P/L from this source alone. In addition, a











substantial fraction of the inorganic phosphorus sorbed to lake

sediments participates in rapid solid phase/aqueous phase exchange

reactions. This reservoir of phosphorus may be more important in

controlling the phosphorus flux resulting from sediment resuspension

than is the pore water phosphorus.

Lam and Jaquet (1976) developed an empirical model to predict the

upward flux of TP from resuspended sediment in Lake Erie. The flux is a

function of shear stress at the sediment-water interface (TB) from

wind-induced waves; TB was calculated from linear wave theory using

the deep-water Sverdrup-Munk-Bretschneider (SMB) wave-hindcasting method

to estimate wave height and period. Regeneration was assumed to arise

primarily from organic and nonapatite inorganic phosphate fractions, in

conjunction with a small quantity of soluble interstitial phosphate.

Apatite [Calo(P04)6(OH)2] was not considered in the regeneration

process because of its relatively high density and large grain size.

Sheng and Lick (1979) developed a similar sediment resuspension

model for Lake Erie; a principal point of departure from the Lam and

Jaquet model was the use of a shallow-water SMB wave-hindcasting model

to predict bottom orbital velocities.



Molecular Diffusion

The existence of concentration gradients across the sediment-water

interface implies that molecular diffusion may be an important mechanism

in describing solute transport. Theoretically, the flux across the

sediment-water interface constituent dissolved in the interstitial

solution arises from advection due to pore water buried by sediment











deposition, as well as from diffusion (Imboden 1975; Lerman 1979).

Neglecting the advective component, the flux equation reduces to Fick's

first law (Berner 1971):


IC
J oDs () (1-3)
z = 0


where Js = flux from the sediment to the overlying water
(g/cm2-s with "s" denoting bulk sediment),

0o = porosity at the interface,
Ds = bulk sediment diffusion coefficient at the interface
(cm2/s), and
ac
(--) = pore water (pw) concentration gradient at the
3z pw
sediment-water interface (i.e., z=0).



This equation is tractable if the assumption is made that Ds is

both spatially and temporally invariant. The flux of a dissolved

species across the sediment-water interface then can be calculated from

the concentration gradient at the interface and molecular diffusivity

(e.g., McCaffrey et al. 1980; Thibodeaux 1979; Vanderborght et al.

1977).

Fluxes as high as 7 mg P/m2-d attributed to diffusion have been

reported by Ulen (1978) for sediment cores from Lake Norvikken in

central Sweden. The average diffusive flux for Lake Norvikken was

1.0 g/m2-y; this contrasts with an external phosphorus loading rate

of 0.53 g/m2-y. Ulen (1978) obtained good agreement between

empirically measured fluxes and calculated rates of exchange using a

bulk sediment diffusion coefficient of 4.4 x 10-6 cm2/s. This value

was first derived by Tessenow (1972) and probably represents an upper












limit for bulk sediment diffusivities (Tessenow 1972, in Kamp-Nielsen

1974). Data of Li and Gregory (1974) indicate that the diffusivities of

H2PO at 25'C and infinite dilution are 7.34 x 10-6 cm2/s and

8.46 x 10-6 cm2/s, respectively; Manheim (1970) suggested that

these values are diminished by factors ranging from 0.05 to 0.5 in

unconsolidated sediments, as a result of porosity and path tortuosity

effects.

According to Kamp-Nielsen (1974), the aerobic release of phosphorus

from sediments in Lake Esrom (Sweden) is controlled by both adsorption

and diffusion. Berner (1976) modified the expression for Fickian

diffusion to quantify the retarding influence of adsorption:


D 0
s o ac
J () z = 0 (1-4)
s 1 +K 3z


where K = nondimensionalized linear adsorption coefficient.



Excellent agreement was obtained by Krom and Berner (1980b) between

empirically-derived effective diffusivities for phosphate, ammonium, and

sulfate ions and molecular diffusivities corrected for tortuosity and

adsorption. The influence of adsorption on the effective or apparent

diffusion coefficient can be quite significant; for example, Krom and

Berner (1980a) cite values for K ranging up to 5,000 for oxic oceanic

sediments.



Summary

Sediments in lakes are generally characterized by high concentra-

tions of nutrient and other chemical constituents (such as heavy metals)











resulting from the deposition and accretion of detrital material. As a

result, sediments may actively participate in regulating the cycling of

a particular constituent in the water column. The extent that the

sedimentary reservoir can ultimately influence material cycling within

the water column depends on the availability of the substance for

exchange. Exchange of nutrients and other solutes across the sediment-

water interface into the overlying water is controlled by four principal

mechanisms: bioturbation, gas ebullition, resuspension of bulk sediment

by wind-induced turbulence, and molecular diffusion. In shallow lakes

characterized by extensive fetches, sediment resuspension is likely to

be a major mechanism for internal nutrient cycling. This research

focuses on the significance of sediment resuspension on rates of inter-

nal nutrient release in two lacustrine systems of similar morphometry,

but quite different trophic state--hypereutrophic Lake Apopka and

mesotrophic Lake Okeechobee. In conjunction with this objective, a

corollary objective of this study was to develop an event-dependent

model to predict the release or removal of phosphorus by wind-induced

sediment resuspension.

















CHAPTER II
SITE DESCRIPTION AND METHODOLOGY



This research focused on two rather large and shallow lakes

important as aquatic resources in Florida--Lake Apopka in central

Florida and Lake Okeechobee in south Florida. By virtue of the broad

fetch and shallow nature of each basin, both Lake Apopka and Lake

Okeechobee are subjected to periodic sediment resuspension due to wind-

induced currents. The trophic state of these two lakes, however, is

quite different. Until the early to mid-1950's, Lake Apopka achieved

national reknown as a bass fishing lake. Currently and primarily

because of cultural inputs of nutrients, Lake Apopka is now widely

regarded as one of the most eutrophic lakes in Florida. Conversely,

Lake Okeechobee is intensively used as a recreational resource and is

critical to the hydrologic and ecological stability of south Florida.

The following section briefly summarizes the limnological characteris-

tics of these two systems and details the specific analytical procedures

used in this study.



Physical Description and Limnology of Lake Okeechobee

After the Laurentian Great Lakes, Lake Okeechobee is the largest

freshwater lake in the United States. Located in south central Florida

between latitudes 26'41' to 27013'N and longitudes 8036' to 81007'W











(Figure II-i), Lake Okeechobee has a surface area of 1,890 km2 at

normal stage (approximately 4.6 m above sea level). The broadest

expanse of open water lies along the north-south axis and approximates

56 km; the maximum east-west dimension is 48 km. The shoreline

development index, which is the ratio of the lake shoreline length to

the circumference of a circle with an equivalent surface area is 1.12

(Brezonik et al. 1979). Summarized in Table II-1 are some of the

important physical characteristics of the watershed.

Lake Okeechobee, which is considered moderately eutrophic (Brezonik

et al. 1979), is quite shallow throughout the entirety of its basin. At

normal stage, the depth of the lake averages only 2.8 m and has a

maximum depth of less than 5 m (Figure II-i). Because of its shallow

depth, Lake Okeechobee is characterized by an extensive littoral zone.

Macrophyte communities occupy nearly 500 km2 or 26 percent of the

total lake surface area along the southern and western shores of the

lake; however, no important macrophyte communities are established on

the eastern portion of the basin (Brezonik et al. 1979).

Lake Okeechobee in its present state originated approximately

6,000 years ago (Brooks 1974). It occupies a shallow depression which,

according to Hutchinson (1957), was formed by epeirogenetic uplift of an

irregular marine surface. Brooks (1974) has hypothesized that

differential subsidence of the thick underlying sequence of Miocene

clays is responsible for the deeper portions of the lake. Extensive

discussions of the origin and early history of Lake Okeechobee have been

presented by Brooks (1974) and Brezonik et al. (1979).




























O II 0 A
S T'-""" -A--


I fi o *io -K-


Figure II-i. Location Map of Lake Okeechobee.











Table II-I. Physical Characteristics of Lake Apopka and Lake Okeechobee.


Lake Apopka*


Lake Okeechobeet


Lake Surface Area (km2)

Maximum Depth, zm (m)

Mean Depth, (m)

Volume, V (m3)

Annual Inflow (m3)


124.0

11.0

1.7

2.14 x 108

5.92 x 107


Theoretical Retention Time (years)

Maximum Length (km)

Maximum Width (km)

Shoreline Length (km)

Shoreline Development Index, DL

Watershed Area (including lake) (km2)

Watershed Land Area (km2)


6.3

14.4

13.8

58.5

1.48

3.11

187


1,891

4.7

2.8

5.24 x 109

1.98 x 109 to
4.68 x 109

1.12 to 2.65

56.4

48

172

1.13

13,007

11,116


* From Brezonik et al. (1978).
t From Brezonik et al. (1979) and Federico et al. (1981).











Excluding the lake surface, the watershed of Lake Okeechobee drains

approximately 11,116 km2. The watershed comprises a series of both

natural and man-made sub-drainage basins (Figure 11-2). Flow is

primarily from north to south; the Kissimmee River, which drains

6,048 km2, is the most important tributary (Davis and Marshall 1975).

Other natural basins of importance include Fisheating Creek (1,194 km2)

and Taylor Creek which drain 1,194 and 477 km2, respectively. Since

the early 1900s, a 785 km2 area of the Everglades immediately south

of the lake has been added to the watershed via an extensive system of

canal and pump stations. This region, otherwise known as the Everglades

Agricultural Area (EAA), is dominated by Everglades muck soils and is

intensively farmed. Flow in the EAA is conducted along the North New

River, Hillsboro, and Miami and, depending on flood control and water

conservation requirements, can be either imported from or exported to

the lake. Hydraulic export from Lake Okeechobee is also conducted by

the St. Lucie, West Palm Beach, and Caloosahatchee canals, whereas

outflow historically occurred as sheet flow over the southern rim into

the Everglades.

Land use within the Lake Okeechobee watershed has been compiled by

McCaffrey et al. (1976). Results from the compilation are presented in

Figure 11-3 and Table 11-2. In general, the watershed is dominated by

some type of agricultural activity. Agricultural lands constitute

48 percent of the watershed and comprise primarily improved and

unimproved pasture land in the northern and northwestern portions of the

basin and croplands in the south. Wetlands are the next most signifi-

cant land use type (18 percent), followed by surface water (18 percent
































































Figure 11-2. Surface Water Drainage Basins in the Lake Okeechobee Watershed.

Note: Areas designated by dashed lines contribute only small amounts of water
at irregular intervals. Drainage basins south of the lake have been
added by man. From Federico et al. (1981).


































































Figure 11-3. Land Use Within the Lake Okeechobee Watershed. From
McCaffrey et al. (1976).



















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including Lake Okeechobee) and forests (6 percent). Urban areas

(5 percent), which include the towns of Clewiston, Belle Glade, and

Okeechobee City, represent the smallest area of the total watershed.



Physical Description and Limnology of Lake Apopka

Lake Apopka is the fourth-largest lake in Florida, encompassing a

surface area of 124 km2. Located approximately 25 km northwest of

Orlando between latitudes 28*33' to 2841' and longitudes 8131' to

81'42', Lake Apopka forms the headwaters of the Oklawaha chain of lakes,

and ultimately the Oklawaha River (Figure 11-4). As indicated by its

shoreline development index (1.39), the lake is somewhat circular in

shape and is characterized by relatively broad fetches extending along

both the north-south and the east-west axes (14.4 and 13.8 km,

respectively). Important physical characteristics of the watershed are

summarized in Table II-1.

Lake Apopka resides at the southernmost end of the physiographic

region known as the Central Valley, which is bounded on the west by the

Lake Wales ridge and to the east by the Mount Dora Ridge. The lake

itself occupies an extremely shallow basin with an average depth of only

1.7 m at normal stage (20.3 m above MSL) (Figure 11-5); consequently,

Lake Apopka is continually well mixed by wind-driven currents. Despite

its shallow nature, the lake is characterized by the virtual non-

existence of rooted macrophytes. This apparently stems from compression

of the photic zone to much less than 1 m (approximately 40 to 60 cm)

because of high algal densities characteristic of the hyper-eutrophic

lake and the instability of the highly flocculent sediments.
































MAGNOLIA
PARK


Location Map of Lake Apopka.


Figure 11-4.




















































II MILE


Figure 11-5. Bathymetric Features of Lake Apopka. Depth Contours Given
in Feet.











The watershed of Lake Apopka is rather small in relation to the

lake surface area, and excluding the lake surface, drains only 187 km2

No natural surface influents of any consequence convey water to the

lake. The major hydraulic input is precipitation falling directly on

the lake surface; the other important natural source is artesian flow

from the Floridan Aquifer at Gourd Neck Springs in the southwest corner

of the lake. During periods of intense rainfall, Johns Lake located

2.0 km to the south may overflow into Lake Apopka. Several other small

streams in the southeastern basin Lake Apopka may flow into the lake;

these streams are also ephemeral, and their existence is a function of

antecedent rainfall.

Similar to Lake Okeechobee, the watershed of Lake Apopka has been

increased by cultural activities. Around 1940, a vast (7,200 ha) marsh

was drained and converted into highly productive muck farmland. Flow,

which historically has been south to north, was diverted from the marsh

by an earthen dike and conducted northward through the Apopka-Beauclair

Canal. A series of canals and pump stations was constructed to regulate

water level within the muck soils. As a result, backpumping for the

for the muck farms has constituted a significant hydraulic (and

nutrient) source; however, since 1976 the magnitude of this source has

diminished considerably with the institution of discharge abatement

measures.

Land use estimates within the Lake Apopka drainage basin have been

developed by the East Central Florida Regional Planning Council (1973)

and are included along with projections for 1990 in Table 11-2. At the

time of the above study, 8 percent of the land area was urbanized, while











citrus growing and muck farming activities occupied 43 and 39 percent of

the land, respectively. As can be seen from Figure 11-6, muck farming

is limited to the northern perimeter of Lake Apopka, and most of the

remaining land in the lake basin is occupied by citrus groves. Urban

areas are primarily limited to sections on the south and west shores of

the lake and include the towns of Winter Garden and Montverde.



Materials and Methods

Physical and Chemical Characterization

Samples were collected at 14 stations within Lake Okeechobee

(Figure 11-7) to determine physical and chemical characteristics of

surficial sediments. Five stations (01, 03, 04, 08, 09) were located

within the littoral zone; five stations (06, 07, 010, 011, 012) were

open-water stations; and the remaining four stations (02, 05, 013, 014)

may be described as transitional. Samples were collected during three

sampling efforts in the summer and fall of 1980. Sediment samples from

Lake Apopka were collected quarterly at nine open lake stations (Al

through A9) and one station at Gourd Neck Springs (A10) (Figure 11-8)

between March 1977 and March 1978. All samples were collected with a

Ponar dredge and placed in wide-mouth one-liter polyethylene bottles,

stored on ice until return to the laboratory and then refrigerated until

analysis.

All sediment samples were homogenized prior to analysis. Water and

volatile solids content were determined on a percent basis according to

standard methods (APHA 1976). Particle size distribution was evaluated

by wet sieve analysis for sand-sized fractions (>63 um) and by pipette










I /
I

*:- -


Citrus Gro e
Apopka -Beauclair Citru o .
Canal..
i ; : : .: :-. .


U.S.G.S. Drainage Boundary .





Figure 11-6. Land Use Within the Lake Apopka Watershed.




























































Figure 11-7. Sediment Sampling Locations in Lake Okeechobee.
































MAGNOUA
PARK


Sediment Sampling Locations in Lake Apopka.


Figure 11-8.











analysis for silt-clay fractions (Guy 1969). In order to characterize

the effective particle size of the whole sediment, sedimentary organic

matter was not removed by oxidation. Effective particle size is

necessary in determining the threshold shear required to induce sediment

resuspension for a particular sediment.

The phosphorus content of selected sediment samples was analyzed in

terms of three fractions: apatite inorganic phosphorus (AI-P),

nonapatite inorganic phosphorus (NAI-P), and organic phosphorus. The

latter fraction was determined as the difference between the total

phosphorus (TP) content and the sum of two inorganic fractions. The

inorganic fractions were estimated on fresh undriedd) sediments by the

analytical method described by Armstrong (1979). This method involves a

two-step sequential extraction scheme: the sample is initially

extracted with 0.1 N NaOH for 16 hours to obtain NAI-P followed by an

extraction with 1 N HCI for 16 hours to determine AI-P. TP was

determined on fresh sediments by the acid-persulfate digestion technique

(APHA 1976). Digested samples were filtered, neutralized, and analyzed

for soluble reactive phosphorus (SRP) using the single reagent

molybdenum blue technique (U.S. EPA 1976).

Interstitial water was extracted at 4C by centrifugation of fresh

sediment samples at 10,000 rpm for 30 minutes. The supernatant was then

filtered through GF/C glass micro-fibre filter paper, and the filtrate

was analyzed for SRP by the single reagent molybdenum blue technique

(U.S. EPA 1976).











Sorption Experiments

A series of adsorption-desorption studies were conducted to

quantify phosphorus exchange dynamics using sediment samples representa-

tive of littoral transitional and profunded zones. Sorption isotherms

were evaluated at 220C; samples were buffered at pH 8.3 with NaHCO3

at an ionic strength of 8.5 x 10-3 M and 5.0 x 10-3 M. These conditions

reflect the average pH and ionic strength observed in Lake Okeechobee

(Federico et al. 1981) and Lake Apopka (Pollman et al. 1980), respec-

tively. Aliquots of wet sediment equivalent to 0.5 g of oven-dried

material were allowed to equilibrate with 100 mL of buffered water for

24 h on a rotary shaker table. The buffered solution was adjusted with

KH2PO4 to yield initial orthophosphorus concentrations of 0, 20,

50, 100, 250, 500, 1,000, and 5,000 ug P/L. After 24 h of shaking, the

suspensions were filtered through GF/C glass micro-fibre filter paper

and subsequently analyzed colorimetrically for SRP using the method

described previously. The quantity of phosphorus adsorbed or desorbed

was determined from the difference between the final and initial

concentrations and was corrected for contributions of SRP from

interstitial water.

















CHAPTER III
PHYSICAL-CHEMICAL SEDIMENT CHARACTERISTICS



Physical Characteristics

Erosion, transportation, and deposition of sediment particles in a

lake are governed by many physical factors, including effective fetch,

water depth, bottom depth, and lake morphometry (Hakanson 1981). The

sediment parameter that probably best indicates the dynamic conditions

prevailing at the sediment-water interface is the water content of

surficial sediment. For example, Migniot (1968) demonstrated an inverse

relationship existed between critical shear stress and sediment water

content for a series of lacustrine muds. Hakanson (1977) derived an

empirical relationship that describes erosion and deposition of

sediments in Lake Vanern (Sweden) as a function of sediment water

content and maximum effective fetch (Peff) (U.S. Coastal Engineering

Research Center 1977). The water content of surficial sediments in

Lakes Okeechobee and Apopka thus was determined to evaluate the physical

nature of the sedimentary environment, i.e., whether a particular

location within a lake basin is a zone of relative quiescence or an area

subject to turbulence and resuspension.

Results for water content analyses for Lake Okeechobee and Lake

Apopka are summarized in Tables III-1 and 111-2, respectively.

Sediments from the central portion of Lake Okeechobee had water contents

ranging from 82.3 to 91.1 percent. With the exception of Station 09,





















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the water content of littoral sediments within Lake Okeechobee was

significantly lower (p <0.05) than that of the central sediments and

averaged only 49.6 percent. Station 09 was in the vicinity of beds of

rooted macrophytes and emergent vegetation, which effectively dampen

wave energy, thus permitting the accumulation of finer particles and

detritus. Water content of sediments from transitional zones was quite

variable, ranging from 26.7 to 88.6 percent, and reflected the gradation

from a highly dynamic environment to more quiescent conditions. The

difference between central-lake and littoral-zone sediments in Lake

Okeechobee is further illustrated by the relationship between the

fractional content of silt- and clay-sized particles and water content

(Figure III-i). A close correlation (r = .94) was found between

sediment water content and particle size distribution:


W = 41.16 + 12.66 (In B) (III-1)


where W = percent water content, and
B = cumulative mass percent (sediment particles < 63 um in
diameter).


Thus, the near-shore sediments within Lake Okeechobee tend to be large

grained with low water content, while the central-lake sediments are

fine grained with high water content. A similar relationship for Lakes

Vanern and Ekoln (Sweden) was derived by Hakanson (1977), who approxi-

mated water content as a function of mean particle size. In general,

water content (or more specifically porosity) increases with decreasing

particle size, as a result of increasingly effective electrostatic

(repulsive) forces between clay minerals (Berner 1971, 1980).





0

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In contrast to Lake Okeechobee, the sediments of Lake Apopka are

relatively homogeneous with respect to water content; this trend implies

a somewhat uniform hydrodynamic environment. Lake Apopka is charac-

terized by extensive deposits of highly flocculent sediments with water

content ranging between 95.1 to 97.0 percent. According to Schneider

and Little (1969), these unconsolidated deposits cover 90 percent of the

lake bottom with an average thickness of 1.5 m. Excluding the immediate

shoreline, stable sediments comprising sand and shells are restricted to

four small areas. The largest of these areas is a 240-ha region in the

northern portion of the lake basin; the second largest region

encompasses 120 ha adjacent to Crown Point near the eastern shoreline.

Sly (1978) suggested a simple approach to evaluate the occurrence

of sediment suspension. According to Sly, the depth at which

wave-induced shear stress exceeds the critical shear stress (Fc) of

bed materials corresponds to 25 percent of the available wavelength

(based on significant wave period). For Lake Okeechobee, the deepest

portion of the basin is approximately 4.3 m deep and has a maximum fetch

of 20 km. Applying linear wave theory (U.S. Coastal Engineering

Research Center 1977) to Sly's (1978) model, one calculates that sedi-

ment transport can be initiated in the deepest part of Lake Okeechobee

at continuous wind velocities of 13.1 m/s (29 mph) or greater. It is

thus apparent that sediment erosion and transport can be induced

throughout Lake Okeechobee and that sediment accumulation tends to be

restricted to relatively protected areas along the southern edge of the

main basin (South and Pelican Bays) and the subbasin in the northern

portion of the lake. The relatively low water content (<90 percent)











generally observed throughout the lake and especially in the deeper

portion are indicative of substantial wave energy extending to the

sediment surface. For example, according to Hakanson's (1981) model for

sediment transport, sediments at a depth of 1 m in accumulating regions

of Lake Okeechobee should exhibit water contents exceeding 80 percent.

In Hakanson's model, the critical water content defining the limit

between sediment transport and accumulation was found empirically to

approximate


Wc = Wmax 10 (111-2)


where Wc = the critical water content, and

Wmax = the water content at the maximum depth.


Of the three samples collected at this approximate depth (Stations 03,

08, and 014) only Station 014 located within South Bay had a water

content suggesting active sediment deposition. Despite the expansive

fetch that extends to South Bay during wind events from the north

(nearly 58 km), South Bay apparently is sufficiently sheltered by Rocky

Reef, macrophyte vegetation, and by the shoreline configuration to serve

as a settling basin. Moreover, prevailing winds originate from the

east, resulting in more limited fetches for South and Pelican Bays.

According to Hakanson's (1981) model, the only other station within

Lake Okeechobee with sediment water content indicative of relatively

undisturbed accumulation is Station 09, which is located within a

protective stand of rooted macrophytes. The central (open-lake)

stations experience a mixture of deposition and erosion, resulting in












the accumulation of silt- to sand-sized particles (Table III-l). In

shallow lakes such as Lake Okeechobee, continued circulation and

upwelling prevent deposition of extremely fine particles, as Sly's

(1978) model predicts.

Excluding several relatively deep and narrow trenches along the

western and southern shores (4.9 and 5.5 m, respectively), the deepest

portion of the major basin of Lake Apopka is only 3.0 m in depth and has

an effective fetch of 7.4 km for winds originating from the east.

Under these conditions, Sly's (1978) model indicates that resuspension

can be initiated at wind velocities of 9.4 m/s (21 mph) or greater.

This result suggests the ease with which sediment resuspension occurs in

Lake Apopka. Because of its morphometry, quiescent regions of

undisturbed deposition are essentially nonexistent. Indeed the water

column of Lake Apopka often assumes a brownish cast because of

wind-induced scouring of the bottom sediments; for example, turbidities

in Lake Apopka often exceed 40 NTU (cf. Brezonik et al. 1978, 1981;

Tuschall et al. 1979; and Pollman et al. 1980) and have been observed as

high as 120 NTU (see Chapter IV).

Use of Sly's (1978) model as a guideline indicates that virtually

the entire basin of Lake Apopka is subjected to resuspension; the only

regions falling outside Sly's (1978) threshold criterion are the

previously mentioned trenches along the western and south shores and the

relatively protected subbasins comprising Gourd Neck Springs. Given the

extremely flocculent nature of the sediments, the existence of such

trenches is unusual. Bush (1974) explained their existence as a result

of scouring created by wave setup. Wind-induced surface currents are












maximized along the longest reaches of a lake, resulting in a piling-up

of surface water toward the leeward shore. The pressure thus exerted

induces a return flow along the bottom in the opposite direction,

scouring the bottom of muck.

These results conflict with a priori conceptions concerning

sediment water content and lake hydrodynamics. The consistent and

extremely high water content of Lake Apopka sediments suggests (in the

absence of other information) that quiescent conditions prevail

throughout the basin. A similar conclusion is reached from Hakanson's

(1981) model, which indicates that water contents in excess of

90 percent reflect an undisturbed depositional environment. It is

apparent from the previous discussion of Sly's (1978) model that the

morphometry of the Apopka basin precludes undisturbed deposition. The

question then arises as to why such high sediment water contents are

observed throughout Lake Apopka.

Lake Apopka, by virtue of its shallow nature and inherently high

rates of algal productivity, constitutes a rather unique depositional

environment that falls beyond the framework of Hakanson's (1981) model.

Average rates of net primary production as high as 465 mg C/m3-hr

have been observed for Lake Apopka, with an annual average rate of

140 mg C/m3-hr reported by Brezonik et al. (1978). Compared with

the threshold level of 95 mg C/m3-hr reported by Brezonik and

Shannon (1971) for eutrophic lakes, these results show that Lake Apopka

is highly eutrophic. As evidenced by sedimentary concentrations of

volatile solids, high rates of deposition of organic detritus are a

direct -consequence of high rates of algal production. Average sedimen-

tary organic content (as defined by volatile solids) ranged between 56.0










and 77.4 percent in Lake Apopka, and the lakeside average was

64.5 percent. Organic detritus, which has a bulk density approaching

water, is readily resuspended and transported by wind-induced currents,

and its accumulation in sedimentary deposits generally reflects not only

rates of biogenic material production but also the vertical distribution

of hydrodynamic forces. In most lakes, then, transitional hydrodynamic

environments occur that result in selective deposition of particles as a

function of size and density. Consequently, sediment-focusing or high

rates of deposition of organic-rich material are observed in the deeper

reaches of these systems (cf. Davis and Ford 1982). However, because of

the configuration of Lake Apopka's basin, relatively undisturbed

depositional zones are non-existent. Consequently, a virtually uniform

layer of highly flocculent, organic sediments prevails throughout the

basin.

Unlike Lake Apopka, the distribution of organic material in Lake

Okeechobee clearly demonstrates a transitional hydrodynamic environment.

Organic content varied with station location in the basin, ranging from

0.3 to 42.7 percent (dry weight) (Table III-i). Sediments in littoral

areas (excluding Station 09) exhibited very low concentrations of

organic matter, averaging only 2.2 percent. Organic content of open-

lake sediments was significantly higher and ranged between 12.7 and

35.1 percent.

Two classes of open-lake sediments in Lake Okeechobee can be

identified from the results of this study. The first class consists of

highly organic muck (Stations 07 and 012), with sediment organic content

approximately 34 percent. These sediments tend to predominate in the












mid to north central portion of the lake basin. The second sediment

association, which occurred at Stations 06, 010, and 011, is less

organic in nature, averaging 15.1 percent, and was found in closer

proximity to the shoals of the western basin. Sediment organic content

was highest at Station 014 in South Bay, with organic matter

constituting 43.2 percent of the total solid content. South Bay, which

previously was identified as a settling basin, receives substantial

portions of inorganic nutrients from the Everglades Agricultural Area

(Federico et al. 1981).

Further evidence supporting the idea that detrital accumulation in

Lake Okeechobee is influenced by hydrodynamic factors is supplied by

evaluating the relationship between organic matter and water content in

Lake Okeechobee sediments. A logarithmic correlation (r = .92) was

observed between water content and organic content (Figure 111-2):


W = 43.21 + 12.37 (In VS) (111-3)


where VS = percent volatile solids dry weight.



A similar logarithmic relationship was noted by Nisson (1975) for

sediments from Lakes Harney, Jessup, and Monroe, on the St. Johns River

in central Florida.

If detrital accretion in sediments primarily reflects hydrodynamic

factors, a correlation also should be observed between particle size and

organic content. This relationship stems from the fact that the

conditions that favor the deposition of fine-grained sediments are also

conducive for the accretion of detritus. For sediments in Lake


















*















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*


*


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0 *
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Okeechobee containing 23 percent organic matter or less, this is indeed

the case (Figure 111-3). In this range, organic content and silt-clay

content are linearly related. With organic content exceeding about

23 percent, however, the fractional silt-clay content remains constant

(approximately 20 percent), suggesting that in quiescent waters local-

ized rates of primary production assume a greater role in controlling

the magnitude of detrital deposition. The fractional silt-clay and

organic content of Station 06 does not conform to this relationship;

unique morphometric and hydrodynamic factors may permit greater

deposition rates of colloidal silt-sized particles at this site.



Chemical Characteristics

The chemical nature of sedimentary phosphorus defines its

availability for exchange and biological uptake. Differential dissolu-

tion schemes have been developed that correlate algal availability with

a particular fraction. Such fractionation schemes were applied to Lake

Okeechobee and Lake Apopka sediments, and results are described in this

section. Total P analyses, on the other hand, provide an estimate of

the total pool of phosphorus that ultimately is available for exchange.

Total phosphorus (TP) concentrations in Lake Okeechobee sediments

were highly variable and ranged from only 142 ug P/g (dry weight) to

1,440 ug P/g. Conversely, the distribution of TP in Lake Apopka

sediments was fairly homogeneous and reflects the uniformity of the

depositional environment in the lake. Concentrations in sediments

within the main basin ranged from 749 ug P/g at Station Al to 1,170 ug

P/g at Station A8. TP concentrations in Station A10 sediments at Gourd








48








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00



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Neck Springs were somewhat higher (1,320 ug P/g) because of the

deposition of large quantities of decaying water hyacinths (Eichornia

crassipes). Phosphorus concentrations in lacustrine sediments may range

as high as 7,500 ug P/g, but typically are less than 2,500 ug P/g (Jones

and Bowser 1978). In a survey of mineralogical constituents comprising

profundal Minnesota lake sediments, Dean and Gorham (1976) observed an

average TP concentration of 1,400 ug P/g in calcareous lakes. Although

it is apparent that TP concentrations in Lake Okeechobee and Lake Apopka

sediments fall somewhat within the lower end of the spectrum, the

observed concentrations approach and in some cases exceed the

1,000 ug P/g threshold level cited by Frink (1967) as indicative of

eutrophication.

Within Lake Okeechobee, the highest TP concentrations generally

were observed in the pelagic reaches of the lake basin. TP concentra-

tions in pelagic sediments ranged from 502 to 1,440 ug P/g with an

average value of 866 ug P/g; concentrations in littoral sediments were

significantly lower (p <0.05), averaging only 294 ug P/g. These results

imply that selective size sorting processes influence sedimentary P

distribution; similar results have been reported by Frink (1967 and

1969) and Hakanson (1981), as well as others. Based on the relatively

poor correlation between water content and TP in Lake Hjalmaren

sediments (r = .68), however, Hakanson (1981) concluded that accretion

of sedimentary P is mediated by anthropogenic and/or chemical factors in

addition to hydrodynamic processes. The weak correlation (r = .57,

Table III-3) between TP and water content in Lake Okeechobee sediments

implies a similar conclusion.












Table III-3. Summary of Sedimentary Component Interrelationships
with Total Phosphorus Concentrations in Lake
Okeechobee Sediments.


Relationship


NAI-P vs. TP .46

AI-P vs. TP .81

Interstitial SRP vs. TP .19

Volatile Solids vs. TP .76

Water Content vs. TP .32











As indicated by linear regression analysis, the accumulation of

sedimentary P in Lake Okeechobee is dictated to a significant extent

(p <0.05) by the deposition of apatite-inorganic P (AI-P) (Table III-3).

With the exception of organic content, correlations between TP and other

sediment parameters were weak and insignificant at the 95 percent

confidence level.

Mechanistically, the functional dependency of sediment TP content

on AI-P and organic content may be explained by evaluating the relation-

ship between calcium-carbonate solubility, calcium-phosphate solubility,

and pH changes induced by primary production. The premise of this

hypothesis is that deposition of AI-P and detrital material are causally

linked to the same forcing function--primary production. For example,

calcium carbonate precipitation may be induced by increases in pH that

result from net primary production (Kelts and Hsu 1978); calcite

surfaces act as heterogeneous nuclei for calcium phosphate minerals

(Stumm and Leckie 1971; Griffin and Jurinak 1973), resulting in removal

of phosphorus from the water column. Similarly, calcium-phosphate

minerals tend to become less soluble with increasing pH (within the

range for most natural waters) owing to a shift in phosphorus speciation

and, in the case of hydroxyapatite CalO(P04)6(OH)2, an increase in

hydroxide concentration. To test the hypothesis that AI-P deposition

may be influenced by biogenically induced pH shifts, the stability of

various calcium phosphate mineral phases in Lake Okeechobee was

evaluated by comparing measured ion activity products (IAP) for the

species with thermodynamic solubility products (Ks0 values). For

purposes of this exercise, two pH values were used: 8.10, the average











pH and 9.10, the maximum pH observed by Federico et al. (1981) for a

series of stations near South Bay. Reported average Ca concentrations

approximated 46.5 mg/L. Average orthophosphorus concentrations at these

stations ranged between 0.020 mg P/L and 0.025 mg P/L; for the purposes

of this exercise a concentration of 0.0225 mg P/L was assumed. Of the

three mineral phases evaluated [CaHPO4, Ca4H(PO4)3, and hydroxy-

apatite, Cal0(PO4)6(OH)21, only hydroxyapatite is stable under ambient

conditions in Lake Okeechobee (see Table 111-4). During periods of high

photosynthetic activity (i.e., high pH), the water column is greatly

supersaturated with respect to hydroxyapatite (IAP/KsO >108), and this

may result in the direct precipitation and subsequent incorporation of

this species in the sediments.

Similar relationships between sedimentary TP and sediment inorganic

P were observed by Wildung et al. (1974) in Upper Klamath Lake, a non-

calcareous lake located in Oregon. Unfortunately, inorganic P as

reported by Wildung et al. (1974) includes both non-apatite inorganic P

(NAI-P) and as AI-P, thus precluding a direct comparison of results. In

direct contrast to the results of this study, however, Williams et al.

(1980) found a weak, negative correlation between TP and AI-P in

sedimentary material derived the drainage basins of Lakes Ontario and

Erie. Williams et al. (1980) demonstrated strong correlations between

variability in TP concentrations and changes in both NAI-P and organic P

fractions; in Lake Okeechobee sediments, however, neither parameter

yielded a significant (p <0.05) correlation with sediment TP.

NAI-P, which is indicative of the sedimentary P reservoir directly

available to algae upon resuspension (Williams et al. 1980; Allan et al.










Table III-4.


Solubility Product Constants (Ks0) and Calculated Ion
Activity Products (IAP) for Several Calcium Phosphate Mineral
Phases as a Function of Ambient Ca (1.16 x 10-3 M), SRP
(7.26 x 10-7 M), and pH Levels in Lake Okeechobee.


pH 8.10 pH 9.10
Solid Phase log Ks0* log IAP log (IAP/Ks0) log IAP log (IAP/Ks0)


CaHPO4 -6.6 -10.1 -3.5 -10.1 -3.5

Ca4H(P04)3 -46.9 -55.1 -8.2 -54.9 -8.0

Ca(P04)6(OH)2 -114 -111.6 2.4 -105.4 8.6


* From Stumm and Morgan (1981).










1980), generally is low in Lake Okeechobee sediments. Concentrations

ranged from 6 to 96 ug P/g and generally constituted less than

9.0 percent of the total P content. From vertical profiles of nutrients

in Lake Okeechobee sediment cores Brezonik et al. (1981) inferred a

sediment suspension or bioturbation depth of 10 cm; for Station 07

sediments this implies a labile P pool of approximately 1.04 g P/m2

which is sufficient to increase existing levels by 305 ug P/L (of labile

P) if the entire reservoir was released in a single pulse event.

Despite the relatively low concentrations of NAI-P, this calculation

illustrates the inherent potential for photosynthetic stimulation

induced by nutrient exchange across the sediment water interface.

Concentrations of NAI-P in Lake Apopka sediment were relatively

uniform and ranged from 83 to 111 ug P/g. Similar to Lake Okeechobee,

this fraction comprises only 10 percent of the total P reservoir.

However, the inherent potential for this reservoir to support primary

production in the water column is considerable. For example, at

Station A5 with average NAI-P and interstitial SRP concentrations of

108 ug P/g and 0.80 mg P/L, respectively, the total potential release of

inorganic phosphorus to the water column by resuspending only 1 cm of

sediment is 49 mg P/m2. This is equivalent to an incremental

addition of 29 ug P/L to the water column. Of this total, the NAI-P

fraction contributes about 84 percent. It is important to note that

resuspension of sediment to a depth of 4.7 cm has been observed in Lake

Apopka during convective disturbances (see Chapter IV) and most likely

extends more deeply into the sediment.










The distribution of SRP concentrations in sediment pore fluids for

both Lake Okeechobee and Lake Apopka was somewhat erratic, and no trends

were discernible between sediment type and interstitial SRP. Inter-

stitial SRP concentrations in Lake Apopka were quite high and ranged

from 0.40 to 2.80 mg P/L. The source of this variability is apparently

a combination of physical, biological, and chemical factors. For

example, the ability of Lake Apopka sediments to further adsorb

phosphorus (and hence buffer interstitial SRP concentrations within a

relatively narrow concentration range) is virtually exhausted and is

limited to extremely high concentrations. Under these conditions, small

changes in the concentration of phosphorus adsorbed to sediment particle

surfaces can result in very large changes in equilibrium SRP concentra-

tions in the pore fluid (see Chapter V). In addition, interstitial SRP

levels increase as organic phosphorus is mineralized by microbial

heterotrophs. Pore fluid concentrations apparently continue to increase

until the sediments are disturbed and this pool is exchanged with the

overlying water. The sediments then reequilibrate at a lower but still

highly elevated (relative to the water column) concentration.

Interstitial SRP concentrations in Lake Okeechobee sediments

ranged from 0.002 mg P/L to 0.942 mg P/L with the maximum concentration

found in the nutrient-enriched, highly organic sediments of Station 014.

On the other hand, the pore water content of SRP in Station 07

sediments, which also were quite organic, was only 0.012 mg P/L, and the

interstitial SRP content of the relatively inorganic, sandy sediments of

Station 04 was substantially higher (0.267 mg P/L). Despite the

variability in concentrations, it is clear that SRP concentrations in










the interstitial fluids were generally quite low, and this fraction

constitutes less than 0.1 percent of the total sedimentary P. The

ability of calcareous sediments (and specifically calcium carbonate) to

adsorb P is well documented (e.g., Griffin and Jurinak 1973 and 1974),

and Morse and Cook (1978) suggested that this mechanism accounts for the

low levels of dissolved interstitial SRP found in carbonate-rich marine

sediments. The small size of the interstitial reservoir implies that

this pool is not a significant source of phosphorus to the overlying

water via advective exchange across the interface. For example,

resuspension of 10 cm of sediments from Station 07 would result in

addition of only 0.3 ug P/L to the water from entrained pore water.
















CHAPTER IV
RELEASE STUDIES



A series of passive exchange studies were conducted to evaluate P

release over a broad range of water content for Lake Apopka sediment.

Sediment was collected from Station A2 and placed in five small aquaria

with surface areas approximately 305 cm2. Aquaria were filled with

fresh sediment to a depth of approximately 13 cm and were exposed to

varying intensities of artificial light from two 150 watt spotlights to

generate a gradient of sediment dessication. Initial water content was

95.1 percent, final values ranged from 13.8 to 88.3 percent (Table IV-1).

As dessication proceeded, surface areas decreased correspondingly and

ranged from reductions of 18.5 to 78.9 percent. Following dessication,

the aquaria were refilled with phosphorus-free 6.0 x 10-4 M CaC03

to simulate ambient alkalinities observed in Lake Apopka and were

allowed to remain quiescent for a 9-day period. Samples of the

overlying water were taken on a daily basis to determine the quantity of

SRP released.

Phosphorus release by simple diffusion is illustrated in

Figure IV-1. The appropriate theoretical (Fickian) expression for

describing the flux of phosphorus across the sediment-water interface is

given by the solution to Fick's first law of diffusion:



dC
J = 0 Ds -- (IV-l)
dz










Table IV-1.


Final Volatile Solids and Water Content of Partially
Dessicated Lake Apopka Sediments Used in Passive
Exchange Experiments.


Water Content Volatile Solids
Aquarium (percent) (percent)


1 88.30 69.99

2 86.18 70.58

3 34.31 68.58

4 13.87 70.90

5 13.82 70.94




















CO


c( >
0










-4 W a)
a.

0 *


S- -O o .



0.- 0 0B









0 *








\) ,Q) 4 0
)




> 0 0 ca
-4 4-


S'' C m

to CI N -W4


/d ~ ~ O ca d



o 0 0 0
10 \J d C- no













-Q


1










where J = flux of phosphorus across the interface (g P/cm2-s),
S= porosity of the sediment dimensionlesss),
Ds = the whole sediment diffusion coefficient (cm2-s),
and
dC
dz= the concentration gradient at the interface (g/cm4).



This simple model assumes that the flux across the interface is strictly

due to molecular diffusion and that deposition or compaction of sediment

particles is negligible (cf. Lerman 1979; Berner 1981). The change in

concentration in the water column at any particular point in time is

simply the flux across the interface divided by the depth of the water

column; this of course assumes that the water column is well mixed and

is a reasonable assumption for the depth scale of these microcosm

studies.

Solution of Equation IV-1 requires that the concentration gradient

at the interface is known at a particular point in time; consequently,

application of Equation IV-1 to the experimental results is extremely

difficult. A more tractable approach is to fit an empirical model to

the data which may then be differentiated to describe the instantaneous

change in concentration as a function of initial concentration. In all

cases, release over the duration of the experiment (9 days) can be

described by the following logarithmic expression:


Ct = a + b In t (IV-2)


where Ct = the uniform concentration (mg/L) in the overlying
water at time t (days), and
a (mg/L) and b (mg/L/d) are both empirically derived constants.











The rate of change in concentration is simply

dC 1
b (IV-3)
dt t


Agreement of the data with the model, expressed in terms of the

correlation coefficient, r, is presented in Table IV-2. Results from

Aquaria 4 and 5 were combined because of the virtually identical water

contents. Equation (IV-3) can be applied to evaluate the instantaneous

change in concentration, and through mass balance considerations, the

instantaneous areal release rate for a specified time. Average and

instantaneous fluxes evaluated at the conclusion of the experiment are

presented in Table IV-3.

Release rates averaged over the 9-day exchange period ranged from

1.57 to 22.7 mg P/m2-d and were inversely related to water content.

These results were somewhat surprising and did not conform to a priori

expectations; that is, if the concentration gradient is independent of

the degree of dessication, sediments with greater water contents should

yield higher diffusive fluxes because of the decreased tortuosity

(Manheim 1970; Lerman 1979). Enhanced rates of detrital mineralization

with increasing dessication and a concomitant elevation of interstitial

SRP levels can account for the observed changes in flux; however,

analyses conducted by Fox et a!. (1977) of interstitial water SRP

content in fresh and dried Lake Apopka sediments demonstrated essen-

tially no difference between the two sediment types and thus do not

substantiate this mechanism. A more likely mechanism is given by

considering the change in sediment mass exposed on an areal basis as

dessication proceeds. As pore water evaporates and the sediments











Table IV-2.


Summary of Empirical
Parameters (Equation


Time-Dependent Concentration Model
IV-2) and Goodness of Fit.


Water Content
Aquarium (percent) a b r2


1 88.30 0.0268 0.0417 .995

2 86.18 0.0231 0.0415 .992

3 34.31 0.0416 0.1137 .986

4 and 5 13.85 0.0032 0.1951 .919







Table IV-3. Calculated Average and Instantaneous Phosphorus Flux at
Time = 9 Days During Passive Exchange Studies.



Average Flux Instantaneous Flux
Aquarium (mg P/m2-d) (mg P/m2-d)


1 1.57 0.55

2 2.18 0.77

3 13.9 5.12

4 and 5 22.7 9.71










consolidate, increasing quantities of underlying sediment particles

become directly exposed at the surface. A change in water content from

88 to 13 percent increases the exposed mass by approximately a factor of

10, which is close to the observed differences between the average flux

from sediments 4 and 5 and from 1 and 2.

Additional experiments to quantify the passive exchange of P

between Lake Apopka sediment and water were conducted using homogenized

sediment from Station A5. Sediment was transferred to a series of three

cylinders in sufficient quantity to yield a final depth of 14.5 cm. The

diameter of the cylinders was 10.2 cm, resulting in a surface area of

81.7 cm2 for exchange. The experiment was also designed to quanti-

tatively evaluate the effects of dessication induced by lake drawdown on

exchange; consequently two of the cylinders, B and C, were partially

dessicated by insolation to yield final water contents of 91.66 and

89.14 percent, respectively. The final cylinder, A, was allowed to

incubate at room temperature (22C) for the duration of the dessication

period (1 week). The water content of Cylinder A sediments was

94.89 percent. Following the dessication period, water was added to all

three cylinders simultaneously. One liter of filtered, aged lake water

from Lake Apopka, with an SRP concentration of 0.004 mg P/L, was added

to each cylinder to serve as the solution phase; this maximized the

concentration gradient across the sediment water interface and also

removed any algae present.

During the incubation period the sediments in Cylinder A began to

compact, lowering the sediment-water interface by approximately 0.7 cm

and forming a residual interfacial pool of SRP enriched water.

Subsequent extraction, filtration and analysis of 10 ml of the










interfacial water indicated an average concentration of 1.35 mg P/L.

The interfacial water comprised a labile pool of approximately 64 ug P

for immediate exchange upon addition of filtered lake water. Care was

taken not to disturb the interface when the filtered lake water was

added to each cylinder. Since the photic zone rarely extends below a

depth of 0.9 m in Lake Apopka, the diffusional phase of the experiment

was conducted in the dark. Free exchange with the atmosphere was

permitted, however, to prevent the system from going anaerobic.

Changes in SRP concentration with time are shown in Figure IV-2 for

all three cylinders. Cylinder A, representing fresh sediment, began the

initial phase of the experiment with the highest concentration of SRP

(0.192 mg P/L). Approximately 32 percent of this pulsed release was

engendered by mixing of the residual interfacial pool with added lake

water; the remaining release may have been induced by a slight

disturbance of mud-water interface and subsequent entrainment of

interstitial P and/or desorption from sediment particles. Because of

this pulsed release, the concentration gradient was reduced and

Cylinder A, consequently, showed the lowest rate of release. Calcula-

tions of release rates were based upon observed changes in SRP in the

overlying water during the initial phase of the experiment, when the

concentration gradient was most extreme and the resulting overlying

water concentrations approximated ambient SRP levels observed in situ.

Release rates were essentially linear during this phase, and as seen in

the previous exchange studies with A2 sediment, release increased as an

inverse function of sediment water content. Release from Cylinder A

approximated 4.3 mg P/m2-d while release from Cylinder C was

calculated to be 28.8 mg P/m2-d, (Table IV-4). Observed rates of





















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release for Cylinders A and B are comparable to other values reported

in the laboratory for undisturbed cores (Table IV-5). In addition to

the previously described hypothesis regarding sediment exposure and

dessication, the observed flux in Cylinder A is also undoubtedly lower

because of the reduced concentration gradient across the sediment-water

interface. This effect can be derived directly from Fick's first law of

diffusion where flux is proportional to the concentration gradient. This

effect has also been empirically verified by Rippey (1977) who observed

that maintenance of low concentrations in the overlying water of

undisturbed cores increased areal release rates by 82 percent.

For Cylinders A and B, the linear phase of rapid release was

confined to an extremely short timeframe of 6 hours beyond which

decreases in concentration approaching initial levels occurred.

Concentrations in Cylinder A remained relatively stable for the next

42 hours, but by 72 hours they showed a further decline to a minimum

concentration of 0.172 mg P/L. After 72 hours, release was again

essentially linear in Cylinder A, and approximated 0.6 mg P/m2-d.

Similar erratic behavior was observed in Cylinder B; between 6 hours and

48 hours SRP levels fell to 0.007 mg P/L and then proceeded to increase

towards a secondary maximum of 0.023 mg P/L at 48 hours. Between 121

and 362 hours, SRP concentrations steadily increased. Release during

this period equalled 0.3 mg P/m2-d. Although the concentration

gradient was more pronounced in Cylinder B, the reduced flux after the

initial equilibration stage is more consistent with the decline in

overall porosity and the formation of a relatively impermeable crust

caused by partial dessication.



























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Fox et al. (1977) conducted similar experiments with Lake Apopka

sediments to determine the effects of dessication on overlying water

quality. A general pattern of increasing P release with extent of

dessication was observed. Release as a function of dessication for both

this study and that of Fox et al. (1977) is presented in Figure IV-3 for

equilibration times of 144.5 hours and 1 week, respectively. Linear

regression analysis indicates a highly significant relationship between

the two variables (r2 = .74, p <0.025), yielding the following model:



SL = 1.44 0.01 W (IV-4)


where SL = concentration (mg P/L) observed after the stated
equilibration times, and
W = percent water content of the sediment.



The most consistent behavior with respect to release was observed

for the most dessicated sediment, Cylinder C. Release was relatively

constant over the initial 24 hours and began to decrease, apparently in

response to the diminished concentration gradient. Net release

continued until a state of dynamic equilibrium was achieved by Day 4,

when uptake essentially balanced release. Equilibrium was maintained

for approximately the next 93 hours following which time concentrations

dropped steadily for the duration of the experiment. The subsequent

decline in SRP is probably biological in nature; the longer period of

exposure to insolation and incident UV radiation undoubtedly had a

pronounced, deleterious effect on the benthic microflora in Cylinder C

relative to the other cylinders. If benthic microbial activity



































































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increased as the experiment progressed a concomitant increase in SRP

uptake should have been observed; this mechanism is consistent with the

observed decline in SRP. It is interesting to note that concentrations

of SRP in both Cylinder A and B remained relatively constant during the

latter stages of the experiment; this may indicate that the benthic

microbial community was stable from the outset.

Calcium and magnesium concentrations were monitored during the

course of the study. Magnesium levels were essentially constant when

corrected for evaporative effects whereas corrected calcium levels show

a substantial increase of approximately 14 mg/L by the end of the

experiment. Sedimentary CaCO3 is apparently solubilized as a result

of heterotrophic activity producing CO2 (Lee et al. 1977). The

magnitude of Ca release for all three cylinders was approximately the

same, and thus solubilization of hydroxyapatite was not the primary

mechanism for release since the amount of P release differed markedly

among the cylinders.



Turbulence Study

As an extension of the quiescent nutrient release study conducted

with sediments from Station A5 in Lake Apopka, the effects of turbulence

on SRP exchange also were evaluated. After equilibration periods of 35

to 45 days to dampen phosphorus concentration changes associated with

diffusion, Cylinders A and C (representing fresh and partially

dessicated sediments, respectively) were subjected to varying degrees of

turbulence with a variable speed rotor with a paddle attachment. The

paddle encompassed a surface area of 21 cm2 and was situated 4.4 cm










above the sediment-water interface. The depth of the water column for

each system was maintained at approximately 11.2 cm.

Samples for SRP were withdrawn immediately preceding the start of

each experiment; subsequent samples were taken at measured time

intervals. All samples were filtered through GF/C glass microfibre

filter paper. Supplemental aliquots also were removed at periodic

intervals to determine suspended sediment concentrations. Equivalent

quantities of aged and filtered water from Lake Apopka were introduced

into each cylinder after each sample was withdrawn in order to maintain

a constant volume.



Fresh Sediment

Results of the turbulence experiment with fresh Lake Apopka

sediment are depicted in Figures IV-4 and IV-5. The initial phase was

conducted at a stirring velocity of 10.5 to 12.2 revolutions per minute

(rpm) sustained for 62 minutes. This velocity was sufficient to entrain

isolated, loose sediment particles at the interface, although the

interface remained essentially intact. Upon lowering the velocity to

9.0 rpm, the suspended particles began to settle. A net decline

(corrected for sample removal) of 0.002 mg P/L was observed during this

period, indicating virtually no effect of stirring at this rate on

equilibrated sediment.

The system was allowed to remain quiescent for another 67 minutes

before initiation of a second turbulence period. Rotor velocity was

increased to a maximum of 67.6 rpm, causing immediate scour of the

interface. Turbulence was maintained for exactly 1 hour. After a















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preliminary reduction in concentration of SRP in the overlying water at

the start of turbulence, SRP levels increased to 0.484 mg P/L and

suspended sediment concentrations reached 12.61 g/L by the conclusion of

the turbulent phase. Mass balance calculations indicate that between

25 to 68 percent of the net release in SRP was the result of entrained

interstitial water; the remaining fraction was apparently desorbed from

suspended particles and was equivalent to desorption of 12.3 to

28.4 ug P/g. More exhaustive studies concerning sorption phenomena are

presented elsewhere (see Section V, Adsorption-Desorption Studies). The

contribution of the pore water to release was calculated by assuming

that the concentration of SRP was between 0.50 to 1.35 mg P/L; the

latter value represents the interfacial water concentration at the

beginning of the diffusion study following exposure whereas the former

value constitutes the interstitial water content of the fresh sediment

prior to exposure. If release during the quiescent study is assumed to

be strictly diffusive, then 33 percent of the observed net release can

be attributed to entrainment of the pore fluid and the magnitude of

desorption approximates 25.6 ug P/g.

After stirring was concluded, resuspended sediment particles began

to settle rapidly, establishing a sharp interface between a clear

supernatant and the settling particles. Migration of the interface was

relatively slow (2.57 x 10-3 cm/s) during the first 7 minutes after

cessation of stirring because of turbulence within the water column

(Figure IV-6). After this equilibration period, water motion within the

cylinder became dampened and the settling velocity was much more rapid.

As the interface approached the point of origin, velocities decreased in




















100

90-

80-

70. o
70
S60

W 50-
z




20
? 40-

S 30- e

20- e

10-

10 20 30 40 50 60 70 80
TIME (MINUTES)

Figure IV-6. Migration of Interface Between Suspended Sediment and
Clear Supernatant with Time Following Cessation of
Turbulence.










response to resistance from the compacted particles. The maximum

observed settling velocity was 1.95 x 10-2 cm/s. Some readsorption

of phosphorus occurred as the entrained sediment particles settled;

however, a significant increase in concentration relative to initial SRP

levels was observed after equilibrium had been established. The final

concentration was 0.355 mg P/L compared with an initial concentration of

0.159 mg P/L. These results conflict with Holdren and Armstrong (1980)

who observed that P release from Lake Mendota sediments generally

increased with increased stirring up to the point where sediment suspen-

sion occurred. Resuspension was found by Holdren and Armstrong (1980)

to decrease overlying water SRP concentrations to levels below those in

unstirred cores. Conversely, resuspension of Lake Apopka sediments

results in a substantial release of SRP, while stirring in the absence

of resuspension has virtually no effect on release. Indeed, in the

experimental system employed in this study, no change in SRP levels was

anticipated with stirring up to the point of bed failure because the

system was allowed to equilibrate for an extended timeframe. In other

words, the initial conditions of the experiment set ac/9t = 0, which

according to Fick's second law implies that any change in the diffu-

sivity constant caused by stirring will not alter the concentration of

SRP in the overlying water. In the cores used by Holdren and Armstrong

(1980), the derivative of the concentration gradient is non-zero; thus,

an increase in diffusivity induced by stirring will result in a

comcomitant increase in cc/lt.










Partially Dessicated Sediment

The effect of turbulence on SRP release from partially dessicated

sediments (Cylinder C) was minimal. As in the preceding experiment on

Cylinder A, the sediments of Cylinder C were subjected to two discrete

periods of turbulence separated by a quiescent phase to isolate any

observed effects. The initial turbulent phase was conducted at an

average rotor velocity of 13.9 rpm maintained for 1 hour. Concentration

changes were somewhat erratic during this phase (Figure IV-7) and when

corrected for sample removal and replacement with filtered Lake Apopka

water were equivalent to a net release of only 0.001 mg P/L. Rotor

velocity was insufficient to disturb the sediment-water interface,

although some unconsolidated particles were entrained.

After a stabilization period of nearly 2 hours during which the

overlying water SRP concentration remained essentially constant,

turbulence was again induced. Rotor velocity was increased to 115.5 rpm

and was allowed to continue for exactly 2 hours. Despite the extreme

amount of turbulence, the interface remained intact except for some

erosion from a small hole initially created by the removal of a sediment

subsample for analysis. A steady-state suspended sediment concentration

of only 0.0720 g/L was observed after 90 minutes. No phosphorus

desorption was detected in association with the minor amount of sediment

resuspended; however, a small but discernible decrease in concentration

of SRP in the overlying water did occur. The net decrease when

corrected for sample removal was 0.004 mg P/L and corresponds qualita-

tively to the initial decrease observed in Cylinder A when resuspension

was induced. The apparent mechanism for the loss from solution is














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adsorption by resuspended particles derived from the surface; due to a

period of desorption and diffusive loss during the month long quiescent

equilibration phase, the surface particles apparently have less

surface-sorbed P than underlying sediments. Because of the reduction in

surface coverage of available binding sites (see Section V, Sorption

Characteristics), the relatively impoverished surface particles are free

to adsorb P upon resuspension. Further scour of underlying sediments

entrains surface-P rich particles which then undergo desorption (e.g.,

Cylinder A). The extent of scour in Cylinder C was apparently not

sufficient to resuspend these enriched particles; thus, a net decline in

SRP was observed.

The results of this experiment indicate that even partially

dessicated sediments are very stable and will not be resuspended even

under conditions of extreme turbulence. However, if the oxidized

surface layer is breached, exposing the less compacted sediments lying

underneath, localized scour is likely to occur.



In-Situ Studies

Water quality and limnological monitoring of Lake Apopka was

conducted between January 1977 and January 1981 as an integral part of

the Lake Apopka restoration program. The results of the study have been

presented elsewhere for individual years (Brezonik et al. 1978; Tuschall

et al. 1979; Pollman et al. 1980; and Brezonik et al. 1981) and will not

be discussed here except to relate observed chemical characteristics

with the phenomenon of internal loading. Monitoring within Lake Apopka

focused on three open-water stations (A2, A5, and A8) and Gourd Neck











Springs for the bulk of the study after a comprehensive survey of

11 stations during the first year showed that adequate spatial

characterization was provided by three sampling points. Sampling was

conducted monthly except for the first year when the frequency was

biweekly.

To evaluate the degree of interaction between SRP concentrations

and other chemical components, a correlation matrix was developed using

SAS (Statistical Analysis System) (Barr et al. 1976). Correlation

analysis compares parameters by way of a linear model to determine if a

significant relationship exists. The degree of linear correlation

between the two variables is given by the correlation coefficient, r,

which by definition must lie between 1 and -1. Selected results of the

analysis are presented in Table IV-6. Using the 5 percent confidence

level as the threshold of significance, significant relationships

between SRP and other abiotic variables were established for only two

parameters: turbidity and inorganic carbon (IC). Although a simple

correlation between two variables is not conclusive evidence of a cause

and effect relationship, this result does suggest that SRP levels within

Lake Apopka are influenced by the resuspension of sediments.

Indirect evidence to substantiate the role of sediment resuspension

of phosphorus dynamics is offered by evaluating the interaction between

turbidity and ammonia variability. Correlation analysis (Table IV-6)

indicates these two parameters are significantly related. Ammonia,

which is generated by heterotrophic bacteria as organic matter is










Table IV-6.


Intercomponent Relationships Between Selected Variables in
Lake Apopka, 1977 to 1980.


Turbidity IC K Ammonia Color


SRP 0.28915 0.38099 0.30150 0.12823 -0.14958
(0.0487) (0.0082) (0.0621) (0.3904) (0.3156)

Turbidity 0.65573 0.72893 0.32722 0.01153
(0.0001) (0.0001) (0.0248) (0.9387)

IC 0.90591 0.10961 -0.20465
(0.0001) (0.4633) (0.1676)

K -0.00954 -0.10756
(0.9541) (0.5145)

Ammonia -0.03957
(0.7917)


Note: ( ) = significance of relationship.












decomposed, accretes in sedimentary pore fluids to quite significant

levels. Concentrations as high as 49.0 mg N/L have been observed in

Lake Apopka sediments (Brezonik et al. 1978). Average interstitial

concentrations exceed typical levels in the overlying water of Lake

Apopka by over two orders of magnitude; thus even minor disturbances of

the sediment-water interface can exert a profound influence on the

nutrient status of the water column. Seasonal dynamics of nitrogen in

Lake Apopka are stochastic in nature, apparently as a result of episodic

disturbances and scouring of the bottom sediments.

SRP was strongly correlated (p = 0.0082) with inorganic carbon and

to a lesser extent (p = 0.0621) with potassium. Similar to ammonia,

inorganic carbon accumulates in the interstitial fluid as detrital

material is metabolized. Likewise, the close relationship between

turbidity and inorganic carbon (r = .656, p = 0.0001) (Table IV-6)

supports a resuspension type of mechanism for controlling to a signifi-

cant extent the concentration of this solute in the overlying water.

Potassium, which in turn is very closely related to inorganic carbon

(r = .729, p = 0.0001), is essential for aerobic life forms (Lehninger

1970) and has been used as a tracer for the migration through soils of

leachate derived from decomposing vegetative material (Ellis 1980).

Wetzel (1975) has observed that potassium levels can be depleted in the

trophogenic zone of highly productive lakes presumably due to biotic

uptake and in turn accumulate in the tropholytic zone. It consequently

seems reasonable to label K as an indicator of internal nutrient cycling

and, in particular, nutrient release induced by sediment resuspension.










If the internal dynamics of phosphorus cycling in Lake Apopka are

dominated by external loading, a simple correlation between external

loading (or a corollary parameter) and SRP can be anticipated. A

nutrient budget developed for the 1977 water year has identified main-

tenance backpumping of the muck farms bordering the northern perimeter

of Lake Apopka as the principal external source of phosphorus. Because

of the high organic content of the muck soils, the nutrient-enriched

soil water is highly colored. This characteristic provides a convenient

marker with which to trace the discharge of agricultural wastes and

associated nutrients to the system. The following example illustrates

this point. During late summer 1979, the Apopka drainage basin

experienced heavy rainfall and high rates of backpumping to the lake and

the Apopka-Beauclair canal were observed. The effect of agricultural

discharge on Lake Apopka and the downstream Oklawaha lakes was striking,

as illustrated in Figure IV-8 which delineates the spatial distribution

of both color and SRP during September 1979. Concentrations in both SRP

and color show an increase with location moving towards the north end of

the lake. Once within the Apopka-Beauclair canal, SRP and color

concentrations increased dramatically. Color and SRP concentrations at

the outfall of a discharge pipe on the Apopka-Beauclair canal were

101 chloroplatinate units (CPU) and 0.523 mg P/L, respectively; this is

contrasted with substantially lower color (65 CPU) and SRP (0.026 mg P/L)

concentrations at the outlet of the lake. Figure IV-8 shows that SRP

concentrations and color appear to be intrinsically linked; both

parameters decrease monotonically with distance downstream from the

source until the Dora canal is reached. Similar observations were made

during August and October 1979.
















































-
- -


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/


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If color is a reasonable label to trace long-term influences of

external loading on SRP dynamics, correlation analysis for the 4-year

data base implies that the relationship is insignificant. A weak,

negative correlation (r = -.14958, p = 0.3156) was derived between color

and SRP, suggesting that SRP variability within Lake Apopka is

independent of external inputs.

On July 11, 1979, a brief but intense convective disturbance was

encountered on Lake Apopka at the beginning of a routine sampling

effort. The storm, which originated from the southwest, began at

approximately 1540 hours. Wind velocities were estimated to range

between 9 and 11 m/s at the advent of the storm and had diminished to

approximately 6.7 m/s by the time the last sample was taken 1 hour

later. As the storm swept across the lake surface, bottom sediments

were immediately resuspended to the surface. The visual change was

rather remarkable; for example, sediment resuspension was observed as a

sharp front normal to and moving across the lake in the same direction

as the wind. Behind the front the water surface assumed a muddy brown

appearance from entrained sediments while on the leeward side, a

characteristic greenish cast typical of the highly productive water

column was observed.

Turbidity and suspended solids measurements from a protected point

along the western edge of the lake near Gourd Neck Springs showed

concentrations of 31 NTU and 0.0865 g/L of suspended material at the

surface. These values were considered to be representative of back-

ground or pre-storm conditions. Surface samples from the open-water

stations (A2, A5, and A8) readily showed the effects of resuspension;











turbidities ranged from 43 to 46 NTU and suspended sediment

concentrations ranged from 0.2053 to 0.2553 g/L.

Composite samples were taken from the surface and 1-m depth at the

open-water points and analyzed for turbidity, SRP, and TP. Results are

presented in Figure IV-9. Turbidity levels at all three stations were

substantially higher in the composite samples relative to surface

specific samples, thus indicating that resuspension was not uniform

throughout the water column but existed in the form of a concentration

gradient. Lakewide average turbidity and SRP concentrations both

exceeded levels observed the previous month during quiescent conditions

by a factor of three (Pollman et al. 1980). SRP levels were particu-

larly high at Station A5 where a concentration 0.277 mg P/L was

observed. Although the composite turbidity concentration was lower at

this station (90 NTU) relative to Stations A2 and A8 (100 and 120 NTU,

respectively), sampling was conducted at this station during the waning

moments of the disturbance. Wind velocity and wave activity were much

reduced at Station A5, allowing entrained particles to resettle.

Kinetic factors may also account for the lower SRP concentrations

observed at Stations A2 and A8.

Although suspended sediment concentrations are unavailable for the

composite samples, an empirical relationship derived in situ between

turbidity and suspended solids for Lake Apopka (Figure IV-10) enables an

estimate of the quantity of sediment resuspended to be made. The

following linear model was derived from least squares regression

analysis (r2 = .96, p <0.025):























































Figure IV-9.


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WIND


Suspended Sediment (g/L) and Total Phosphorus (TP)
(mg P/L) Concentrations in Lake Apopka During a Convective
Disturbance, July 11, 1979. TP and Suspended Sediment
Concentrations are Presented as the Upper and Lower
Figures, Respectively, at Each Sampling Site.









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Turbidity = 12.04 + 141.2 (SS) (IV-5)


where SS is the concentration of suspended solids (g/L), and turbidity

is given in NTU. On the basis of this model, the sediment concentration

yielding the observed composite turbidity levels can be determined.

Using data from Station A5 as an example, a turbidity concentration of

90 NTU equates to a suspended solids concentration of 0.5521 g/L. The

suspended sediment concentration at 1-m depth must therefore be equal to

0.8973 g/L. Assuming the concentration of suspended matter increases

linearly with depth gives an average concentration for the water column

(z = 176 cm) of 0.8145 g/L.

Correcting for the background contribution from ambient phyto-

plankton and suspended sediment concentrations (0.0696 g/L) yields a

net resuspension of 0.7449 g/L. The areal loss across the sediment

water interface is therefore 1,311 g/m2. Since the average bulk

density and water content of Lake Apopka sediments is 1.024 g/cm3

and 96.04 percent, respectively, the depth of the sediment entrainment

is estimated to be 3.2 cm. Calculated depths of sediment resuspension

for Stations A2 and A8 are summarized in Table IV-7.











Table IV-7.


Estimated Depth of Sediment Resuspension of Lake Apopka During a
Convective Disturbance, July 11, 1979.


Calculated Composite Surface Suspended Calculated
Composite Suspended Sediment Sediment Depth of
Depth Turbidity Concentration Concentration Resuspension
Station (cm) (NTU) (g/L) (g/L) (cm)


A2 180 110 0.6937 0.1616 4.7

A5 176 90 0.5521 0.2069 3.2

A8 150 120 0.7645 0.2553 3.5

















CHAPTER V
NUTRIENT RELEASE MODEL



A series of adsorption-desorption isotherm studies were conducted

in the laboratory to determine net release or uptake of phosphorus as a

function of ambient levels of SRP in the water under well-mixed

(equilibrium) conditions. Experimental results indicate that Lake

Okeechobee sediments vary appreciably in their ability to adsorb (or

desorb) phosphorus. A characteristic curvilinear relationship between

initial concentration and the magnitude of sorption was observed for all

stations except Station 013, which showed little ability to desorb or

absorb phosphorus over the experimental range of concentrations

(Figures V-1 through V-4). Similar curvilinear results were reported by

Harter and Foster (1976) and Mayer and Gloss (1980). A station-by-

station comparison of adsorption at an initial concentration of

1,000 ug/L suggests that Station 07 sediments have greatest affinity for

phosphorus at high concentrations; these sediments also have the

greatest silt-clay content, and thus the greatest surface area available

for exchange. Because of their relative homogeneity with respect to

chemical and physical characteristics, sorption studies with Lake Apopka

sediments were limited to sediment from Station A5. A curvilinear

relationship between the extent of sorption and initial concentration

was also observed for Station A5 sediment; the results, however,













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INGEST IEID E9CQAP2T7_ELELLW INGEST_TIME 2011-08-29T15:49:01Z PACKAGE AA00003437_00001
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INTERNAL LOADING IN SHALLOW LAKES
By
CURTIS DEVIN POLLMAN
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1983

ACKNOWLEDGMENTS
Support from numerous sources assisted me during the course of my
studies. Charles Hendry, Tom Belanger, Carl Miles, and Charles Fellows
all sacrified time and effort in accommodating me during the field
portion of my studies. Further appreciation is extended to Charles
Hendry for introducing me to some of the subtle nuances of the
analytical equipment critical to the success of this research.
I am deeply grateful for the support of my chairman, Dr. P.L.
Brezonik whose continued faith, gentle proddings, and guidance helped
provide the inspiration to complete my research. I also wish to
acknowledge the ocher members of my graduate committee, Dr. Wayne
C. Huber, Dr. Donald Graetz, Dr. Edward S. Deevey, and Dr. Thomas
L. Crisman, whose critical review and comments helped produce the final
manuscript. Dr. A.J. Mehta and Dr. Gregory M. Powell both provided
invaluable help in approaching the development of a sediment
resuspension model.
Eileen Town, who typed the initial and final manuscripts, deserves
special recognition for her professional competence and perserverance.
Appreciation is also extended to Carla S. Jones and Marlene A. Hobel for
putting up with the neurotic antics of an expectant author and providing
the finishing touches to the manuscript.
11

I particularly wish to thank my parents, Mr. and Mrs. Ralph
C. Pollman, and my grandmother, Mrs. Harold G. Lengerich, for
spiritually and financially sustaining me during the ebb moments of my
graduate career. My wife Kathleen sacrificed much for the sake of my
work and is especially deserving of recognition. Without her support
and encouragement, this work doubtlessly would have remained
uncompleted. Finally, I wish to dedicate this work to the memory of my
twin brother, Chris, whose recent
author a sense of necessity. His
is profoundly missed.
departure from life instilled in the
spiritual guidance was prodigious and

TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS ii
ABSTRACT vi
CHAPTER
I INTRODUCTION 1
Forms of Sedimentary Phosphorus 2
Mechanisms of Nutrient Release 6
Bioturbation 6
Gas Ebullition 8
Sediment Resuspension 10
Molecular Diffusion 13
Summary 15
II SITE DESCRIPTION AND METHODOLOGY 17
Physical Description and Limnology of Lake Okeechobee. 17
Physical Description and Limnology of Lake Apopka. . . 25
Materials and Methods 29
Physical and Chemical Characterization 29
Sorption Experiments 34
III PHYSICAL-CHEMICAL SEDIMENT CHARACTERISTICS 35
Physical Characteristics 35
Chemical Characteristics 47
IV RELEASE STUDIES 57
Turbulence Study 71
Fresh Sediment 72
Partially Dessicated Sediment 78
In Situ Studies 80
IV

PAGE
V NUTRIENT RELEASE MODEL 92
Development of a Langmuir Sorption Model 97
Application of Langmuir Model to Lake Apopka and
Lake Okeechobee Sediments 108
Equilibrium Phosphorus Concentration (EPC) 118
Desorption 123
Effect of pH on P Sorption by Lake Okeechobee
and Lake Apopka Sediments 127
Development of a Sediment Resuspension Phosphorus
Release Model for Lake Okeechobee and Lake Apopka. . 131
Nutrient Release Submodel 131
Sediment Resuspension Model 133
Calculation of Wind-Induced Stress at the
Sediment-Water Interface 134
Determination of Critical Shear Stress 137
Calculation of Sediment Resuspension Rates 142
Sediment Dispersion Model 144
Application of the Integrated Model to Lake Apopka
and Lake Okeechobee 150
VI DISCUSSION AND SUMMARY 161
Discussion of Internal Loading 161
Summary 174
REFERENCES 178
BIOGRAPHICAL SKETCH 191
v

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
INTERNAL LOADING IN SHALLOW LAKES
By
Curtis Devin Pollman
April 1983
Chairman: Patrick. L. Brezonik
Major Department: Environmental Engineering Sciences
Internal loading of phosphorus may be significant in shallow lakes
where physical and biological processes can accelerate rates of solute
exchange across the sediment-water interface. Sediments were collected
from Lake Apopka and Lake Okeechobee located in central and south
Florida, respectively, and characterized with respect to physical and
chemical attributes of the sediment that control the dynamics of
phosphorus exchange between sediments and overlying water. A series of
experiments were conducted on bulk sediment to determine rates of
phosphorus exchange across the sediment-water interface as a function
of turbulence and physical state of the sediment.
The shallow nature and broad fetch characteristic of Lake Apopka
(mean depth 1.7 m) and Lake Okeechobee (mean depth 2.8 m) suggest that
vi

sediment resuspension and the concomitant release of sorbed phosphorus
may be important in the cycling of phosphorus. As a result, a model
was developed incorporating physical, chemical, and hydrodynamic
components to predict internal loading of phosphorus in shallow lakes.
Phosphorus releases are predicted on a single event basis in response
to wind-driven turbulent mixing in the water column. The model
synthesizes a wave-hindcasting submodel to determine sediment resus¬
pension rates and a phosphorus release submodel derived from Langmuir
sorption theory. Langmuir constants used as model inputs were derived
from adsorption-desorption isotherms conducted at controlled pH.
Under ambient conditions in the water column, both Lake Apopka and
Lake Okeechobee sediments are predicted to release phosphorus upon
resuspension. Results indicate that a moderate wind event (approxi¬
mately 8.9 ra/s) can more than double mid-lake concentrations of ortho¬
phosphorus in Lake Apopka from 20 to 61 ug P/L at pH 8.3. Phosphorus
release from Lake Okeechobee sediments is substantially lower
(approximately 3 to 8 ug P/L). Release is primarily from desorptive
processes; however, in Lake Apopka entrained pore fluid contributes
approximately 30 percent of the net release. The cumulative effect of
cyclonic and convective disturbances during the course of a year
results in internal loading in Lake Apopka easily exceeding external
loading rates. Sediment resuspension is less significant in Lake
Okeechobee, and the nutrient regime appears to be controlled primarily
by external loading.
VI L

CHAPTER I
INTRODUCTION
Within the past two decades, the widespread recognition of lake
degradation as an issue of major environmental concern has spawned
accelerated efforts to better protect and manage lake resources (Born
1979; U.S. EPA 1979). Phosphorus has been implicated as the most
important nutrient limiting phytoplankton growth and production in many
freshwater lakes (Vollenweider 1968; Vallentyne 1970; Likens 1972;
Schindler 1977). Consequently, many lake restoration schemes have
focused on curbing external or cultural input of phosphorus to
lacustrine systems.
In the decade following the introduction of Vollenweider's (1968)
now classic paper, great emphasis was placed by limnologists on modeling
one factor that influences productivity—external nutrient loading. In
spite of the emphasis given this narrow focal point, interest also has
been sustained in observing the contribution of phosphorus from lake
sediments to the overlying water—"internal loading" (cf. Golterman
1977). Much of this work was inspired by early studies such as those
conducted by Mortimer (1941, 1942) on Esthwaite Water in the English
Lake District. More recent work by Fee (1979) has rekindled speculation
and debate on the importance of lake morphometry to lake trophic state
and, in particular, the area of sediment in direct contact with the
trophogenic zone.

2
Lacustrine sediments may consist of up to 0.75 percent (by weight)
phosphorus (Jones and Bowser 1978). This nutrient pool represents a
potential source of supply to the overlying water column approximating
8 g P/m^ for each centimeter of the sediment column actively engaged
in exchange across the mud-water interface. In view of the work of
Vollenweider (1968, 1975, 1976) and others in defining critical areal
nutrient loading rates, it becomes quite evident that the availability
of sedimentary phosphorus for exchange and the factors that mediate the
transfer of the available pool, both abiotic and biotic, may have a
profound influence on the nutrient regime of a lake. The following
section is thus devoted to a discussion of the chemical nature of
sedimentary phosphorus and its availability for exchange and biological
uptake; factors that control the rate of exchange across the sediment-
water interface are discussed in subsequent sections.
Forms of Sedimentary Phosphorus
A variety of analytical techniques has been used to fractionate
sedimentary P with the ultimate objective of correlating a defined
fraction with biological response (i.e., nutrient uptake). Early
fractionation schemes borrowed heavily from the pioneering work in soils
by Chang anc Jackson (1957). Perhaps the most definitive work on
lacustrine sediments to supersede the efforts of Chang and Jackson
(1957) in soils was that of Williams et al. (1971). Williams et al.
(1971) operationally defined four distinct species of sedimentary P:
(1) inorganic ? present as orthophosphate ion sorbed on surfaces of P
retaining minerals (nonoccluded P); (2) inorganic P present as a

coprecipitate or minor component of an amorphous phase (occluded P);
(3) orthophosphate P present in discrete phosphatic minerals such as
apatite Ca^Q(P0^)^X2, where X = OH-, F-, or 1/2 CC^-^; and (4) P present
as an organic ester or directly bonded to carbon atoms (organic P). The
chemical nature of sedimentary P has been the subject of rather
intensive reviews by both Jones and Bowser (1978) and Armstrong (1979).
In more recent work by Williams et al. (1976), the categorization
of sedimentary P has been reduced to three major types: (1) organic
phosphorus, which encompasses all P associated with organic material via
C-O-P or C-P bonds; (2) apatite P comprised of orthophosphorus present
within the crystalline lattice of apatite grains; and (3) nonapatite
inorganic phosphorus (NAI-P) which consists of the remaining ortho¬
phosphorus fraction including that dissolved in the interstitial
solution. This latter category constitutes the most labile fraction and
is generally associated with exchangeability. In the development of
these categories, Williams et al. (1976) assumed that nonorthophosphatic
forms of inorganic phosphorus, e.g., polyphosphate ions (including
pyrophosphate) are present in sediments only in negligible quantities.
Isotopic exchange studies conducted by Li et al. (1972) using P-32
as a tracer provide direct evidence that a substantial fraction of the
total inorganic pool in sediments is exchangeable. Exchangeable P
comprised 19 to 43 percent of the total inorganic content of sediments
derived from four Wisconsin lakes. Total exchangeable P was found to be
relatively insensitive to changes in redox status; this was interpreted
by Li et al. (1972) as consistent with the retention of inorganic P in
sediments by an iron-rich gel complex (Williams et al. 1971;

Shukla et al. 1971). Li et al. (1972) hypothesize that although the
equilibrium position between sorbed and free inorganic P for the iron
gel complex is altered by a shift in redox potential, the total
exchangeable pool is not appreciably increased. Conversely, if
inorganic P existed primarily as a discrete crystalline iron phosphate
such as strengite, total exchangeable P should increase significantly
upon dissolution of tne solid phase during anaerobios is.
The high rate of exchangeability observed by Li et al. (1972)
provides further qualitative evidence of the retention of inorganic P as
a nonoccluded form through sorption to an amorphous iron hydrous oxide
gel. Incorporation of P into a crystalline lattice is anticipated to
yield a reduction of exchangeability primarily because of a reduction in
surface area. Additional experiments conducted by Li et al. (1972) with
sediments equilibrated with solutions spiked with orthophosphorus
demonstrated equivalent degrees of exchange for sorbed and native
inorganic P. The authors suggested that native inorganic P is retained
in sediments by the same mechanism as sorbed P.
Alternative fractionation schemes to define the labile fraction of
sedimentary P available for algal uptake have involved chelating agents,
anion and cation exchange resins (Wildung and Schmidt 1973; Huettl
et al. 1979), and desorption potential (Schaffner and Oglesby 1978)
Golterman (1973) used 0.01 N NTA (nitrilotriacetic acid) to extract P
associated with calcium (Ca) and iron (Fe) constituents in sediments.
Golterman (1977) found essentially no difference between NTA extracted
phosphorus and calculated P uptake by the alga Scenedesmus utilizing the

5
same sediment as its solitary source of P. Grobler and Davies (1979)
found NTA-extractable P to generally be a more applicable measure of
algal available P than total inorganic P. Although both fractions were
correlated with algal uptake, the most consistent relationship was
observed between P availability and NTA extractable P. A non-linear
relationship between algal available P and sediment inorganic P was
demonstrated by Grobler and Davies (1978); the fraction of inorganic P
comprised by algal available P varied with sediment type and was quite
variable, ranging from 4 to 98 percent.
As a means for differentiating between Fe-bound and Ca-bound P,
Golterman (1977) proposed an initial extraction with 0.01 M Ca-NTA.
According to Golterman, coraplexation of the added Ca by NTA prevents the
solubilization of sedimentary calcium and extraction of Ca-P; conse¬
quently only iron-bound P is extracted. Subsequent work by Williams
et al. (1980), however, does not support Golterman's contention that
Ca-NTA selectively extracts Fe- and aluminum (Al)-associated P, leaving
Ca-P intact. Extraction with a neutral, noncomplexing agent (1 M NaCl)
yielded extraction efficiencies comparable with Ca-NTA. Extraction with
Ca-NTA rarely exceeded 10 percent of the NAI-P fraction. Uptake of
sedimentary P by S. quadricauda was closely correlated with the NAI-P
fraction by Williams et al. (1980) and constituted approximately
75 percent of NAI-P; apatite-P was virtually unavailable for algal
growth. These results qualitatively confirm the earlier observations of
Grobler and Davies (1978), who noted that algal available P generally
exceeded NTA-extractable P by a factor of approximately 5 to 6.

6
Mechanisms of Nucrient Release
As detritus accumulates in surficial deposits, metabolism of
accreting organic material results in the buildup of interstitial
phosphorus concentration to levels that exceed those in the overlying
water by as much as several orders of magnitude. Release to the
overlying wacer is accomplished by burrowing and irrigation activities
of benthic organism (i.e., bioturbation), by gas ebullition from
anaerobic decomposition processes, by scouring of the interface by
wind-induced waves, and simply by molecular diffusion across the
concentration gradient that usually exists between sediment pore water
and the overlying water.
Bioturbation
Based on analysis of Pb-210 and Cs-137 profiles in recent sediments
of the Great Lakes, Robbins and Edgington (1975) suggested that post-
depositional movement of material occurred within the surficial 10 cm of
sediment. Robbins and Edgington (1975) postulated that the observed
redistribution of sediment particles was the result of either physical
mixing or bioturbation. Activities of the benthic community, such as
burrowing, irrigation, and feeding, result in fluid and particle
transport near (and across) the sediment-water interface. Bioturbation
increases the influx of water into sediments; this is balanced by an
equal volume of interstitial solution flushed from sediments (Petr
1977). Hargrave and Connolly (1978) observed a great deal of spatial
variability in nutrient and gas fluxes across the sediment-water inter¬
face of a subtidal flat and attributed the variability to heterogeneity

7
in Che distribution of benthic macrofauna. Berger and Heath (1968)
suggested that homogenization of the upper sediments is complete when
sediment displacement by benthic infauna is comparable to or exceeds the
sediment accumulation rate. As an extension of this assumption,
Krezoski et al. (1978) used average defecation rates for Tubifex tubifex
to estimate sediment displacement, and they calculated that oligochaete
densities in profundal sediments of Lake Huron were sufficient to
displace the upper 3 to 6 cm. Neame (1977) observed a total phosphorus
flux of approximately 650 ug/m^-d across the surficial 3 cm of
sediment in Castle Lake, California, and attributed this flux to
chironomid activity in the oxidized zone of the sediments. Under anoxic
conditions, the phosphorus flux across the interface decreased by two
orders of magnitude.
Aller (1978) reported that benthic infauna activity affects the
dynamics of the sediment-water exchange process by altering the geometry
and kinetics of molecular diffusion in sediments. In the absence of
benthic activity, Aller (1978) successfully used Berner's (1976) one¬
dimensional diffusion model to describe the vertical distribution of
interstitial phosphorus in marine sediments. In the presence of the
highly mobile protobranch, Yoldia limatula, pore-water transport was
characterized by a non-steady-state two-layer model in which an
effective (or biogenic) diffusion coefficient acts in the zone of
feeding and molecular diffusion controls transport in underlying
sediment. The effective biogenic diffusion coefficient for pore-water
transport by Yoldia is approximately 1 x 10-5 Cm^/s; this compares
to a tortuosity-corrected molecular diffusion coefficient of

3
2.4 x 10~6 cm^/s for HPO^-^ undisturbed sediments. Lerman (1979)
indicated that effective diffusion coefficients in the mixed layer are
on the order of 5 to 100 times greater than the molecular diffusion
coefficient.
Several other types of models have been developed to describe the
effects of bioturbation on sediment-water solute exchange. Grundmanis
and Murray (1977) and McCaffrey et al. (1980) suggested that transport
can be simulated by assuming that interstitial water is biogenically
advected between discrete well-mixed reservoirs within the sediment and
overlying water. Aller (1978, 1980) proposed an idealized model which
assumes that specific changes in the average geometry of molecular
diffusion result from the presence of irrigated tube and burrow
structures. Average interstitial concentrations, solute flux, and
apparent one-dimensional diffusion coefficients are influenced by both
the size and spacing of burrows.
Gas Ebullition
The evolution of gas bubbles within anaerobic sediments also may
enhance the flux of dissolved constituents across the sediment-water
interface. Two mechanisms have been proposed by Klump and Martens
(1981) to explain the influence of gas ebullition on exchange. Bubble
tube structures alter the geometry of the sediment-water interface and
increase the surface area available for chemical exchange. If flow
patterns at the interface permit unrestricted import and export of
overlying water within tube structures, the total flux across the
interface is increased. Conversely, restriction of flow may allow

9
solute concentrations within the tube to reach high concentrations,
establishing a sharp vertical concentration gradient across the
interface. Mixing of the pore water in the tube by rising bubbles can
mix the pore water along the length of the tube, creating a sharp
gradient immediately at the interface. Enhancement of diffusive
transport consequently would result from the increased gradient.
The critical concentration at which bubbles form is dependent on
hydrostatic pressure; thus, resistance to bubble formation increases
with increasing depths (Hutchinson 1957). Gas ebullition generally
occurs in shallow aquatic systems such as marshes and littoral zones,
where surficial sediments are the site of intensive anaerobic metab¬
olism. In hypereutrophic Wintergreen Lake, a shallow ("z = 3.5 m),
hardwater lake in Michigan, Strayer and Tiedje (1978) measured an
average methane ebullition rate of 21 mmol/m^-d during the period
late May through August. Maximum rates of methane loss by bubble
evolution (35 mmol/m^-d) occurred in late summer. In comparison,
Martens and Klump (1980) measured an average methane ebullition of
16.8 mmol/m^-d from intertidal sediments on the North Carolina
coast between June and October. Ebullition was initiated by the release
of hydrostatic pressure that accompanied low tide. High fluxes of total
dissolved phosphorus were observed; rates up to 120 umol/m^-hr were
observed from these sediments during the summer months. The high rates
were attributed to increased mass transport associated with bubble tubes
maintained by methane gas ebullition.

10
Sediment Resuspension
A third mechanism that may contribute to internal loading is the
resuspension of sediments in shallow lakes by wind-induced currents
(e.g., Sheng and Lick 1979). Fee (1979) correlated rates of primary
production in unfertilized ELA lakes in western Ontario to the fraction
of epilimnetic surface area in direct contact with bottom sediments.
Fee concluded that epilimnetic nutrient recycling is dominated by
processes occurring at the sediment-water interface and not within the
sediment. Chapra (1982) recently showed that resuspension becomes an
important mechanism in lakes with mean depths less than 9.2 m. Lastein
(1976) showed that approximately 24 percent of the material collected in
sediment traps in Lake Esrom (Sweden) (z = 12.3 m) was from resuspended
sediment, and he hypothesized that wind-induced circulation alone was
sufficient to explain the phenomenon. In Lake Uttran (Sweden), a
shallow (T = 5.7 m), eutrophic lake, Ryding and Forsberg (1977)
correlated aqueous total phosphorus concentrations with the force and
duration of winds blowing lengthwise across the lake, and they suggested
that resuspension played a dominant role in regulating water quality.
In a study of six shallow Danish lakes, Andersen (1974) concluded that
wind-induced mixing of surficial sediments is the most probable
mechanism for the high phosphorus release rates he observed (up to
1.2 g/m^-month). Concomitant laboratory studies indicated that
maximum rates of release from quiescent, undisturbed sediments were of
the order of 0.25 g/m^-month.
Surface waves produce an oscillatory motion which translates in
shallow water to elliptical orbits extending to the sediment-water

11
interface (Bascom 1964; U.S. Coastal Engineering Research Center 1977).
Near the boundary layer, the elliptical motion is reduced to a simple,
reciprocating horizontal motion, and the maximum horizontal velocity
associated with the oscillating motion is given by
it Hg
Um “ Ts sinh (2r d Ld) ' (I_1)
where Hs = significant wave height;
Ts = significant wave period;
d = depth of the water column;
= wavelength for the particular depth, d; and
um = maximum orbital velocity (Komar and Miller 1973).
When the shear force exceeds the bulk shear strength of the
surficial sediment deposits, sediment resuspension occurs (Alishahi and
Krone 1964; Komar and Miller 1973, 1975; Terwindt 1977; Sternberg 1972).
Scour may be accomplished either by detachment and entrainment of
individual floes (surface erosion) or removal of aggregated material
(mass erosion) (Lonsdale and Southard 1974; Terwindt 1977). In deposits
where shear strength increases with depth, erosion continues to the
depth where the applied stress is equivalent to the bed strength (Krone
1976). Below the threshold or critical shear velocity, the bed remains
intact and no resuspension occurs. Migniot (1968) demonstrated that for
various types of cohesive sediments, the critical shear velocity may be
expressed as an inverse function of water content. These findings
subsequently were confirmed for marine sediments by Southard et al.
(1971) and Lonsdale and Southard (1974).

12
The critical shear velocity also is influenced by the mechanical
structure of the sediment, which in turn is related to the solute
characteristics and ionic strength of the interstitial solution and the
eroding solution (Arulananden et al. 1975). For example, critical shear
stress decreases with increasing sodium adsorption ratio (SAR)
(Arulananden et al. 1975):
SAR
[Na+]
([Ca2+] + [Mg2+])1/2
(1-2)
This observation correlates with the Schulze-Hardy valence rule,
which indicates that the critical coagulation concentration of mono-,
di-, and trivalent ions are in the ratio of z~6, where z is the
charge of the ion (Stumm and Morgan 1981). Increasing the solute
concentration of the pore fluid destabilizes colloidal suspensions
(i.e., agglomerates them and tends to remove them from solution). The
net effect is an increase in bed strength (Arulanandan et al. 1975).
The effects of electrolyte concentration and SAR on critical shear
stress and erosion rates were reviewed in detail by Terwindt (1977).
Mixing or agitation of surficial sediments due to wave action
entrains interstitial (pore) water, resulting in release of solutes such
as nitrogen and phosphorus species to the overlying water (Gahler 1969;
Lam and Jaquet 1976). For example, resuspension of the upper 1 cm of a
flocculent sediment with a porosity of 80 percent and a pore water
phosphorus content of 1 mg/L would result in a release of 8 mg P/m^.
This is sufficient phosphorus to supply a shallow lake (e.g., ~z = 3 m)
with an additional 2.7 ug P/L from this source alone. In addition, a

13
substantial fraction of the inorganic phosphorus sorbed to lake
sediments participates in rapid solid phase/aqueous phase exchange
reactions. This reservoir of phosphorus may be more important in
controlling the phosphorus flux resulting from sediment resuspension
than is the pore water phosphorus.
Lam and Jaquet (1976) developed an empirical model to predict the
upward flux of TP from resuspended sediment in Lake Erie. The flux is a
function of shear stress at the sediment-water interface (tg) from
wind-induced waves; ig was calculated from linear wave theory using
the deep-water Sverdrup-Munk-Bretschneider (SMB) wave-hindcasting method
to estimate wave height and period. Regeneration was assumed to arise
primarily from organic and nonapatite inorganic phosphate fractions, in
conjunction with a small quantity of soluble interstitial phosphate.
Apatite [Ca^gipO^/^ÍOH^] was not considered in the regeneration
process because of its relatively high density and large grain size.
Sheng and Lick (1979) developed a similar sediment resuspension
model for Lake Erie; a principal point of departure from the Lam and
Jaquet model was the use of a shallow-water SMB wave-hindcasting model
to predict bottom orbital velocities.
Molecular Diffusion
The existence of concentration gradients across the sediment-water
interface implies that molecular diffusion may be an important mechanism
in describing solute transport. Theoretically, the flux across the
sediment-water interface constituent dissolved in the interstitial
solution arises from advection due to pore water buried by sediment

14
deposition, as well as from diffusion (Imboden 1975; Lerman 1979).
Neglecting the advective component, the flux equation reduces to Fick's
first law (Berner 1971):
J
s
= - 0 D
o s
(2£)
d Z ^
z = 0
(1-3)
where
(f£)
3z pw
= flux from the sediment to the overlying water
(g/cm^-s with "s" denoting bulk sediment),
= porosity at the interface,
= bulk sediment diffusion coefficient at the interface
(cm^/s)y and
= pore water (pw) concentration gradient at the
sediment-water interface (i.e., z=0).
This equation is tractable if the assumption is made that Ds is
both spatially and temporally invariant. The flux of a dissolved
species across the sediment-water interface then can be calculated from
the concentration gradient at the interface and molecular diffusivity
(e.g., McCaffrey et al. 1980; Thibodeaux 1979; Vanderborght et al.
1977).
Fluxes as high as 7 mg P/m^-d attributed to diffusion have been
reported by Ulen (1978) for sediment cores from Lake Norvikken in
central Sweden. The average diffusive flux for Lake Norvikken was
1.0 g/m^-y; this contrasts with an external phosphorus loading rate
of 0.53 g/m^-y. Ulen (1978) obtained good agreement between
empirically measured fluxes and calculated rates of exchange using a
bulk sediment diffusion coefficient of 4.4 x 10“° cm^/s. This value
was first derived by Tessenow (1972) and probably represents an upper

15
limit for bulk sediment diffusivities (Tessenow 1972, in Kamp-Nie1 sen
1974). Data of Li and Gregory (1974) indicate that the diffusivities of
H2PO4 at 25°C and infinite dilution are 7.34 x lO-^ Cm^/s and
8.46 x 10-6 cm^/s, respectively; Manheim (1970) suggested that
these values are diminished by factors ranging from 0.05 to 0.5 in
unconsolidated sediments, as a result of porosity and path tortuosity
effects.
According to Kamp-Nielsen (1974), the aerobic release of phosphorus
from sediments in Lake Esrom (Sweden) is controlled by both adsorption
and diffusion. Berner (1976) modified the expression for Fickian
diffusion to quantify the retarding influence of adsorption:
J
s
D 0
s o
1 + K
(1C)
3z
z = 0
(1-4)
where K = nondimensionalized linear adsorption coefficient.
Excellent agreement was obtained by Krom and Berner (1980b) between
empirically-derived effective diffus ivities for phosphate, ammonium, and
sulfate ions and molecular diffusivities corrected for tortuosity and
adsorption. The influence of adsorption on the effective or apparent
diffusion coefficient can be quite significant; for example, Krom and
Berner (1980a) cite values for K ranging up to 5,000 for oxic oceanic
sediments.
Sediments in lakes are generally characterized by high concentra¬
tions of nutrient and other chemical constituents (such as heavy metals)

16
resulting from the deposition and accretion of detrital material. As a
result, sediments may actively participate in regulating the cycling of
a particular constituent in the water column. The extent that the
sedimentary reservoir can ultimately influence material cycling within
the water column depends on the availability of the substance for
exchange. Exchange of nutrients and other solutes across the sediment-
water interface into the overlying water is controlled by four principal
mechanisms: bioturbation, gas ebullition, resuspension of bulk sediment
by wind-induced turbulence, and molecular diffusion. In shallow lakes
characterized by extensive fetches, sediment resuspension is likely to
be a major mechanism for internal nutrient cycling. This research
focuses on the significance of sediment resuspension on rates of inter¬
nal nutrient release in two lacustrine systems of similar morphometry,
but quite different trophic state—hypereutrophic Lake Apopka and
mesotrophic Lake Okeechobee. In conjunction with this objective, a
corollary objective of this study was to develop an event-dependent
model to predict the release or removal of phosphorus by wind-induced
sediment resuspension.

CHAPTER II
SITE DESCRIPTION AND METHODOLOGY
This research focused on two rather large and shallow lakes
important as aquatic resources in Florida—Lake Apopka in central
Florida and Lake Okeechobee in south Florida. By virtue of the broad
fetch and shallow nature of each basin, both Lake Apopka and Lake
Okeechobee are subjected to periodic sediment resuspension due to wind-
induced currents. The trophic state of these two lakes, however, is
quite different. Until the early to mid-1950's, Lake Apopka achieved
national reknown as a bass fishing lake. Currently and primarily
because of cultural inputs of nutrients, Lake Apopka is now widely
regarded as one of the most eutrophic lakes in Florida. Conversely,
Lake Okeechobee is intensively used as a recreational resource and is
critical to the hydrologic and ecological stability of south Florida.
The following section briefly summarizes the limnological characteris¬
tics of these two systems and details the specific analytical procedures
used in this study.
Physical Description and Limnology of Lake Okeechobee
After the Laurentian Great Lakes, Lake Okeechobee is the largest
freshwater lake in the United States. Located in south central Florida
between latitudes 26°41' to 27°13'N and longitudes 80°36' to 81°07'W
17

13
o
(Figure II-l), Lake Okeechobee has a surface area of 1,890 kmz at
normal stage (approximately 4.6 m above sea level). The broadest
expanse of open water lies along the north-south axis and approximates
56 km; the maximum east-west dimension is 48 km. The shoreline
development index, which is the ratio of the lake shoreline length to
the circumference of a circle with an equivalent surface area is 1.12
(Brezonik et al. 1979). Summarized in Table II-l are some of the
important physical characteristics of the watershed.
Lake Okeechobee, which is considered moderately eutrophic (Brezonik
et al. 1979), is quite shallow throughout the entirety of its basin. At
normal stage, the depth of the lake averages only 2.8 m and has a
maximum depth of less than 5 ra (Figure II-l). Because of its shallow
depth, Lake Okeechobee is characterized by an extensive littoral zone.
Macrophyte communities occupy nearly 500 km^ or 26 percent of the
total lake surface area along the southern and western shores of the
lake; however, no important macrophyte communities are established on
the eastern portion of the basin (Brezonik et al. 1979).
Lake Okeechobee in its present state originated approximately
6,000 years ago (Brooks 1974). It occupies a shallow depression which,
according to Hutchinson (1957), was formed by epeirogenetic uplift of an
irregular marine surface. Brooks (1974) has hypothesized that
differential subsidence of the thick underlying sequence of Miocene
clays is responsible for the deeper portions of the lake. Extensive
discussions of the origin and early history of Lake Okeechobee have been
presented by Brooks (1974) and Brezonik et al. (1979).

19
Figure II-l. Location Map of Lake Okeechobee

20
Table 11— 1. Physical Characteristics of Lake Apopka and Lake Okeechobee.
Lake Apopka*
Lake Okeechobeet
o
Lake Surface Area (knr)
124.0
1,891
Maximum Depth, Zjj, (m)
11.0
4.7
Mean Depth, "z (m)
1.7
2.8
Volume, V (m8)
2.14 x 108
5.24 x 109
Annual Inflow (m8)
5.92 x 107
1.98 x 109 to
4.68 x 109
Theoretical Retention Time (years)
6.3
1.12 to 2.65
Maximum Length (km)
14.4
56.4
Maximum Width (km)
13.8
48
Shoreline Length (km)
58.5
172
Shoreline Development Index, D^
1.48
1.13
Watershed Area (including lake) (km^)
3.11
13,007
Watershed Land Area (km^)
187
11,116
* From Brezonik et al. (1978).
t From Brezonik et al. (1979) and Federico et al. (1981).

21
Excluding Che lake surface, the watershed of Lake Okeechobee drains
approximately 11,116 km^. The watershed comprises a series of both
natural and man-made sub-drainage basins (Figure II-2). Flow is
primarily from north to south; the Kissimmee River, which drains
6,048 km^, is the most important tributary (Davis and Marshall 1975).
Other natural basins of importance include Fisheating Creek (1,194 km^)
and Taylor Creek which drain 1,194 and 477 km^, respectively. Since
the early 1900s, a 785 km^ area of the Everglades immediately south
of the lake has been added to the watershed via an extensive system of
canal and pump stations. This region, otherwise known as the Everglades
Agricultural Area (EAA), is dominated by Everglades muck soils and is
intensively farmed. Flow in the EAA is conducted along the North New
River, Hillsboro, and Miami and, depending on flood control and water
conservation requirements, can be either imported from or exported to
the lake. Hydraulic export from Lake Okeechobee is also conducted by
the St. Lucie, West Palm Beach, and Caloosahatchee canals, whereas
outflow historically occurred as sheet flow over the southern rim into
the Everglades.
Land use within the Lake Okeechobee watershed has been compiled by
McCaffrey et al. (1976). Results from the compilation are presented in
Figure II-3 and Table II-2. In general, the watershed is dominated by
some type of agricultural activity. Agricultural lands constitute
48 percent of the watershed and comprise primarily improved and
unimproved pasture land in the northern and northwestern portions of the
basin and croplands in the south. Wetlands are the next most signifi¬
cant land use type (18 percent), followed by surface water (18 percent

22
Figure II-2. Surface Water Drainage Basins in the Lake Okeechobee Watershed.
Note: Areas designated by dashed lines contribute only small amounts of water
at irregular intervals. Drainage basins south of the lake have been
added by man. From Federico et al. (1981).

23
Figure II-3. Land Use Within the Lake Okeechobee Watershed. From
McCaffrey et al. (1976).

Table 11-2. Land Use
for Lake
within the
Okeechobee
Lake Okeechobee and Lake Apopka Watersheds (included are 1980 projections
and 1990 projections for Lake Apopka).
Land Use
Lake Okeechobee*
Lake Apopkat
1970
1980
1972
1990
Area
(km^ )
Percent
of Total
Area
(km^)
Percent
of Total
Area
(ha)
Percent
of To t a 1
Area
(ha)
Pe rcent
of Total
Urban
62 3
4.0
813
5.3
1,578
5. 1
3,035
9.8
Agricultural
7,636
49.5
7,442
48. 1
17,118
55.0
15,661
50.4
Crop 1 and
1,153
7.5
959
6.2
7,284
23.4
7,284
23.4
Citrus
688
4.4
660
4.2
8,094
26.0
8,094
26.0
Improved pasture
3,221
21.0
3,541
22.9
—
—
--
—
Unimproved pasture
2,570
16.7
2,286
14.8
1,740**
5.6
283**
0.9
Forest
870
5.6
935
6. 1
—
—
—
—
We tland s
2,865
18.5
2,813
18.2
—
—
—
—
Barren/ld le
708
4.6
700
4.5
—
—
—
—
Surface Water
2,756
17.8
2,756
17.8
12,400
39.9
12,400
39.9
* From McCaffrey et al. 1976.
t From ECFRPC 1973.
** Swamp and pasture.

25
including Lake Okeechobee) and forests (6 percent). Urban areas
(5 percent), which include the towns of Clewiston, Belle Glade, and
Okeechobee City, represent the smallest area of the total watershed.
Physical Description and Limnology of Lake Apopka
Lake Apopka is the fourth-largest lake in Florida, encompassing a
surface area of L24 km^. Located approximately 25 km northwest of
Orlando between latitudes 28°33' to 28°41' and longitudes 81031' to
81°421, Lake Apopka forms the headwaters of the Oklawaha chain of lakes,
and ultimately the Oklawaha River (Figure II-4). As indicated by its
shoreline development index (1.39), the lake is somewhat circular in
shape and is characterized by relatively broad fetches extending along
both the north-south and the east-west axes (14.4 and 13.8 km,
respectively). Important physical characteristics of the watershed are
summarized in Table II-l.
Lake Apopka resides at the southernmost end of the physiographic
region known as the Central Valley, which is bounded on the west by the
Lake Wales ridge and to the east by the Mount Dora Ridge. The lake
itself occupies an extremely shallow basin with an average depth of only
1.7 m at normal stage (20.3 m above MSL) (Figure 11—5); consequently,
Lake Apopka is continually well mixed by wind-driven currents. Despite
its shallow nature, the lake is characterized by the virtual non¬
existence of rooted macrophytes. This apparently stems from compression
of the photic zone to much less than 1 m (approximately 40 to 60 cm)
because of high algal densities characteristic of the hyper-eutrophic
lake and the instability of the highly flocculent sediments.

26
Figure II-4. Location Map of Lake Apopka.

27
/N
1*
I MILE
Figure II-5. Bathymetric Features of Lake Apopka,
in Feet.
Depth Contours Given

28
The watershed of Lake Apopka is rather small in relation to the
• • 9
lake surface area, and excluding the lake surface, drains only 187 km .
No natural surface influents of any consequence convey water to the
lake. The major hydraulic input is precipitation falling directly on
the lake surface; the other important natural source is artesian flow
from the Floridan Aquifer at Gourd Neck Springs in the southwest corner
of the lake. During periods of intense rainfall, Johns Lake located
2.0 km to the south may overflow into Lake Apopka. Several other small
streams in the southeastern basin Lake Apopka may flow into the lake;
these streams are also ephemeral, and their existence is a function of
antecedent rainfall.
Similar to Lake Okeechobee, the watershed of Lake Apopka has been
increased by cultural activities. Around 1940, a vast (7,200 ha) marsh
was drained and converted into highly productive muck farmland. Flow,
which historically has been south to north, was diverted from the marsh
by an earthen dike and conducted northward through the Apopka-Beauclair
Canal. A series of canals and pump stations was constructed to regulate
water level within the muck soils. As a result, backpumping for the
for the muck farms has constituted a significant hydraulic (and
nutrient) source; however, since 1976 the magnitude of this source has
diminished considerably with the institution of discharge abatement
measures.
Land use estimates within the Lake Apopka drainage basin have been
developed by the East Central Florida Regional Planning Council (1973)
and are included along with projections for 1990 in Table II-2. At the
time of the above study, 8 percent of the land area was urbanized, while

29
citrus growing and muck farming activities occupied 43 and 39 percent of
the land, respectively. As can be seen from Figure II-6, muck farming
is limited to the northern perimeter of Lake Apopka, and most of the
remaining land in the lake basin is occupied by citrus groves. Urban
areas are primarily limited to sections on the south and west shores of
the lake and include the towns of Winter Garden and Montverde.
Materials and Methods
Physical and Chemical Characterization
Samples were collected at 14 stations within Lake Okeechobee
(Figure II-7) to determine physical and chemical characteristics of
surficial sediments. Five stations (01, 03, 04, 08, 09) were located
within the littoral zone; five stations (06, 07, 010, Oil, 012) were
open-water stations; and the remaining four stations (02, 05, 013, 014)
may be described as transitional. Samples were collected during three
sampling efforts in the summer and fall of 1980. Sediment samples from
Lake Apopka were collected quarterly at nine open lake stations (Al
through A9) and one station at Gourd Neck Springs (A10) (Figure II-8)
between March 1977 and March 1978. All samples were collected with a
Ponar dredge and placed in wide-mouth one-liter polyethylene bottles,
stored on ice until return to the laboratory and then refrigerated until
analysis.
All sediment samples were homogenized prior to analysis. Water and
volatile solids content were determined on a percent basis according to
standard methods (APHA 1976). Particle size distribution was evaluated
by wet sieve analysis for sand-sized fractions (>63 um) and by pipette

30
Apopka -Beauclair
Canal —
U.S.G.S. Drainage Boundary
/'
Figure II-6. Land Use Within the Lake Apopka Watershed.

31
Figure II-7. Sediment Sampling Locations in Lake Okeechobee

32
LAKE
COUNTY
MAGNOLIA
PARK
Figure II-8. Sediment Sampling Locations in Lake Apopka.

33
analysis for silt-clay fractions (Guy 1969). In order to characterize
the effective particle size of the whole sediment, sedimentary organic
matter was not removed by oxidation. Effective particle size is
necessary in determining the threshold shear required to induce sediment
resuspension for a particular sediment.
The phosphorus content of selected sediment samples was analyzed in
terms of three fractions: apatite inorganic phosphorus (AI-P),
nonapatite inorganic phosphorus (NAI-P), and organic phosphorus. The
latter fraction was determined as the difference between the total
phosphorus (TP) content and the sum of two inorganic fractions. The
inorganic fractions were estimated on fresh (undried) sediments by the
analytical method described by Armstrong (1979). This method involves a
two-step sequential extraction scheme: the sample is initially
extracted with 0.1 N NaOH for 16 hours to obtain NAI-P followed by an
extraction with 1 _N HCl for 16 hours to determine AI-P. TP was
determined on fresh sediments by the acid-persulfate digestion technique
(APHA 1976). Digested samples were filtered, neutralized, and analyzed
for soluble reactive phosphorus (SRP) using the single reagent
molybdenum blue technique (U.S. EPA 1976).
Interstitial water was extracted at 4°C by centrifugation of fresh
sediment samples at 10,000 rpm for 30 minutes. The supernatant was then
filtered through GF/C glass micro-fibre filter paper, and the filtrate
was analyzed for SRP by the single reagent molybdenum blue technique
(U.S. EPA 1976).

34
Sorption Experiments
A series of adsorption-desorption studies were conducted to
quantify phosphorus exchange dynamics using sediment samples representa¬
tive of littoral transitional and profunded zones. Sorption isotherms
were evaluated at 22°C; samples were buffered at pH 8.3 with NaHCC^
at an ionic strength of 8.3 x lO-^ ^ anc[ 5.0 x 10~3 These conditions
reflect the average pH and ionic strength observed in Lake Okeechobee
(Federico et al. 1981) and Lake Apopka (Pollman et al. 1980), respec¬
tively. Aliquots of wet sediment equivalent to 0.5 g of oven-dried
material were allowed to equilibrate with 100 mL of buffered water for
24 h on a rotary shaker table. The buffered solution was adjusted with
KH2?0^ to yield initial orthophosphorus concentrations of 0, 20,
50, 100, 250, 500, 1,000, and 5,000 ug P/L. After 24 h of shaking, the
suspensions were filtered through GF/C glass micro-fibre filter paper
and subsequently analyzed colorimetrically for SRP using the method
described previously. The quantity of phosphorus adsorbed or desorbed
was determined from the difference between the final and initial
concentrations and was corrected for contributions of SRP from
interstitial water.

CHAPTER III
PHYSICAL-CHEMICAL SEDIMENT CHARACTERISTICS
Physical Characteristics
Erosion, transportation, and deposition of sediment particles in a
lake are governed by many physical factors, including effective fetch,
water depth, bottom depth, and lake morphometry (Hakanson 1981). The
sediment parameter that probably best indicates the dynamic conditions
prevailing at the sediment-water interface is the water content of
surficial sediment. For example, Migniot (1968) demonstrated an inverse
relationship existed between critical shear stress and sediment water
content for a series of lacustrine muds. Hakanson (1977) derived an
empirical relationship that describes erosion and deposition of
sediments in Lake Vanern (Sweden) as a function of sediment water
content and maximum effective fetch (Peff) (U.S. Coastal Engineering
Research Center 1977). The water content of surficial sediments in
Lakes Okeechobee and Apopka thus was determined to evaluate the physical
nature of the sedimentary environment, i.e., whether a particular
location within a lake basin is a zone of relative quiescence or an area
subject to turbulence and resuspension.
Results for water content analyses for Lake Okeechobee and Lake
Apopka are summarized in Tables III—1 and III-2, respectively.
Sediments from the central portion of Lake Okeechobee had water contents
ranging from 82.3 to 91.1 percent. With the exception of Station 09,
35

Table 111 — 1 . Summary of Physical and Chemical Characteristics of Lake Okeechobee Sediments.
Substrate
Descript ion
Station
Approximate
Depth, d
(m)
Percent
h2o
Percent
VS
Mean
Particle Size
(u)
Interstitial
P (mg P/L)
NAI-P
(ug P/g)
Apatite P
(ug P/g)
Total P
(ug P/g)
Pelagic
06
2.6
89.15
15.31
0.003
14
29
502
010
2.4
91.07
12.66
0.004
12
12
—
01 1
3.4
82.28
17.21
270
0.002
35
349
527
012
3.4
85.16
33.41
170
0.075
39
564
995
07
3.4
83.16
35.05
59
0.012
66
783
1,440
Littoral
09
0.6
71.52
14.98
0. 148
43
74
248
08
1.1
42.32
1.18
0.004
8
17
407
01
0.6
58.28
1.44
0.008
9
54
142
03
0.9
45.68
2.74
24
158
284
04
0.8
52.21
3.48
208
0.267
30
199
388
Transí tion
013
1.8
26.73
0.26
275
0.027
6
81
518
014
0.9
88.60
42.69
149
0.942
96
1,078
1, 190
02
2.4
76.65
23.07
151
0.076
23
665
1,043
03
2.3
66.98
11.86
0.002
26
50
—

Table III-2. Summary of Physical and Chemical Characteristics of Lake Apopka Sediments
Approximate
Depth, d
Percent
Percent
Mean
Particle Size
Interstitial
NAI-P
Total P
Station
(m)
h2o
VS
(u)
P (mg P/L)
(ug P/g)
(ug P/g)
A1
1.8
96.97
68.91
2.80
749
A2
0.9
96.58
55.96
181
0.40
83
902
A3
0.5
95.27
62.06
1.20
1,140
A4
1.8
96.07
63.35
1.90
1,100
A5
1.8
96.25
62.21
143
0.80
108
953
A6
1.8
96.32
65.01
1.70
1,020
A7
2.7
95.57
61.35
0.40
922
A8
1.5
96.88
61.69
0. 70
111
1,170
A9
1.4
95.38
66.72
0.40
883
A10
12.2
95.08
77.38
0.40
1,320
OJ

38
the water content of littoral sediments within Lake Okeechobee was
significantly lower (p <0.05) than that of the central sediments and
averaged only 49.6 percent. Station 09 was in the vicinity of beds of
rooted macrophytes and emergent vegetation, which effectively dampen
wave energy, thus permitting the accumulation of finer particles and
detritus. Water content of sediments from transitional zones was quite
variable, ranging from 26.7 to 88.6 percent, and reflected the gradation
from a highly dynamic environment to more quiescent conditions. The
difference between central-lake and littoral-zone sediments in Lake
Okeechobee is further illustrated by the relationship between the
fractional content of silt- and clay-sized particles and water content
(Figure III-l). A close correlation (r = .94) was found between
sediment water content and particle size distribution:
W = 41.16 + 12.66 (In B) (III-l)
where W = percent water content, and
B = cumulative mass percent (sediment particles < 63 um in
diameter).
Thus, the near-shore sediments within Lake Okeechobee tend to be large
grained with low water content, while the central-lake sediments are
fine grained with high water content. A similar relationship for Lakes
Vanern and Ekoln (Sweden) was derived by Hakanson (1977), who approxi¬
mated water content as a function of mean particle size. In general,
water content (or more specifically porosity) increases with decreasing
particle size, as a result of increasingly effective electrostatic
(repulsive) forces between clay minerals (Berner 1971, 1980).

Figure 11C — 1. Relationship Between Water Content and Fractional Silt-Clay Content for
Surficial Lake Okeechobee Sediments.

40
In contrast to Lake Okeechobee, the sediments of Lake Apopka are
relatively homogeneous with respect to water content; this trend implies
a somewhat uniform hydrodynamic environment. Lake Apopka is charac¬
terized by extensive deposits of highly flocculent sediments with water
content ranging between 95.1 to 97.0 percent. According to Schneider
and Little (1969), these unconsolidated deposits cover 90 percent of the
lake bottom with an average thickness of 1.5 m. Excluding the immediate
shoreline, stable sediments comprising sand and shells are restricted to
four small areas. The largest of these areas is a 240-ha region in the
northern portion of the lake basin; the second largest region
encompasses 120 ha adjacent to Crown Point near the eastern shoreline.
Sly (1978) suggested a simple approach to evaluate the occurrence
of sediment suspension. According to Sly, the depth at which
wave-induced shear stress exceeds the critical shear stress (Tc) of
bed materials corresponds to 25 percent of the available wavelength
(based on significant wave period). For Lake Okeechobee, the deepest
portion of the basin is approximately 4.3 m deep and has a maximum fetch
of 20 km. Applying linear wave theory (U.S. Coastal Engineering
Research Center 1977) to Sly's (1978) model, one calculates that sedi¬
ment transport can be initiated in the deepest part of Lake Okeechobee
at continuous wind velocities of 13.1 m/s (29 mph) or greater. It is
thus apparent that sediment erosion and transport can be induced
throughout Lake Okeechobee and that sediment accumulation tends to be
restricted to relatively protected areas along the southern edge of the
main basin (South and Pelican Bays) and the subbasin in the northern
portion of the lake. The relatively low water content (<90 percent)

41
generally observed throughout the lake and especially in the deeper
portion are indicative of substantial wave energy extending to the
sediment surface. For example, according to Hakanson's (1981) model for
sediment transport, sediments at a depth of 1 m in accumulating regions
of Lake Okeechobee should exhibit water contents exceeding 80 percent.
In Hakanson's model, the critical water content defining the limit
between sediment transport and accumulation was found empirically to
approximate
Wc = "max " 10 (III-2)
where Wc = the critical water content, and
Wmax = ^e water content at the maximum depth.
Of the three samples collected at this approximate depth (Stations 03,
08, and 014) only Station 014 located within South Bay had a water
content suggesting active sediment deposition. Despite the expansive
fetch that extends to South Bay during wind events from the north
(nearly 58 km), South Bay apparently is sufficiently sheltered by Rocky
Reef, macrophyte vegetation, and by the shoreline configuration to serve
as a settling basin. Moreover, prevailing winds originate from the
east, resulting in more limited fetches for South and Pelican Bays.
According to Hakanson's (1981) model, the only other station within
Lake Okeechobee with sediment water content indicative of relatively
undisturbed accumulation is Station 09, which is located within a
protective stand of rooted macrophytes. The central (open-lake)
stations experience a mixture of deposition and erosion, resulting in

42
the accumulation of silt- to sand-sized particles (Table 111 — 1). In
shallow lakes such as Lake Okeechobee, continued circulation and
upwelling prevent deposition of extremely fine particles, as Sly's
(1978) model predicts.
Excluding several relatively deep and narrow trenches along the
western and southern shores (4.9 and 5.5 m, respectively), the deepest
portion of the major basin of Lake Apopka is only 3.0 m in depth and has
an effective fetch of 7.4 km for winds originating from the east.
Under these conditions, Sly's (1978) model indicates that resuspension
can be initiated at wind velocities of 9.4 m/s (21 mph) or greater.
This result suggests the ease with which sediment resuspension occurs in
Lake Apopka. Because of its morphometry, quiescent regions of
undisturbed deposition are essentially nonexistent. Indeed the water
column of Lake Apopka often assumes a brownish cast because of
wind-induced scouring of the bottom sediments; for example, turbidities
in Lake Apopka often exceed 40 NTU (cf. Brezonik et al. 1978, 1981;
Tuschall et al. 1979; and Pollman et al. 1980) and have been observed as
high as 120 NTU (see Chapter IV).
Use of Sly's (1978) model as a guideline indicates that virtually
the entire basin of Lake Apopka is subjected to resuspension; the only
regions falling outside Sly's (1978) threshold criterion are the
previously mentioned trenches along the western and south shores and the
relatively protected subbasins comprising Gourd Neck Springs. Given the
extremely flocculent nature of the sediments, the existence of such
trenches is unusual. Bush (1974) explained their existence as a result
of scouring created by wave setup. Wind-induced surface currents are

43
maximized along the longest reaches of a lake, resulting in a piling-up
of surface water toward the leeward shore. The pressure thus exerted
induces a return flow along the bottom in the opposite direction,
scouring the bottom of muck.
These results conflict with a. priori conceptions concerning
sediment water content and lake hydrodynamics. The consistent and
extremely high water content of Lake Apopka sediments suggests (in the
absence of other information) that quiescent conditions prevail
throughout the basin. A similar conclusion is reached from Hakanson's
(1981) model, which indicates that water contents in excess of
90 percent reflect an undisturbed depositional environment. It is
apparent from the previous discussion of Sly's (1978) model that the
morphometry of the Apopka basin precludes undisturbed deposition. The
question then arises as to why such high sediment water contents are
observed throughout Lake Apopka.
Lake Apopka, by virtue of its shallow nature and inherently high
rates of algal productivity, constitutes a rather unique depositional
environment that falls beyond the framework of Hakanson's (1981) model.
Average rates of net primary production as high as 465 mg C/m^-hr
have been observed for Lake Apopka, with an annual average rate of
140 mg C/m^-hr reported by Brezonik et al. (1978). Compared with
the threshold level of 95 mg C/m-^-hr reported by Brezonik and
Shannon (1971) for eutrophic lakes, these results show that Lake Apopka
is highly eutrophic. As evidenced by sedimentary concentrations of
volatile solids, high rates of deposition of organic detritus are a
direct -consequence of high rates of algal production. Average sedimen¬
tary organic content (as defined by volatile solids) ranged between 56.0

44
and 77.4 percent in Lake Apopka, and the lakewide average was
64.5 percent. Organic detritus, which has a bulk density approaching
water, is readily resuspended and transported by wind-induced currents,
and its accumulation in sedimentary deposits generally reflects not only
rates of biogenic material production but also the vertical distribution
of hydrodynamic forces. In most lakes, then, transitional hydrodynamic
environments occur that result in selective deposition of particles as a
function of size and density. Consequently, sediment-focusing or high
rates of deposition of organic-rich material are observed in the deeper
reaches of these systems (cf. Davis and Ford 1982). However, because of
the configuration of Lake Apopka's basin, relatively undisturbed
depositional zones are non-existent. Consequently, a virtually uniform
layer of highly flocculent, organic sediments prevails throughout the
basin.
Unlike Lake Apopka, the distribution of organic material in Lake
Okeechobee clearly demonstrates a transitional hydrodynamic environment.
Organic content varied with station location in the basin, ranging from
0.3 to 42.7 percent (dry weight) (Table III-l). Sediments in littoral
areas (excluding Station 09) exhibited very low concentrations of
organic matter, averaging only 2.2 percent. Organic content of open-
lake sediments was significantly higher and ranged between 12.7 and
35.1 percent.
Two classes of open-lake sediments in Lake Okeechobee can be
identified from the results of this study. The first class consists of
highly organic muck (Stations 07 and 012), with sediment organic content
approximately 34 percent. These sediments tend to predominate in the

45
mid to north central portion of the lake basin. The second sediment
association, which occurred at Stations 06, 010, and Oil, is less
organic in nature, averaging 15.1 percent, and was found in closer
proximity to the shoals of the western basin. Sediment organic content
was highest at Station 014 in South Bay, with organic matter
constituting 43.2 percent of the total solid content. South Bay, which
previously was identified as a settling basin, receives substantial
portions of inorganic nutrients from the Everglades Agricultural Area
(Federico et al. 1981).
Further evidence supporting the idea that detrital accumulation in
Lake Okeechobee is influenced by hydrodynamic factors is supplied by
evaluating the relationship between organic matter and water content in
Lake Okeechobee sediments. A logarithmic correlation (r = .92) was
observed between water content and organic content (Figure III-2):
W = 43.21 + 12.37 (In VS) (III-3)
where VS = percent volatile solids dry weight.
A similar logarithmic relationship was noted by Nisson (1975) for
sediments from Lakes Harney, Jessup, and Monroe, on the St. Johns River
in central Florida.
If detrital accretion in sediments primarily reflects hydrodynamic
factors, a correlation also should be observed between particle size and
organic content. This relationship stems from the fact that the
conditions that favor the deposition of fine-grained sediments are also
conducive for the accretion of detritus. For sediments in Lake

Figure III-2. Relationship Between Sediment Water Content and Organic Content Expressed
as Volatile Solids for Surficial Lake Okeechobee Sediments.

47
Okeechobee containing 23 percent organic matter or less, this is indeed
the case (Figura III-3). In this range, organic content and silt-clay
content are linearly related. With organic content exceeding about
23 percent, however, the fractional silt-clay content remains constant
(approximately 20 percent), suggesting that in quiescent waters local¬
ized rates of primary production assume a greater role in controlling
the magnitude of detrital deposition. The fractional silt-clay and
organic content of Station 06 does not conform to this relationship;
unique morphometric and hydrodynamic factors may permit greater
deposition rates of colloidal silt-sized particles at this site.
Chemical Characteristics
The chemical nature of sedimentary phosphorus defines its
availability for exchange and biological uptake. Differential dissolu¬
tion schemes have been developed that correlate algal availability with
a particular fraction. Such fractionation schemes were applied to Lake
Okeechobee and Lake Apopka sediments, and results are described in this
section. Total P analyses, on the other hand, provide an estimate of
the total pool of phosphorus that ultimately is available for exchange.
Total phosphorus (TP) concentrations in Lake Okeechobee sediments
were highly variable and ranged from only 142 ug P/g (dry weight) to
1,440 ug P/g. Conversely, the distribution of TP in Lake Apopka
sediments was fairly homogeneous and reflects the uniformity of the
depositional environment in the lake. Concentrations in sediments
within the main basin ranged from 749 ug P/g at Station A1 to 1,170 ug
P/g at Station A8. TP concentrations in Station A10 sediments at Gourd

50
b 40
co
cB
Sj 30
o
LÜ
O
QC
LJ
o.
20
10
-* 1 1 1
10 20
VOLATILE SOLIDS (%)
30
40
4>
00
Figure III — 3. Relationship Between Fractional Silt-Clay Content and Organic Content
Expressed as Volatile Solids for Surficial Lake Okeechobee Sediments.

49
Neck Springs were somewhat higher (1,320 ug P/g) because of the
deposition of large quantities of decaying water hyacinths (Eichornia
crassipes). Phosphorus concentrations in lacustrine sediments may range
as high as 7,500 ug P/g, but typically are less than 2,500 ug P/g (Jones
and Bowser 1978). In a survey of mineralogical constituents comprising
profundal Minnesota lake sediments, Dean and Gorham (1976) observed an
average TP concentration of 1,400 ug P/g in calcareous lakes. Although
it is apparent that TP concentrations in Lake Okeechobee and Lake Apopka
sediments fall somewhat within the lower end of the spectrum, the
observed concentrations approach and in some cases exceed the
1,000 ug P/g threshold level cited by Frink (1967) as indicative of
eutrophication.
Within Lake Okeechobee, the highest TP concentrations generally
were observed in the pelagic reaches of the lake basin. TP concentra¬
tions in pelagic sediments ranged from 502 to 1,440 ug P/g with an
average value of 866 ug P/g; concentrations in littoral sediments were
significantly lower (p <0.05), averaging only 294 ug P/g. These results
imply that selective size sorting processes influence sedimentary P
distribution; similar results have been reported by Frink (1967 and
1969) and Hakanson (1981), as well as others. Based on the relatively
poor correlation between water content and TP in Lake Hjalmaren
sediments (r = .68), however, Hakanson (1981) concluded that accretion
of sedimentary P is mediated by anthropogenic and/or chemical factors in
addition to hydrodynamic processes. The weak correlation (r = .57,
Table III-3) between TP and water content in Lake Okeechobee sediments
implies a similar conclusion.

50
Table III-3. Summary of Sedimentary Component Interrelationships
with Total Phosphorus Concentrations in Lake
Okeechobee Sediments.
Relationship r^
NAI-P vs
. TP
.46
AI-P vs.
TP
.81
Interst i t
:ial SRP vs
. TP
.19
Volatile
Solids vs.
TP
.76
Water Content vs. TP
.32

51
As indicated by linear regression analysis, the accumulation of
sedimentary P in Lake Okeechobee is dictated to a significant extent
(p <0.05) by the deposition of apatite-inorganic P (AI-P) (Table III-3).
With the exception of organic content, correlations between TP and other
sediment parameters were weak and insignificant at the 95 percent
confidence level.
Mechanistically, the functional dependency of sediment TP content
on AI-P and organic content may be explained by evaluating the relation¬
ship between calcium-carbonate solubility, calcium-phosphate solubility,
and pH changes induced by primary production. The premise of this
hypothesis is that deposition of AI-P and detrital material are causally
linked to the same forcing function—primary production. For example,
calcium carbonate precipitation may be induced by increases in pH that
result from net primary production (Kelts and Hsu 1978); calcite
surfaces act as heterogeneous nuclei for calcium phosphate minerals
(Stumm and Leckie 1971; Griffin and Jurinak 1973), resulting in removal
of phosphorus from the water column. Similarly, calcium-phosphate
minerals tend to become less soluble with increasing pH (within the
range for most natural waters) owing to a shift in phosphorus speciation
and, in the case of hydroxyapatite Ca^o^PO^fctOH^, an increase in
hydroxide concentration. To test the hypothesis that AI-P deposition
may be influenced by biogenically induced pH shifts, the stability of
various calcium phosphate mineral phases in Lake Okeechobee was
evaluated by comparing measured ion activity products (lAP) for the
species with thermodynamic solubility products (Ksq values). For
purposes of this exercise, two pH values were used: 8.10, the average

52
pH and 9.10, the maximum pH observed by Federico et al. (1981) for a
series of stations near South Bay. Reported average Ca concentrations
approximated 46.5 mg/L. Average orthophosphorus concentrations at these
stations ranged between 0.020 mg P/L and 0.025 mg P/L; for the purposes
of this exercise a concentration of 0.0225 mg P/L was assumed. Of the
three mineral phases evaluated [CaHPO^, Ca^HiPO^)^, and hydroxy¬
apatite, Ca^giPO^) g,(0H) 2 1 , only hydroxyapatite is stable under ambient
conditions in Lake Okeechobee (see Table III-4). During periods of high
photosynthetic activity (i.e., high pH), the water column is greatly
supersaturated with respect to hydroxyapatite (IAP/Ksq >10®), and this
may result in the direct precipitation and subsequent incorporation of
this species in the sediments.
Similar relationships between sedimentary TP and sediment inorganic
P were observed by Wildung et al. (1974) in Upper Klamath Lake, a non-
calcareous lake located in Oregon. Unfortunately, inorganic P as
reported by Wildung et al. (1974) includes both non-apatite inorganic P
(NAI-P) and as AI-P, thus precluding a direct comparison of results. In
direct contrast to the results of this study, however, Williams et al.
(1980) found a weak, negative correlation between TP and AI-P in
sedimentary material derived the drainage basins of Lakes Ontario and
Erie. Williams et al. (1980) demonstrated strong correlations between
variability in TP concentrations and changes in both NAI-P and organic P
fractions; in Lake Okeechobee sediments, however, neither parameter
yielded a significant (p <0.05) correlation with sediment TP.
NAI-P, which is indicative of the sedimentary P reservoir directly
available to algae upon resuspension (Williams et al. 1980; Allan et al.

53
Table III-4. Solubility Product Constants (Ksq) and Calculated Ion
Activity Products (IAP) for Several Calcium Phosphate Mineral
Phases as a Function of Ambient Ca (1.16 x 10“^ M), SRP
(7.26 x 10”7 M), and pH Levels in Lake Okeechobee.
Solid Phase
log Ks0*
pH 8.10
pH 9.10
log IAP
log (IAP/Ks0)
log IAP
log (IAP/Ks0)
CaHP04
-6.6
-10.1
-3.5
-10.1
-3.5
Ca4H(P04)3
-46.9
-55.1
-8.2
-54.9
-8.0
Ca(P04)6(0H)2
-114
-111.6
2.4
-105.4
8.6
* From Stumm and Morgan (1981).

54
1980), generally is low in Lake Okeechobee sediments. Concentrations
ranged from 6 to 96 ug P/g and generally constituted less than
9.0 percent of the total P content. From vertical profiles of nutrients
in Lake Okeechobee sediment cores Brezonik et al. (1981) inferred a
sediment suspension or bioturbation depth of 10 cm; for Station 07
sediments this implies a labile P pool of approximately 1.04 g P/m^
which is sufficient to increase existing levels by 305 ug P/L (of labile
P) if the entire reservoir was released in a single pulse event.
Despite the relatively low concentrations of NAI-P, this calculation
illustrates the inherent potential for photosynthetic stimulation
induced by nutrient exchange across the sediment water interface.
Concentrations of NAI-P in Lake Apopka sediment were relatively
uniform and ranged from 83 to 111 ug P/g. Similar to Lake Okeechobee,
this fraction comprises only 10 percent of the total P reservoir.
However, the inherent potential for this reservoir to support primary
production in the water column is considerable. For example, at
Station A5 with average NAI-P and interstitial SRP concentrations of
108 ug P/g and 0.80 mg P/L, respectively, the total potential release of
inorganic phosphorus to the water column by resuspending only 1 cm of
sediment is 49 mg P/m . This is equivalent to an incremental
addition of 29 ug P/L to the water column. Of this total, the NAI-P
fraction contributes about 84 percent. It is important to note that
resuspension of sediment to a depth of 4.7 cm has been observed in Lake
Apopka during convective disturbances (see Chapter IV) and most likely
extends more deeply into the sediment.

55
The distribution of SRP concentrations in sediment pore fluids for
both Lake Okeechobee and Lake Apopka was somewhat erratic, and no trends
were discernible between sediment type and interstitial SRP. Inter¬
stitial SRP concentrations in Lake Apopka were quite high and ranged
from 0.40 to 2.80 mg P/L. The source of this variability is apparently
a combination of physical, biological, and chemical factors. For
example, the ability of Lake Apopka sediments to further adsorb
phosphorus (and hence buffer interstitial SRP concentrations within a
relatively narrow concentration range) is virtually exhausted and is
limited to extremely high concentrations. Under these conditions, small
changes in the concentration of phosphorus adsorbed to sediment particle
surfaces can result in very large changes in equilibrium SRP concentra¬
tions in the pore fluid (see Chapter V). In addition, interstitial SRP
levels increase as organic phosphorus is mineralized by microbial
heterotrophs. Pore fluid concentrations apparently continue to increase
until the sediments are disturbed and this pool is exchanged with the
overlying water. The sediments then reequilibrate at a lower but still
highly elevated (relative to the water column) concentration.
Interstitial SRP concentrations in Lake Okeechobee sediments
ranged from 0.002 mg P/L to 0.942 mg P/L with the maximum concentration
found in the nutrient-enriched, highly organic sediments of Station 014.
On the other hand, the pore water content of SRP in Station 07
sediments, which also were quite organic, was only 0.012 mg P/L, and the
interstitial SRP content of the relatively inorganic, sandy sediments of
Station 04 was substantially higher (0.267 mg P/L). Despite the
variability in concentrations, it is clear that SRP concentrations in

36
the interstitial fluids were generally quite low, and this fraction
constitutes less than 0.1 percent of the total sedimentary P. The
ability of calcareous sediments (and specifically calcium carbonate) to
adsorb P is well documented (e.g., Griffin and Jurinak 1973 and 1974),
and Morse and Cook (1978) suggested that this mechanism accounts for the
low levels of dissolved interstitial SRP found in carbonate-rich marine
sediments. The small size of the interstitial reservoir implies that
this pool is not a significant source of phosphorus to the overlying
water via advective exchange across the interface. For example,
resuspension of 10 cm of sediments from Station 07 would result in
addition of only 0.3 ug P/L to the water from entrained pore water.

CHAPTER IV
RELEASE STUDIES
A series of passive exchange studies were conducted to evaluate P
release over a broad range of water content for Lake Apopka sediment.
Sediment was collected from Station A2 and placed in five small aquaria
with surface areas approximately 305 cm^. Aquaria were filled with
fresh sediment to a depth of approximately 13 cm and were exposed to
varying intensities of artificial light from two 150 watt spotlights to
generate a gradient of sediment dessication. Initial water content was
95.1 percent, final values ranged from 13.8 to 88.3 percent (Table IV-1).
As dessication proceeded, surface areas decreased correspondingly and
ranged from reductions of 13.5 to 78.9 percent. Following dessication,
the aquaria were refilled with phosphorus-free 6.0 x 10“^ M CaCÜ3
to simulate ambient alkalinities observed in Lake Apopka and were
allowed to remain quiescent for a 9-day period. Samples of the
overlying water were taken on a daily basis to determine the quantity of
SRP released.
Phosphorus release by simple diffusion is illustrated in
Figure IV-1. The appropriate theoretical (Fickian) expression for
describing the flux of phosphorus across the sediment-water interface is
given by the solution to Fick's first law of diffusion:
J = Os ^ (IV-1)
s dz
57

5b
Table IV-1. Final Volatile Solids and Water Content of Partially
Dessicated Lake Apopka Sediments Used in Passive
Exchange Experiments.
Aquarium
Water Content
(percent)
Volatile Solids
(percent)
88.30
69.99
86.18
70.58
34.31
68.58
13.87
70.90
13.82
70.94
5

SRP (mg P/L)
TIME ( DAYS )
Figure IV-1. Passive (Diffusive) Release of SRP by Partially Dessicated Lake Apopka
Sediments (Station A2) with Time. Release Studies were Conducted at Various
Levels of Dessication Designated by Graph Number. See Table IV-1 for Water
Content Concentrations.

60
where J = flux of phosphorus across the interface (g P/cm^-s),
0 = porosity of the sediment (dimensionless),
Ds = the whole sediment diffusion coefficient (cm^-s),
and
dC
= the concentration gradient at the interface (g/cm^).
This simple model assumes that the flux across the interface is strictly
due to molecular diffusion and that deposition or compaction of sediment
particles is negligible (cf. Lerman 1979; Berner 1981). The change in
concentration in the water column at any particular point in time is
simply the flux across the interface divided by the depth of the water
column; this of course assumes that the water column is well mixed and
is a reasonable assumption for the depth scale of these microcosm
studies.
Solution of Equation IV-1 requires that the concentration gradient
at the interface is known at a particular point in time; consequently,
application of Equation IV-1 to the experimental results is extremely
difficult. A more tractable approach is to fit an empirical model to
the data which may then be differentiated to describe the instantaneous
change in concentration as a function of initial concentration. In all
cases, release over the duration of the experiment (9 days) can be
described by the following logarithmic expression:
Ct = a + b In t (IV-2)
where Ct = the uniform concentration (mg/L) in the overlying
water at time t (days), and
a (mg/L) and b (mg/L/d) are both empirically derived constants.

61
The rate of change in concentration is simply
dC , 1
dF = b • 7
(IV-3)
Agreement of the data with the model, expressed in terms of the
correlation coefficient, r, is presented in Table IV-2. Results from
Aquaria 4 and 5 were combined because of the virtually identical water
contents. Equation (IV-3) can be applied to evaluate the instantaneous
change in concentration, and through mass balance considerations, the
instantaneous areal release rate for a specified time. Average and
instantaneous fluxes evaluated at the conclusion of the experiment are
presented in Table IV-3.
Release rates averaged over the 9-day exchange period ranged from
1.57 to 22.7 mg P/m^-d and were inversely related to water content.
These results were somewhat surprising and did not conform to a priori
expectations; that is, if the concentration gradient is independent of
the degree of dessication, sediments with greater water contents should
yield higher diffusive fluxes because of the decreased tortuosity
(Manheim 1970; Lerman 1979). Enhanced rates of detrital mineralization
with increasing dessication and a concomitant elevation of interstitial
SRP levels can account for the observed changes in flux; however,
analyses conducted by Fox et a!. (1977) of interstitial water SRP
content in fresh and dried Lake Apopka sediments demonstrated essen¬
tially no difference between the two sediment types and thus do not
substantiate this mechanism. A more likely mechanism is given by
considering the change in sediment mass exposed on an areal basis as
dessication proceeds. As pore water evaporates and the sediments

ó 2
Table IV-2. Summary of Empirical Time-Dependent Concentration Model
Parameters (Equation IV-2) and Goodness of Fit.
Aquarium
Water Content
(percent)
a
b
r2
1
88.30
0.0268
0.0417
.995
2
86.18
0.0231
0.0415
.992
3
34.31
0.0416
0.1137
.986
4 and 5
13.85
0.0032
0.1951
.919
Table IV-3.
Calculated Average and Instantaneous Phosphorus Flux at
Time = 9 Days During Passive Exchange Studies.
Aquarium
Average Flux Instantaneous Flux
(mg P/m^-d) (mg P/m^-d)
1.57
0.55
2.18
0.77
13.9
5.12
22.7
9.71
4 and 5

63
consolidate, increasing quantities of underlying sediment particles
become directly exposed at the surface. A change in water content from
88 to 13 percent increases the exposed mass by approximately a factor of
10, which is close to the observed differences between the average flux
from sediments 4 and 5 and from 1 and 2.
Additional experiments to quantify the passive exchange of P
between Lake Apopka sediment and water were conducted using homogenized
sediment from Station A5. Sediment was transferred to a series of three
cylinders in sufficient quantity to yield a final depth of 14.5 cm. The
diameter of the cylinders was 10.2 cm, resulting in a surface area of
81.7 cmz for exchange. The experiment was also designed to quanti¬
tatively evaluate the effects of dessication induced by lake drawdown on
exchange; consequently two of the cylinders, B and C, were partially
dessicated by insolation to yield final water contents of 91.66 and
89.14 percent, respectively. The final cylinder, A, was allowed to
incubate at room temperature (22°C) for the duration of the dessication
period (1 week). The water content of Cylinder A sediments was
94.89 percent. Following the dessication period, water was added to all
three cylinders simultaneously. One liter of filtered, aged lake water
from Lake Apopka, with an SRP concentration of 0.004 mg P/L, was added
to each cylinder to serve as the solution phase; this maximized the
concentration gradient across the sediment water interface and also
removed any algae present.
During the incubation period the sediments in Cylinder A began to
compact, lowering the sediment-water interface by approximately 0.7 cm
and forming a residual interfacial pool of SRP enriched water.
Subsequent extraction, filtration and analysis of 10 ml of the

04
interfacial water indicated an average concentration of 1.35 mg P/L.
The interfacial water comprised a labile pool of approximately 64 ug P
for immediate exchange upon addition of filtered lake water. Care was
taken not to disturb the interface when the filtered lake water was
added to each cylinder. Since the photic zone rarely extends below a
depth of 0.9 m in Lake Apopka, the diffusional phase of the experiment
was conducted in the dark. Free exchange with the atmosphere was
permitted, however, to prevent the system from going anaerobic.
Changes in SRP concentration with time are shown in Figure IV-2 for
all three cylinders. Cylinder A, representing fresh sediment, began the
initial phase of the experiment with the highest concentration of SRP
(0.192 mg P/L). Approximately 32 percent of this pulsed release was
engendered by mixing of the residual interfacial pool with added lake
water; the remaining release may have been induced by a slight
disturbance of mud-water interface and subsequent entrainment of
interstitial P and/or desorption from sediment particles. Because of
this pulsed release, the concentration gradient was reduced and
Cylinder A, consequently, showed the lowest rate of release. Calcula¬
tions of release rates were based upon observed changes in SRP in the
overlying water during the initial phase of the experiment, when the
concentration gradient was most extreme and the resulting overlying
water concentrations approximated ambient SRP levels observed in situ.
Release rates were essentially linear during this phase, and as seen in
the previous exchange studies with A2 sediment, release increased as an
inverse function of sediment water content. Release from Cylinder A
o
approximated 4.3 mg P/m -d while release from Cylinder C was
calculated to be 28.8 mg P/m^-d, (Table IV-4). Observed rates of

TIME (DAYS)
Figure IV-2.
Passive (Diffusive) Release of SRP by Fresh (A) and Partially Dessicated
(B and C) Lake Apopka Sediments (Station A5) with Time. See Text for Water
Content Concentrations.

Table IV-4.
Calculated Porosities,
for Fresh and Partially
Diffusive Fluxes, and Tortuosity Corrected Molecular
Dessicated Lake Apopka Sediment.
Diffusivities
Cylinder
Water Content
(percent)
Poros it y
(Dimensionless)
Corrected
Molecular Diffusivity
D (cm^/s x 10^)*
Dif fus ive FIux
Observed
(mg P/m2-d)
A
94.89
0.9716
6.93
4.3
B
91 .66
0.9551
6.70
6.9
C
89.14
0.9408
6.50
28.8
* Using a DQ
value of 7.34 x 10-^ cm
2/s for HP04-2 at 25°C
(Li and Gregory 1974).

67
release for Cylinders A and B are comparable to other values reported
in the laboratory for undisturbed cores (Table IV-5). In addition to
the previously described hypothesis regarding sediment exposure and
dessication, the observed flux in Cylinder A is also undoubtedly lower
because of the reduced concentration gradient across the sediment-water
interface. This effect can be derived directly from Fick's first law of
diffusion where flux is proportional to the concentration gradient. This
effect has also been empirically verified by Rippey (1977) who observed
that maintenance of low concentrations in the overlying water of
undisturbed cores increased areal release rates by 82 percent.
For Cylinders A and B, the linear phase of rapid release was
confined to an extremely short timeframe of 6 hours beyond which
decreases in concentration approaching initial levels occurred.
Concentrations in Cylinder A remained relatively stable for the next
42 hours, but by 72 hours they showed a further decline to a minimum
concentration of 0.172 mg P/L. After 72 hours, release was again
essentially linear in Cylinder A, and approximated 0.6 mg P/m^-d.
Similar erratic behavior was observed in Cylinder B; between 6 hours and
48 hours SRP levels fell to 0.007 mg P/L and then proceeded to increase
towards a secondary maximum of 0.023 mg P/L at 48 hours. Between 121
and 362 hours, SRP concentrations steadily increased. Release during
this period equalled 0.3 mg P/m^-d. Although the concentration
gradient was more pronounced in Cylinder B, the reduced flux after the
initial equilibration stage is more consistent with the decline in
overall porosity and the formation of a relatively impermeable crust
caused by partial dessication.

Table IV-5. Summary of Reported Sediment Phosphorus Release Rates Measured In Situ and in the Laboratory.
Lake
Experimental
System
Temper¬
ature
(°c)
Release Rate
(mg P/m^-d)
Aerobic Anaerobic
Re ference
Norvikken
Laboratory-core
4
1.7 - 3.0
3.0 - 4.6
Ulen (1978)
Laboratory-core
10
2.4 - 6.7
4.0 - 7.3
Calculated
—
1
.0
(diffus ion)
Es rom
Undisturbed cores
-8 -
16.2
Karnp-Nielsen (1975)
Es rom
Undisturbed core
7
-1.4
12.3
Kamp-Nielsen (1974)
Gnadensee
4
1.2
Banoub (1975)
10
2.2
15
3.5
25
9.4
Sodra Bergundas-Jon
Undisturbed core
8
36
Bengtsson (1975)
Lough Neagh
Undisturbed core
10
10.4
Rippey ( 1977)
Agitated core
14.8
Lough Neagh
Calculated
14
- 48
Stevens and Gibson (1977)
Warner
In situ-calculated
26
Digiano and Snow (1977)
Castle
In situ
0.65
Neame (1977)
Mohegan
Homogenized
20
-2.5
2-3
Fill os and Biswas ( 19 76 )
Mendot a
Undis turbed
2-23
-1.9 - 83
0.67 - 65
lloldren and Armstrong (1980)
Wingra
Undis t urbed
4-21
-0.56 - 3.4
0.95 - 2.9
Holdren and Armstrong (1980)
Minocqua
Undis turbed
3-18
<0.02 - 0.37
0.03 - 3.1
Holdren and Armstrong (1980)
Little John
Undis t urbed
4-16
<0.02 -1.1
0.02 - 3.8
Holdren and Armstrong (1980)
Shagawa
In situ
7
Sonzogni et al. (1977)

69
Fox et al. (1977) conducted similar experiments with Lake Apopka
sediments to determine the effects of dessication on overlying water
quality. A general pattern of increasing P release with extent of
dessication was observed. Release as a function of dessication for both
this study and that of Fox et al. (1977) is presented in Figure IV-3 for
equilibration times of 144.5 hours and 1 week, respectively. Linear
regression analysis indicates a highly significant relationship between
the two variables (r^ = .74, p <0.025), yielding the following model:
SL = 1.44 - 0.01 W (IV-4)
where SL = concentration (mg P/L) observed after the stated
equilibration times, and
W = percent water content of the sediment.
The most consistent behavior with respect to release was observed
for the most dessicated sediment, Cylinder C. Release was relatively
constant over the initial 24 hours and began to decrease, apparently in
response to the diminished concentration gradient. Net release
continued until a state of dynamic equilibrium was achieved by Day 4,
when uptake essentially balanced release. Equilibrium was maintained
for approximately the next 93 hours following which time concentrations
dropped steadily for the duration of the experiment. The subsequent
decline in SRP is probably biological in nature; the longer period of
exposure to insolation and incident UV radiation undoubtedly had a
pronounced, deletorious effect on the benthic microflora in Cylinder C
relative to the other cylinders. If benthic microbial activity

SRP (mg P/L)
PERCENT WATER, W
Figure IV-3. Equilibrium SRP Levels in Laboratory Aquaria (Fox et al. 1977) and Cylinders (this
study) as a Function of Sediment Dessication. Concentrations Represent
Equilibration Times of 168 Hours and 144.5 Hours, Respectively.

71
increased as the experiment progressed a concomitant increase in SRP
uptake should have been observed; this mechanism is consistent with the
observed decline in SRP. It is interesting to note that concentrations
of SRP in both Cylinder A and B remained relatively constant during the
latter stages of the experiment; this may indicate that the benthic
microbial community was stable from the outset.
Calcium and magnesium concentrations were monitored during the
course of the study. Magnesium levels were essentially constant when
corrected for evaporative effects whereas corrected calcium levels show
a substantial increase of approximately 14 mg/L by the end of the
experiment. Sedimentary CaCC>3 is apparently solubilized as a result
of heterotrophic activity producing CO2 (Lee et al. 1977). The
magnitude of Ca release for all three cylinders was approximately the
same, and thus solubilization of hydroxyapatite was not the primary
mechanism for release since the amount of P release differed markedly
among the cylinders.
Turbulence Study
As an extension of the quiescent nutrient release study conducted
with sediments from Station A5 in Lake Apopka, the effects of turbulence
on SRP exchange also were evaluated. After equilibration periods of 35
to 45 days to dampen phosphorus concentration changes associated with
diffusion, Cylinders A and C (representing fresh and partially
dessicated sediments, respectively) were subjected to varying degrees of
turbulence with a variable speed rotor with a paddle attachment. The
paddle encompassed a surface area of 21 cm^ and was situated 4.4 cm

72
above Che sediment-water interface. The depth of the water column for
each system was maintained at approximately 11.2 cm.
Samples for SRP were withdrawn immediately preceding the start of
each experiment; subsequent samples were taken at measured time
intervals. All samples were filtered through GF/C glass microfibre
filter paper. Supplemental aliquots also were removed at periodic
intervals to determine suspended sediment concentrations. Equivalent
quantities of aged and filtered water from Lake Apopka were introduced
into each cylinder after each sample was withdrawn in order to maintain
a constant volume.
Fresh Sediment
Results of the turbulence experiment with fresh Lake Apopka
sediment are depicted in Figures IV-4 and IV-5. The initial phase was
conducted at a stirring velocity of 10.5 to 12.2 revolutions per minute
(rpm) sustained for 62 minutes. This velocity was sufficient to entrain
isolated, loose sediment particles at the interface, although the
interface remained essentially intact. Upon lowering the velocity to
9.0 rpm, the suspended particles began to settle. A net decline
(corrected for sample removal) of 0.002 mg P/L was observed during this
period, indicating virtually no effect of stirring at this rate on
equilibrated sediment.
The system was allowed to remain quiescent for another 67 minutes
before initiation of a second turbulence period. Rotor velocity was
increased to a maximum of 67.6 rpm, causing immediate scour of the
interface. Turbulence was maintained for exactly 1 hour. After a

CONCENTRATION ( mg P/L)
12 3 4 5
TIME ( HOURS )
Figure IV-4. Effect of Turbulence on SRF Concentrations in Water Overlying Fresh Lake Apopka
Sediraents.
* Sediment not resuspended,
t Sediment resuspended.

Figure IV-5. Effect of Turbulence on SRP Release Rates from Fresh Lake Apopka Sediments
(Station A5).

75
preliminary reduction in concentration of SRP in the overlying water at
the start of turbulence, SRP levels increased to 0.484 mg P/L and
suspended sediment concentrations reached 12.61 g/L by the conclusion of
the turbulent phase. Mass balance calculations indicate that between
25 to 68 percent of the net release in SRP was the result of entrained
interstitial water; the remaining fraction was apparently desorbed from
suspended particles and was equivalent to desorption of 12.3 to
28.4 ug P/g. More exhaustive studies concerning sorption phenomena are
presented elsewhere (see Section V, Adsorption-Desorption Studies). The
contribution of the pore water to release was calculated by assuming
that the concentration of SRP was between 0.50 to 1.35 mg P/L; the
latter value represents the interfacial water concentration at the
beginning of the diffusion study following exposure whereas the former
value constitutes the interstitial water content of the fresh sediment
prior to exposure. If release during the quiescent study is assumed to
be strictly diffusive, then 33 percent of the observed net release can
be attributed to entrainment of the pore fluid and the magnitude of
desorption approximates 25.6 ug P/g.
After stirring was concluded, resuspended sediment particles began
to settle rapidly, establishing a sharp interface between a clear
supernatant and the settling particles. Migration of the interface was
relatively slow (2.57 x 10“^ Cm/s) during the first 7 minutes after
cessation of stirring because of turbulence within the water column
(Figure IV-6). After this equilibration period, water motion within the
cylinder became dampened and the settling velocity was much more rapid.
As the interface approached the point of origin, velocities decreased in

76
10 20 30 40 50 60 70 80
TIME (MINUTES)
Figure IV-6. Migration of Interface Between Suspended Sediment
Clear Supernatant with Time Following Cessation of
Turbul ence.
and

77
response to resistance from the compacted particles. The maximum
observed settling velocity was 1.95 x 10-^ cm/s. Some readsorption
of phosphorus occurred as the entrained sediment particles settled;
however, a significant increase in concentration relative to initial SRP
levels was observed after equilibrium had been established. The final
concentration was 0.355 mg P/L compared with an initial concentration of
0.159 mg P/L. These results conflict with Holdren and Armstrong (1980)
who observed that P release from Lake Mendota sediments generally
increased with increased stirring up to the point where sediment suspen¬
sion occurred. Resuspension was found by Holdren and Armstrong (1980)
to decrease overlying water SRP concentrations to levels below those in
unstirred cores. Conversely, resuspension of Lake Apopka sediments
results in a substantial release of SRP, while stirring in the absence
of resuspension has virtually no effect on release. Indeed, in the
experimental system employed in this study, no change in SRP levels was
anticipated with stirring up to the point of bed failure because the
system was allowed to equilibrate for an extended timeframe. In other
words, the initial conditions of the experiment set 3c/3t = 0, which
according to Fick's second law implies that any change in the diffu-
sivity constant caused by stirring will not alter the concentration of
SRP in the overlying water. In the cores used by Holdren and Armstrong
(1980), the derivative of the concentration gradient is non-zero; thus,
an increase in diffusivity induced by stirring will result in a
comcomitant increase in 3c/3t.

78
Partially Dessicated Sediment
The effect of turbulence on SRP release from partially dessicated
sediments (Cylinder C) was minimal. As in the preceding experiment on
Cylinder A, the sediments of Cylinder C were subjected to two discrete
periods of turbulence separated by a quiescent phase to isolate any
observed effects. The initial turbulent phase was conducted at an
average rotor velocity of 13.9 rpm maintained for 1 hour. Concentration
changes were somewhat erratic during this phase (Figure IV-7) and when
corrected for sample removal and replacement with filtered Lake Apopka
water were equivalent to a net release of only 0.001 mg P/L. Rotor
velocity was insufficient to disturb the sediment-water interface,
although some unconsolidated particles were entrained.
After a stabilization period of nearly 2 hours during which the
overlying water SRP concentration remained essentially constant,
turbulence was again induced. Rotor velocity was increased to 115.3 rpm
and was allowed to continue for exactly 2 hours. Despite the extreme
amount of turbulence, the interface remained intact except for some
erosion from a small hole initially created by the removal of a sediment
subsample for analysis. A steady-state suspended sediment concentration
of only 0.0720 g/L was observed after 90 minutes. No phosphorus
desorption was detected in association with the minor amount of sediment
resuspended; however, a small but discernible decrease in concentration
of SRP in the overlying water did occur. The net decrease when
corrected for sample removal was 0.004 mg P/L and corresponds qualita¬
tively to the initial decrease observed in Cylinder A when resuspension
was induced. The apparent mechanism for the loss from solution is

SRP (mg P/L)
Figure IV-7. Effect of Turbulence on SRP Concentrations in Water Overlying Partially Dessicated
(Cylinder C) Lake Apopka Sediments.

so
adsorption by resuspended particles derived from the surface; due to a
period of desorption and diffusive loss during the month long quiescent
equilibration phase, the surface particles apparently have less
surface-sorbed P than underlying sediments. Because of the reduction in
surface coverage of available binding sites (see Section V, Sorption
Characteristics), the relatively impoverished surface particles are free
to adsorb P upon resuspension. Further scour of underlying sediments
entrains surface-P rich particles which then undergo desorption (e.g.,
Cylinder A). The extent of scour in Cylinder C was apparently not
sufficient to resuspend these enriched particles; thus, a net decline in
SRP was observed.
The results of this experiment indicate that even partially
dessicated sediments are very stable and will not be resuspended even
under conditions of extreme turbulence. However, if the oxidized
surface layer is breached, exposing the less compacted sediments lying
underneath, localized scour is likely to occur.
In-Situ Studies
Water quality and limnological monitoring of Lake Apopka was
conducted between January 1977 and January 1981 as an integral part of
the Lake Apopka restoration program. The results of the study have been
presented elsewhere for individual years (Brezonik et al. 1978; Tuschall
et al. 1979; Pollman et al. 1980; and Brezonik et al. 1981) and will not
be discussed here except to relate observed chemical characteristics
with the phenomenon of internal loading. Monitoring within Lake Apopka
focused on three open-water stations (A2, A5, and A8) and Gourd Neck

81
Springs for Che bulk, of the study after a comprehensive survey of
11 stations during the first year showed that adequate spatial
characterization was provided by three sampling points. Sampling was
conducted monthly except for the first year when the frequency was
biweekly.
To evaluate the degree of interaction between SRP concentrations
and other chemical components, a correlation matrix was developed using
SAS (Statistical Analysis System) (Barr et al. 1976). Correlation
analysis compares parameters by way of a linear model to determine if a
significant relationship exists. The degree of linear correlation
between the two variables is given by the correlation coefficient, r,
which by definition must lie between 1 and -1. Selected results of the
analysis are presented in Table IV-6. Using the 5 percent confidence
level as the threshold of significance, significant relationships
between SRP and other abiotic variables were established for only two
parameters: turbidity and inorganic carbon (IC). Although a simple
correlation between two variables is not conclusive evidence of a cause
and effect relationship, this result does suggest that SRP levels within
Lake Apopka are influenced by the resuspension of sediments.
Indirect evidence to substantiate the role of sediment resuspension
of phosphorus dynamics is offered by evaluating the interaction between
turbidity and ammonia variability. Correlation analysis (Table IV-6)
indicates these two parameters are significantly related. Ammonia,
which is generated by heterotrophic bacteria as organic matter is

82
Table IV-6. Intercomponent Relationships Between Selected Variables in
Lake Apopka, 1977 to 1980.
Turbidity
IC
K
Ammonia
Color
SRP
0.28915
(0.0487)
0.38099
(0.0082)
0.30150
(0.0621)
0.12823
(0.3904)
-0.14958
(0.3156)
Turbidity
0.65573
(0.0001)
0.72893
(0.0001)
0.32722
(0.0248)
0.01153
(0.9387)
IC
0.90591
(0.0001)
0.10961
(0.4633)
-0.20465
(0.1676)
K
-0.00954
(0.9541)
-0.10756
(0.5145)
Ammonia
-0.03957
(0.7917)
Note: ( ) = significance of relationship.

83
decomposed, accretes in sedimentary pore fluids to quite significant
levels. Concentrations as high as 49.0 mg N/L have been observed in
Lake Apopka sediments (Brezonik et al. 1978). Average interstitial
concentrations exceed typical levels in the overlying water of Lake
Apopka by over two orders of magnitude; thus even minor disturbances of
the sediment-water interface can exert a profound influence on the
nutrient status of the water column. Seasonal dynamics of nitrogen in
Lake Apopka are stochastic in nature, apparently as a result of episodic
disturbances and scouring of the bottom sediments.
SRP was strongly correlated (p = 0.0082) with inorganic carbon and
to a lesser extent (p = 0.0621) with potassium. Similar to ammonia,
inorganic carbon accumulates in the interstitial fluid as detrital
material is metabolized. Likewise, the close relationship between
turbidity and inorganic carbon (r = .656, p = 0.0001) (Table IV-6)
supports a resuspension type of mechanism for controlling to a signifi¬
cant extent the concentration of this solute in the overlying water.
Potassium, which in turn is very closely related to inorganic carbon
(r = .729, p = 0.0001), is essential for aerobic life forms (Lehninger
1970) and has been used as a tracer for the migration through soils of
leachate derived from decomposing vegetative material (Ellis 1980).
Wetzel (1975) has observed that potassium levels can be depleted in the
trophogenic zone of highly productive lakes presumably due to biotic
uptake and in turn accumulate in the tropholytic zone. It consequently
seems reasonable to label K as an indicator of internal nutrient cycling
and, in particular, nutrient release induced by sediment resuspension.

84
If the internal dynamics of phosphorus cycling in Lake Apopka are
dominated by external loading, a simple correlation between external
loading (or a corollary parameter) and SRP can be anticipated. A
nutrient budget developed for the 1977 water year has identified main¬
tenance backpumping of the muck farms bordering the northern perimeter
of Lake Apopka as the principal external source of phosphorus. Because
of the high organic content of the muck soils, the nutrient-enriched
soil water is highly colored. This characteristic provides a convenient
marker with which to trace the discharge of agricultural wastes and
associated nutrients to the system. The following example illustrates
this point. During late summer 1979, the Apopka drainage basin
experienced heavy rainfall and high rates of backpumping to the lake and
the Apopka-Beauclair canal were observed. The effect of agricultural
discharge on Lake Apopka and the downstream Oklawaha lakes was striking,
as illustrated in Figure IV-8 which delineates the spatial distribution
of both color and SRP during September 1979. Concentrations in both SRP
and color show an increase with location moving towards the north end of
the lake. Once within the Apopka-Beauclair canal, SRP and color
concentrations increased dramatically. Color and SRP concentrations at
the outfall of a discharge pipe on the Apopka-Beauclair canal were
101 chloroplatinate units (CPU) and 0.523 mg P/L, respectively; this is
contrasted with substantially lower color (65 CPU) and SRP (0.026 rag P/L)
concentrations at the outlet of the lake. Figure IV-8 shows that SRP
concentrations and color appear to be intrinsically linked; both
parameters decrease monotonically with distance downstream from the
source until the Dora canal is reached. Similar observations were made
during August and October 1979.

Chain of Lakes, September 28, 1979. From Pollman et al. (1980).
SRP (mg P/L)

86
If color is a reasonable label Co trace long-term influences of
external loading on SRP dynamics, correlation analysis for the 4-year
data base implies that the relationship is insignificant. A weak,
negative correlation (r = -.14958, p = 0.3156) was derived between color
and SRP, suggesting that SRP variability within Lake Apopka is
independent of external inputs.
On July 11, 1979, a brief but intense convective disturbance was
encountered on Lake Apopka at the beginning of a routine sampling
effort. The storm, which originated from the southwest, began at
approximately 1540 hours. Wind velocities were estimated to range
between 9 and 11 m/s at the advent of the storm and had diminished to
approximately 6.7 m/s by the time the last sample was taken 1 hour
later. As the storm swept across the lake surface, bottom sediments
were immediately resuspended to the surface. The visual change was
rather remarkable; for example, sediment resuspension was observed as a
sharp front normal to and moving across the lake in the same direction
as the wind. Behind the front the water surface assumed a muddy brown
appearance from entrained sediments while on the leeward side, a
characteristic greenish cast typical of the highly productive water
column was observed.
Turbidity and suspended solids measurements from a protected point
along the western edge of the lake near Gourd Neck Springs showed
concentrations of 31 NTU and 0.0865 g/L of suspended material at the
surface. These values were considered to be representative of back¬
ground or pre-storm conditions. Surface samples from the open-water
stations (A2, A5, and A8) readily showed the effects of resuspension;

87
turbidities ranged from 43 to 46 NTU and suspended sediment
concentrations ranged from 0.2053 to 0.2553 g/L.
Composite samples were taken from the surface and 1-m depth at the
open-water points and analyzed for turbidity, SRP, and TP. Results are
presented in Figure IV-9. Turbidity levels at all three stations were
substantially higher in the composited samples relative to surface
specific samples, thus indicating that resuspension was not uniform
throughout the water column but existed in the form of a concentration
gradient. Lakewide average turbidity and SRP concentrations both
exceeded levels observed the previous month during quiescent conditions
by a factor of three (Pollman et al. 1980). SRP levels were particu¬
larly high at Station A5 where a concentration 0.277 mg P/L was
observed. Although the composited turbidity concentration was lower at
this station (90 NTU) relative to Stations A2 and A8 (100 and 120 NTU,
respectively), sampling was conducted at this station during the waning
moments of the disturbance. Wind velocity and wave activity were much
reduced at Station A5, allowing entrained particles to resettle.
Kinetic factors may also account for the lower SRP concentrations
observed at Stations A2 and A8.
Although suspended sediment concentrations are unavailable for the
composited samples, an empirical relationship derived in situ between
turbidity and suspended solids for Lake Apopka (Figure IV-10) enables an
estimate of the quantity of sediment resuspended to be made. The
following linear model was derived from least squares regression
analysis (r^ = .96, p <0.025):

88
Figure IV-9. Suspended Sediment (g/L) and Total Phosphorus (TP)
(mg P/L) Concentrations in Lake Apopka During a Convective
Disturbance, July 11, 1979. TP and Suspended Sediment
Concentrations are Presented as the Upper and Lower
Figures, Respectively, at Each Sampling Site.

100
90
80
70
60
50
40
30
20
10
-i 1 ^ — i . ■ . . .
.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0
SUSPENDED SOLIDS ( g/L)
Ou
Figure IV-10.
Turbidity (NTU) in Lake Apopka as a Function of Suspended Solids (g/L).

90
Turbidity = 12.04 + 141.2 (SS) (IV-5)
where SS is the concentration of suspended solids (g/L), and turbidity
is given in NTU. On the basis of this model, the sediment concentration
yielding the observed composite turbidity levels can be determined.
Using data from Station A5 as an example, a turbidity concentration of
90 NTU equates to a suspended solids concentration of 0.5521 g/L. The
suspended sediment concentration at 1-m depth must therefore be equal to
0.8973 g/L. Assuming the concentration of suspended matter increases
linearly with depth gives an average concentration for the water column
(z = 176 cm) of 0.8145 g/L.
Correcting for the background contribution from ambient phyto¬
plankton and suspended sediment concentrations (0.0696 g/L) yields a
net resuspension of 0.7449 g/L. The areal loss across the sediment
water interface is therefore 1,311 g/m . Since the average bulk
density and water content of Lake Apopka sediments is 1.024 g/cra^
and 96.04 percent, respectively, the depth of the sediment entrainment
is estimated to be 3.2 cm. Calculated depths of sediment resuspension
for Stations A2 and A8 are summarized in Table IV-7.

91
Table IV-7. Estimated Depth of Sediment Resuspension of Lake Apopka During a
Convective Disturbance, July 11, 1979.
Calculated Composite Surface Suspended Calculated
Station
Depth
(cm)
Composite
Turbidity
(NTU)
Suspended Sediment
Concentrat ion
(g/L)
Sediment
Concentration
(g/L)
Depth of
Resuspension
(cm)
A2
180
110
0.6937
0. 1616
4.7
A5
176
90
0.5521
0.2069
3.2
A8
150
120
0.7645
0.2553
3.5

CHAPTER V
NUTRIENT RELEASE MODEL
A series of adsorption-desorption isotherm studies were conducted
in the laboratory to determine net release or uptake of phosphorus as a
function of ambient levels of SRP in the water under well-mixed
(equilibrium) conditions. Experimental results indicate that Lake
Okeechobee sediments vary appreciably in their ability to adsorb (or
desorb) phosphorus. A characteristic curvilinear relationship between
initial concentration and the magnitude of sorption was observed for all
stations except Station 013, which showed little ability to desorb or
absorb phosphorus over the experimental range of concentrations
(Figures V-l through V-4). Similar curvilinear results were reported by
Harter and Foster (1976) and Mayer and Gloss (1980). A station-by¬
station comparison of adsorption at an initial concentration of
1,000 ug/L suggests that Station 07 sediments have greatest affinity for
phosphorus at high concentrations; these sediments also have the
greatest silt-clay content, and thus the greatest surface area available
for exchange. Because of their relative homogeneity with respect to
chemical and physical characteristics, sorption studies with Lake Apopka
sediments were limited to sediment from Station A5. A curvilinear
relationship between the extent of sorption and initial concentration
was also observed for Station A5 sediment; the results, however,
92

SORPTION (-«g P/g)
DESORPTION ADSORPTION
Figure V-l.
SKP Sorption by Station OLD Sediments as a Function of Initial SRP Concentration.

Figure V-2.
SRP Sorption by Station 07 and Station 014 Sediments as a Function of Initial SRP
Concentration.

SORPTION (-*g P/g)
DESORPTION ADSORPTION
Figure V-3.
SRP Sorption by Station Oil Sediments as a Function of Initial SRP Concentration

SORPTION P/g)
DESORPTION ADSORPTION
Figure V-4. SRP Sorption by Station 013 Sediments as a Function of Initial SRP Concentration.

97
indicate that these sediments have little affinity for phosphorus
(Figure V-5).
Phosphorus sorption data for Lakes Okeechobee and Apopka sediments
were evaluated for goodness of fit to the Langmuir adsorption isotherm
model. This model was selected in favor of the Freundlich model, which
has been widely used to describe phosphorus adsorption in solids and
sediments (e.g., Carritt and Goodgal 1954; Fitter and Sutton 1975; Hwang
et al. 1976; Pollman 1977). The Freundlich model is purely empirical,
however, and it has no theoretical significance (Yariv and Cross 1979).
The Langmuir model was derived by Langmuir (1918) from the kinetic
theory of gases to describe gas adsorption on solids; it was applied to
soils by Olsen and Watanabe (1957) and has been used extensively for
lake sediments (e.g., Kuo and Lotse 1974; Edzwald 1977; Ku et al. 1978;
Holm et al. 1979). Sposito (1979) applied statistical mechanics to
verify the applicability of the Langmuir model to soils. The following
section describes the assumptions inherent in the derivation of the
Langmuir model and the implications of applying the Langmuir model to
the prediction of adsorption and desorption processes.
Development of a Langmuir Sorption Model
The Langmuir model assumes that sorption is restricted to a mono-
molecular layer and that all sites are equally susceptible for soption,
i.e., the exchange sites are homogeneous with respect to surface energy.
The Langmuir model can be written in the following linear form:
1 C
+
r br r
OO CO
(V-l)

SORPTION (¿uq P/L)
DESORPTION ADSORPTION
40
30
20
10
0.500
INITIAL CONCENTRATION (mg P/L)
—i—/ k-
1.00
Figure V-5. SRP Sorption by Station A5 Sediment as a Function of Initial SRP Concentration

99
where C = the equilibrium concentration of adsorbate (ug/L),
F = the quantity of adsorbate adsorbed per unit mass of
adsorbent (ug/g),
= the adsorption capacity of the adsorbent (ug/g), and
b(L/ug) = a constant directly related to the binding energy.
Application of the Langmuir model to sorption phenomena has been
restricted almost exclusively to describe adsorption. Theoretically,
however, the model represents the synthesis of two competing processes,
viz., adsorption and desorption, and thus can be employed to model
desorption. This can be demonstrated by reviewing the derivation of the
Langmuir equation for gaseous adsorption by solids. The isotherm can be
developed on the basis of molecular kinetic theory which gives the
number of molecules striking a unit surface area per unit time as
(Jaycock and Parfitt 1981):
1 1/2
(2tt rakT)
where P = the partial pressure of the gas or vapor of interest,
m = the mass of an adsorbed molecule,
k = Boltzmann's constant, and
T = temperature.
Adsorption is restricted to a certain fraction, j, of molecules
striking unoccupied surface sites. If the fraction of sites already
occupied is $ (i.e., $ = Na/Ns where Ns is the total number of
exchange sites per unit surface area, and Na is the number of sites
occupied by adsorbed molecules), then the rate of adsorption is ijíl-í*).

100
Kinetic studies indicate that the rate of desorption per unit surface
area and unit time will be $Ngv exp (-t|j/kT), where v is a constant
of proportionality determining the probability of desorption, and ^ is
the activation energy of desorption (Yariv and Cross 1979). The
probability of desorption is thus v exp (~\p/kT). At equilibrium the
number of adsorbed molecules at the surface must be constant (i.e., the
rate of adsorption equals the rate of desorption). Therefore
-1/2
(1 - $) j P (2t mkT) = Ns 4>v exp (-i|>/kT) (V-3)
This equation may be written as
bP
1 + bP
(V-4)
where b = j (Ng$v exp (~ij;/kT)) ^ (2tt mkT)-^/^.
Replacement of the equilibrium gas pressure with the equilibrium
adsorbate concentration, C, and substituting the surface coverage, F,
and the adsorption capacity, T , for Na and Ns, respectively, gives
or, alternatively
T = bC
r i + be
oo
C = _1_ + C
r br r
(V-5)
(V—1)
Application of the Langmuir equation to predict the extent and
direction of sorption (i.e., adsorption or desorption) can be developed
as follows. Suppose a mass, m, of an adsorbing solid with an initial
surface coverage of ro is introduced to a solution containing an

101
initial concentration, CQ, of adsorbate. At equilibrium, the
adsorbate concentration in solution may be expressed as
C = C + — (V-6)
o v
where Ax = the net change of adsorbate mass in solution, and
v = the solution volume.
Furthermore, conservation of mass requires that
r = r - — (v—7)
o m
The sign of Ax indicates the direction of sorption. If Ax >0, then
desorption occurs whereas adsorption is indicated if Ax <0. Assuming
that sorption is restricted to a monomolecular layer restricts the
magnitude of adsorption such that
-Ax _< m (Too -r0) (V-8)
Furthermore, the extent of desorption cannot exceed the initial surface
coverage of adsorbate
Ax _< m F0
(V-9)
With these constraints imposed, the change in equilibrium position of
and C may be determined by substituting Equations V-6 and V-7 into the
Langmuir equation
_ Ax
C + —
o v
r Ax
c +
o
br„
o
m
+
Ax
v
(V-10)

102
Since all other parameters have been previously specified according to
the initial conditions, only one variable, Ax, is unknown in the above
equation.
Rearranging terms yields a quadratic expression which may be solved
explicitly for Ax:
2
br , bC
. i 0 1
+ Ax (
-bAx
CO
O
â– ) + (r - be r + be r ) = o (v-n)
o
O <*>
O O
v m m
v
m
Depending upon CQ and T0 > no net sorption may occur. Under these
conditions, which incidentally define the equilibrium adsorbate
concentration (EAC), Ax equals zero.
From Equation V-ll, it follows that the EAC (i.e., CQ ac Ax = 0)
is theoretically related to the number of unoccupied exchange sites
r
o
(V-12)
EAC
Expressed in terras of fractional surface coverage, 4> , Equation V-12
gives
(V—13)
Thus, according to the Langmuir relationship, complete coverage of all
exchange sites (i.e., $ = 1) is mathematically unattainable. Thus,
complete surface coverage is approached asymptotically. Figure V-6
illustrates the variability of EAC as a function of for several
different values of the Langmuir adsorption constant, b. The slope of

EAC (/íMOLES/L)
103
SU RFACE COVE RAG E, § (DI M E N SI 0 N LESS)
Figure -V-6. Theoretical Relationship Between Equilibrium Adsorbate
Concentration (EAC) (umole/L) and Fractional Surface
Coverage, $, for Various Values of the Langmuir Adsorption
Constant, b (L/umole).

104
the curve is a measure of the buffering capacity of the system. More
explicitly, when surface coverage ($) is low, EAC is restricted to a
relatively narrow (and low) concentration range; as i> increases, the
ability of the adsorbate-adsorbent system to buffer against perturba¬
tions (e.g., introduction of additional solute) decreases according to
the following equation:
d EAC _ 1
d$ b (1 - $ )^
(V-14)
The relationship also illustrates the influence of b on the buffering
capabilities of the system. The slope of change in EAC relative to $ is
an inverse function of b; thus as b increases, concomitant increases in
buffering capacity result (Figure V-6). By definition, however, b is
invariant for a particular surface; a change in b thus represents a
change in system conditions (e.g., pH or temperature) or surfaces.
In using the Langmuir relationship to interpret sorption phenomena
near or below the EAC, it is critical that the initial surface coverage
T0 be accounted for. Failure to include ro renders Equation V-10
indeterminate at the EAC when no net sorption is observed.
The effect of excluding ro may be evaluated by considering a
model system comprising 1 g of adsorbent, F equal to 125 ug/g, T
oo
equal to 1,000 ug/g, b equal to 1/2625, and a solution volume of
1 liter. Table V-l summarizes calculations for C/T values for a series
of different initial adsorbate concentrations in solution, including
both values for C/r corrected for initial surface coverage, rQ (i.e.,
actual C/r , where V = T0 - Ax/m), and excluding ro (i.e., apparent C/r,

105
Table V-l. Apparent and Corrected C/F Values for a Model Adsorbate-
Adsorbent System for Varying Initial Concentrations of
Adsorbate. (See text for specification of Langmuir
constants and system dimensions.)
Initial
Concentration
(ug/L)
Ax/v
(ug/L)
Final
Concentrat ion
(ug/L)
Apparent
c/r
(g/L)
Corrected
c/r
(g/L)
0
91.4
91.4
-1.00
2.72
50
78.4
128
-1.64
2.75
100
65.6
166
-2.52
2.79
150
53.2
203
-3.82
2.83
250
28.9
279
-9.65
2.90
375
0
375
*
3.00
500
-27.5
473
17.2
3.10
1,000
-125
875
7.00
3.50
1,500
-205
1,295
6.31
3.92
2,000
-272
1,728
6.35
4.35
2,500
-328
2,172
6.63
4.80
* Indeterminate.

106
where r = _Ax/m). Plotting actual C/r versus equilibrium or final
concentrations generates the typical Langmuir plot in accordance with
Equations V-l and V-10 (Figure V-7). At an initial concentration of
0 ug/L of adsorbate, a net release of 91.4 ug adsorbate is calculated;
thus, the isotherm extends linearly from a lower limit of C/T equal to
2.72. Plotting apparent C/T versus C shows that at concentrations
above, but approaching the model becomes increasingly unstable; at
concentrations well beyond the EPC, C/r approaches its true value
asymptotically with increasing C.
Failure to correct T for initial surface coverage not only leads to
spurious values of adsorption parameters, but may result in the mis¬
interpretation of observed sorption phenomena. This is evidenced by the
work of McAllister and Logan (1978) and Green et al. (1978) who
described the phosphorus adsorption-desorption characteristics of soils
and sediments derived from the Maumee River basin in Ohio. Adsorption
was described by the Langmuir model, although the authors apparently
neglected to correct for initial surface coverage. Their results show
what the authors described as a characteristic "check-mark" shaped curve
which closely parallels the curvilinear relationship obtained by
plotting apparent C/r versus C. The authors hypothesized that the
apparently anomalous departure from linearity near the equilibrium
phosphorus concentration (EPC) reflects the strong buffering capacity of
the system against increases in phosphorus concentration. Although
buffering capacity does increase with decreasing concentration, such a
mechanism is implicitly contained within the Langmuir model and does not
result in a deviation from linearity. Similar analytical anomalies near

EQUILIBRIUM CONCENTRATION, C(,ug/L)
Figure V-7. Langmuir Plot of Apparent and Corrected C/r Versus Equilibrium Concentration for
Model Adsorbate-Adsorbent System. See Text for Specification of Langmuir Constants
and System Dimensions.

108
and below the EPC have been alluded to by Fitter and Sutton (1975) in
fitting experimental sorption isotherms to the Freundlich model.
Application of Langmuir Model to Lake Apopka
and Lake Okeechobee Sediments
Sorption by sediments from Stations A5, OlO, Oil, and 013 was well
described by the Langmuir model; correlation coefficients for the model
were highly significant (p <0.01) and exceeded r = .97 (Table V-2;
Figures V-8 and V-9). Sediments from Stations 07 and 014 showed poor
agreement between observed results and the model, particularly at
concentrations below the EPC. Both sediments showed a pronounced
tendency at low initial P concentrations to buffer sediment-water sus¬
pensions at equilibrium concentrations approximating 160 to 170 ug P/L
(Figures V-10 and V-ll). The observed deviations at low equilibrium P
levels may indicate error introduced by underestimation of the initial
(calculated) surface concentration. Adsorption maxima for Lake
Okeechobee sediments varied with sediment type, ranging from 6.5 ug/g
for Station 013, which primarily comprises quartz sands, to 229 ug/g for
the organic sediments of 014. Adsorption maxima for Lake Okeechobee
sediments were significantly correlated (r^ = .96, p <0.05) with
sediment fractional silt-clay content. With the exception of
Stations 07 and 014, the observed adsorption maxima constitute some of
the lowest values reported in the literature (Table V-3). These results
suggest that adsorption maxima are controlled by substrate type (Stumm
and Leckie 1971). Edzwald et al. (1976) determined, for example, that
solid silica has no capacity to adsorb phosphorus. Conversely, clay

109
Table V-2. Summary of Langmuir Sorption Parameters and Goodness of
Fit to Model for Lake Okeechobee and Lake Apopka Sediments
(pH 8.30) .
Sample
NAI-P
r
00
b (x 10^)
EPC*
r
P
(ug P/L)
(ug P/g)
(L/ug)
(ug P/L)
(Probability)
Oil
35
91.0
7.51
83.2
.982
<0.01
010
12
27.4
4.29
181
.982
<0.01
013
6
6.5
20.8
577
.970
<0.01
014
96
228.6
1.74
416
.719
t
07
66
irk
irk
irk
**
t
A 5
108
108.4
34.6
7,190
.999
<0.01
* Calculated,
t P > 0.05.
** Not included because of lack of significant data.

Figure V-8. Langmuir Sorption Isotherm Plot for Lake Apopka Sediment (Station A5).
110

vr (g/L)
ni
Figure V-9. Langmuir Sorption Isotherm Plot for Lake Okeechobee
Sediments (Stations 010, Oil, and 013).

80
60 •
40 â– 
Q_
o:
o
^ in
.cn Q
a: < 20
D>
Q_
o:
o
if)
CL
o:
o
LU
Q
0
20
40
60
80
0.500
EQUILIBRIUM CONCENTRATION (mg p/l)
Figure V-10. SKP Sorption by Station 07 Sediments as a Function of Final (equilibrium)
Concentration.
1.000
112

60
° 40
LU
Q
60 •
SRP Sorption by Station 014 Sediments as a Function of Final (equilibrium)
Concentration.
Figure V—11.
113

114
Table V-3. Values of Langmuir Sorption Parameters for Phosphate
Sorption by Different Substrates.
Substrate
Type/Source
b(L/ug) x 10^
Too
(ug/g)
Reference
Sao Gabriel
4.8
73
Syers et al. 1973
Cambal
4.7
224
Syers et al. 1973
Durox
10.9
990
Syers et al. 1973
Pierre Clay
0.45
255
Olsen and Watanabe 1957
Owyhee Silt Loam
0.31
136.3
Olsen and Watanabe 1957
Calcite
—
9.17
Griffin and Jurinak 1973
Lake Warner
1.06
1,125
Ku et al. 1974
Lake Wyola
1.26
1,209
Ku et al. 1974
Maumee River Basin
0.68-
1.55
222-
4,870
McAllister and Logan 1978
Menominee River
2.15
282
Holm et al. 1979
Kaolinite #3
—
91
Edzwald et al. 1976
MontmorilIonite #21
—
746
Edzwald et al. 1976
11 lite #36
—
2,510
Edzwald et al. 1976

115
substrates, and in particular 2:1 clays, have relatively high adsorptive
capacities for phosphorus (e.g., Edzwald et al. 1976, Table V-3).
Since particle surface area increases with decreasing particle
size, a conjugate interpretation of these results is that the extent of
adsorption is controlled by particle surface area. Olsen and Watanabe
(1957) previously demonstrated the functional dependency of adsorption
maximum on surface area for both acidic and alkaline soils.
Krom and Berner (1980b) evaluated P adsorption in anoxic estuarine
sediments in terms of a simple distribution coefficient, K*, that
relates adsorption to equilibrium concentration in a linear fashion:
T = K* C (V—15)
This equation can be derived from the Langmuir model under the limiting
condition that the inverse of the adsorption constant, 1/b, greatly
exceeds C. If this condition is not met, then K* will vary inversely
with increasing concentration. An advantage of Equation V-15 is that it
provides direct information about sediment buffering capacity; i.e., the
greater the distribution coefficient, the more pronounced the affinity
of the substrate for the solute. Distribution coefficients for a
variety of substrate types were summarized by Krom and Berner (see
Table V-4). The relatively high adsorption energies of Lake Okeechobee
and Lake Apopka sediments precludes use of Equation V-15, except at
extremely low concentrations. K* decreases with increasing C because
the high energy binding sites quickly become saturated. An example of
the decrease in K* (and thus in buffer capacity) with increasing C is
presented for Lake Okeechobee Station 013 in Figure V-12.

116
Table V-4. Values of Mass of Phosphate Adsorbed/Equilibrium Concentration
for Different Substrates. (Data should be used for approximate
comparisons. Experimental conditions varied between studies
and often data have been fitted to nonlinear isotherms.)
Substrate
Oxygen Status
in Experiment
K*
Reference
Iron oxide (goethite)
ox ic
3,000
Hingston et al. 1974
11 lite
oxic
230
Edzwald et al. 1976
MontmorilIonite
oxic
100
Edzwald et al. 1976
Kaolinite
oxic
20
Edzwald et al. 1976
Calcium carbonate
oxic
20
Cole et al. 1953
(syn. powder)
Calcite
oxic
10
de Kanel and Morse 1978
Aragonite
oxic
175
de Kanel and Morse 1978
Soil
oxic
15-70
Olsen and Watanabe 1957
Soils
oxic and anoxic
80
Khalid et al. 1977
Lake sediment
oxic
40
Hwang et al. 1976
Lake sediment
Calcareous
oxic
35
Li et al. 1972
Noncalcareous
oxic
25-35
Li et al. 1972
Lake Sediment
Calcareous
anoxic
6
Li et al. 1972
Noncalcareous
anoxic
1-5
Li et al. 1972
Estuarine sediment
oxic
3,750
Jitts 1959
Estuarine sediment
oxic
250
Pomeroy et al . 1965
Estuarine sediment
oxic
50
Krom and Berner 1980a
Oceanic sediment
oxic
500-5,000
Berner 1973
Estuarine sediment
anoxic
1
Krom and Berner 1980b
Source: Krom and Berner 1980a.

0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900
EQUILIBRIUM CONCENTRATION (mg p/|_)
Figure V-12. Variation of the Phosphorus Mass Distribution Coefficient, K*, Related to
Equilibrium Concentration Levels of Phosphorus for Station 013 Sediments.
117

lis
Equilibrium Phosphorus Concentration (EPC)
By definition, the EPC indicates the direction of sorption response
when a sediment particle is resuspended and brought into contact with
the overlying water column. Release or uptake occurs as the solid phase
equilibrates with the aqueous phase (which generally has a dissolved
phosphorus level different from that of the interstitial solution with
which the sediment particle had originally equilibrated). EPC values
observed for Lake Okeechobee sediments were rather variable (Table V-5),
ranging from 80 ug P/L to 520 ug P/L. Mean annual SRP concentrations at
various locations throughout Lake Okeechobee between 1973 and 1979 never
exceeded 58 ug P/L (Federico et al. 1981); thus Lake Okeechobee
sediments will function as a nutrient source if resuspended. EPC levels
for Lake Okeechobee sediments correspond to EPCs observed for other
Florida lakes; for example, Nisson (1975) observed EPC values ranging
from 80 to 2,000 ug P/L for Lakes Harney, Jessup, and Monroe. EPC
values as low as 2 ug P/L have been reported by Meyer (1979) for Bear
Brook in the Hubbard Brook watershed. McAllister and Logan (1978) and
Mayer and Gloss (1980) also observed somewhat lower EPC levels than
those found for Florida lakes. Reported values for bottom sediments
ranged respectively from 24 to 54 ug P/L (Maumee River, Ohio) and 15 and
70 ug P/L (Colorado River, Utah).
EPC for the Lake Apopka sediment was quite variable and difficult
to define accurately. Figure V-5, which represents the synthesis of two
separate adsorption-desorption experiments, indicates EPC values of
470 ug P/L and approximately 2,000 ug P/L. Between 1977 and 1980,
lakewide monthly average SRP concentrations for Lake Apopka were always

119
Table V-5. Observed and Calculated EPC (ug P/L) Values for Lake
Okeechobee and Lake Apopka Sediments (pH 8.30).
Station

Observed EPC
(ug P/L)
Calculated EPC
(ug/L)
07
*
225
*
010
0.438
217
181
Oil
0.385
80
83
013
0.923
520
577
014
0.420
230
417
A5
0.996
470-2,000
7,190
* Not calculated.

120
below 200 ug P/L; thus the net direction of sorption upon sediment
resuspension will be release of SRP to the water column. Although the
broad disparity between measured EPC values was initially surprising,
the observed variance is consistent with the inherent decrease in
buffering capacity predicted by the Langmuir model as surface coverage
approaches complete saturation. As described previously (Equation V-13),
EPC is theoretically an inverse function of the number of exchange sites
unoccupied by phosphate anions.
Using this relationship described by Equation V-13, predicted EPC
as a function of fractional surface coverage for Station A5 sediment is
plotted in Figure V-13. Initial surface coverage was estimated at
108 ug P/g compared with an adsorption maximum of 108.4 ug P/g
(Table V-2); $ consequently assumes a value of 0.99b and EPC is pre¬
dicted to be 7,200 ug P/L. As evidenced by Figure V-13, EPC is highly
sensitive in this range to small changes in surface coverage (and hence
í>); for example, a decrease in T of only 2 ug P/g yields $ equal to
0.978 and a predicted EPC of 1,280 ug P/L. A further reduction in T to
102 ug/g results in a predicted EPC of 461 ug P/L. Because the capacity
of these sediments to further adsorb phosphorus is virtually exhausted,
interstitial water concentrations of SRP are highly variable and can
reach quite elevated levels. Above i> equal to approximately 0.85,
larger increases in SRP concentrations are required to effect even a
small increase in T. Thus, despite their limited ability to buffer
against increases in interstitial SRP concentrations, the sediments
exert a profound influence in maintaining SRP concentrations at high
levels. For example, the minimum equilibrium SRP concentration in the

121
SURFACE COVERAGE, $ (DIMENSIONLESS)
Figure V-13. Theoretical Relationship Between Fractional Surface
Coverage ($) and EPC for Station A3 Sediments.

122
pore fluid of Station A5 sediments can be calculated as follows. The
sediments are characterized by a particulate density of 1.935 g/cm^
and an average water content of 96.5 percent. Assuming that the
interstitial fluid has no dissolved phosphorus as an initial condition
(a situation which is approximated when overlying water of low SRP
concentration is buried along with newly deposited sediment particles),
application of Equation V—11 predicts an equiliorium interstitial fluid
SRP concentration of 233 ug P/L when T0 equals 108 ug P/g.
Regression analysis was performed to determine whether EPC is a
linear function of labile inorganic P content (NAI-P), and results
indicate that these two parameters are not significantly correlated
(r^ = .14, p >0.05). McAllister and Logan (1978) had a similar lack
of success in correlating Bray PI "available" P with EPC in sediments
from the Maumee River. The lack of correlation between EPC and NAI-P
directly follows from the theoretical relationship between EPC and $
described by Equation V-13. The relationship is valid if the model is
restricted to a single sediment and if all binding sites are
energetically homogeneous (i.e., b is invariant). Because the adsorp¬
tion constant (b) varies with sediment type, a simple correlation
between and EPC for different sediments is not likely to exist.
Indeed, such a correlation for Lake Okeechobee sediments yielded a
rather poor relationship (r^ = .52). Green et al. (1978) and
McAllister and Logan (1978) empirically demonstrated a negative
correlation between EPC and adsorption energy; similarly, Holford (1978)
indicated that ease of desorption (and implicitly EPC) is inversely
related to the binding energy.

123
Desorption
The greatest magnitude of desorption from Lake Okeechobee sediments
was observed for sediments from Stations 07 and 014. At initial aqueous
concentrations of 0 ug P/L, these two sediments released 52.9 ug P/g and
52.6 ug P/g sediment, respectively (corrected for interstitial P
content). Release from other Lake Okeechobee sediments was substan¬
tially lower under the same initial conditions and ranged from
0.8 ug P/g (Station 013) to 8.1 ug P/g (Station Oil). Desorption from
Lake Apopka was relatively moderate, with an average observed release of
25.4 ug P/g. Williams et al. (1970) observed that sediments with the
highest adsorptive capacity usually release the least P in subsequent
desorption experiments. The converse is true for the sediments
evaluated in this study; the most pronounced release was observed in the
sediments with the greatest adsorptive capacities (Stations A5, 07, and
014). The results of Williams et al. (1970) cannot be extrapolated
directly to Lake Okeechobee sediments because of the broad disparity in
sorptive characteristics between the major sediment types.
A more useful parameter indicative of the ease of desorption is
obtained from the fraction of the initial surface coverage (NAI-P) that
can be readily desorbed. Release of surface sorbed P is more easily
induced from Stations 07 and 014 sediments than from other sediments
(Table V-6). A comparison of Langmuir binding constants and fractional
desorption suggests that release is inversely related to bonding energy
(Figure V-14). This relationship may be described by the following
empirically derived model (r = .815):

124
Table V-6. Desorption Characteristics of Lake Okeechobee and Lake
Apopka Sediments in Buffered (pH 8.30), Phosphorus-Free
Water.
Sample
NAI-P
(ug P/g)
Release
(ug P/g)
Frac tional
Release
(dimensionless)
b(L/ug) x 10^
07
65
52.9
0.81
*
010
12
4.0
0.33
4.59
Oil
35
8.1
0.23
7.51
013
6.0
0.8
0.13
20.9
014
96
52.6
0.55
1.74
A5
108
25.4
0.24
34.6
* Not calculated.

Figure V-14. Relationship Between Fractional Desorption, 3, and Langmuir Binding Constant, b,
for Lake Okeechobee (•) and Lake Apopka (■ ) Sediments.

126
S = 0.0472 b“0-361 (V—16 )
where S = the fraction of labile P desorbed.
These results concur with those of Holford and Mattingly (1976) and
McAllister and Logan (1978), who demonstrated an inverse relationship
between the ease of desorption of labile P and bonding energy. Desorbed
P probably represents low-energy, electrostatically-bound P, as opposed
to specifically sorbed P (Hingston et al. 1976; Obihara and Russel 1972;
and Kuo and Lotse 1974). Williams et al. (1970) found that calcareous
sediments generally release greater fractional quantities of adsorbed P
than do noncalcareous sediments; furthermore, the adsorption data of
Holford and Mattingly (1975) indicate that carbonate particles have
lower bonding energy sites than do iron hydrous oxides.
For sediments characterized by both relatively high water content
and interstitial SRP concentrations, entrainment of pore fluid during
sediment resuspension complements desorption as a significant mechanism
of phosphorus release. For example, at an initial aqueous concentration
of 0 ug P/L, net release from Station A5 sediment averaged 36.5 ug P/g;
of this total, 30 percent was directly the result of entrained pore
fluid. In Lake Okeechobee sediments, which are generally characterized
by lower sedimentary water contents and lower pore fluid concentrations
of SRP (Table III-l), this mechanism is for the most part insignificant.
Station 014 sediments are somewhat anomalous with respect to this
general rule; even so, entrainment of pore fluid accounts for less than
12 percent of the observed total release.

127
Effect of pH on P Sorption by Lake Okeechobee and Lake Apopka Sediments
An experiment was conducted to evaluate the influence of pH on
sorption phenomena. Adsorption-desorption isotherms were conducted on
sediment from Station 014 at pH 7.20, 8.30, and pH 9.30 and from
Station A5 at pH 7.10, 8.30, and 9.30 to bracket the observed vari¬
ability of pH in Lakes Okeechobee and Apopka. The results (Figures V-15
and V-15a) indicate that minimum desorption for both sediments occurs at
pH 8.3; pH shifts in either direction enhance desorption. Increased
desorption at pH 7.10 to 7.20 probably reflects dissolution of
hydroxyapatite (CajQÍPO^^ÍOH^) or other calcium phosphate solid phases.
This mechanism does not explain the increase of P release at pH 9.30.
Specific adsorption of P is inhibited by increasing hydroxide
concentration (Atkinson et al. 1974; Lijklema 1977 and 1980; Hingston
et al. 1967); consequently, the functional response of desorption with
varying pH in Lake Okeechobee and Lake Apopka sediments appears to
represent a compromise between the decreasing solubility of hydroxy¬
apatite (or other Ca-P phases) with increasing pH and enhanced
competition of hydroxide ions for exchange sites.
The variability of EPC with pH is more complex and difficult to
interpret, with different trends observed for the two sediments. As
seen in Figure V-15b, the distribution of EPC with pH for Station A5
sediment is precisely the inverse of the observed change in desorption;
EPC shows a pronounced maximum (470 to 2,000 ug P/L) at pH 8.3
contrasted with minimal desorption of the same pH. These results are
contrary to a priori expectations; that is for sediments that conform to
the Langmuir model, implicit in an increase in EPC is greater ease of

EPC (mg P/L) DESORPTION (,ag P/g)
128
Figure V-15. Phosphorus Desorption (ug P/g) (A) and EPC (mg P/L) (B)
as a Function of pH for Station A5 (•) and 014 ( ®)
Sediments.

129
desorption. This follows from Equation V-13, which shows that increases
in EPC are a direct consequence of increases in the extent of relative
surface coverage $. In addition, Equation V—11 demonstrates that the
magnitude of desorption increases with increasing ro (implicitly $)
and/or decreasing b. Comparison of calculated Langmuir constants for
Station A5 sediment at different pH shows that the adsorption b at
pH 7.10 is reduced by more than 50 percent relative to pH 8.3
(Table V-7). However, the adsorption maximum, T , is substantially
higher at pH 7.10, resulting in a predicted EPC of 174 ug P/L. This
predicted value agrees rather well with the observed EPC of 150 ug P/L.
Furthermore, the increase in agrees conceptually with results of
Hingston et al. (1967), who demonstrated that the adsorption maximum of
a particular surface for polybasic acids such as phosphoric acid
increases with decreasing pH. This effect is attributed to a change in
speciation of the anion resulting in less development of a negative
change by the surface upon specific adsorption of the anion. The net
result is, therefore, an effective increase in the anion exchange
capacity of the sediment. The observed decrease in EPC at pH 9.30
(relative to pH 8.30) can not be explained in terms of the Langmuir
model and, as evidenced by the poorer fit of the Langmuir model to the
experimental data (r^ = .729, Table V-7), probably represents
heterogeneous nucleation with calcite or precipitation as hydroxyapatite
(Stumm and Leckie 1971; Griffin and Jurinak 1974).
EPC variability with pH for Station 014 sediment parallels the
variability of maximum desorption (Figure V—15). The minimum EPC
(230 ug P/L) was observed at pH 8.30; shifts in pH in either direction

130
Table V-7. Calculated Langmuir Constants for Station A5 Sediment
as a Function of pH.
pH
^ (ug P/g)
b(L/ug) x 10^
r2
7.10
149.9
14.8
.972
8.30
108.4
34.6
.999
9.30
108.2
8.51
.729

131
result in corresponding increases in EPC. The lack of fit of the
sorption data to the Langmuir model precludes its use in interpreting
the results and suggests that other mechanisms in addition to sorption
contribute to the observed results.
Development of a Sediment Resuspension Phosphorus Release Model
for Lake Okeechobee and Lake Apopka
Nutrient Release Submodel
From the Langmuir relationship, a nutrient release submodel
applicable to sediment resuspension can be developed to predict P
release and equilibrium P concentrations as a function of initial
conditions. From mass-balance considerations, the equilibrium
concentration of phosphorus in a sediment-water suspension, C, is
equivalent to
C = C + ^ (V-6)
o V
where CQ = initial concentration (ug/L) in the water column,
Ax = amount of P desorbed (ug), and
V = volume of lake water under consideration (L).
Since CQ Can be specified, the solution to this equation requires
the determination of Ax, which in turn may be solved from the following
quadratic expression derived previously from the Langmuir model:
-bAx2
msV
+ Ax
bC
o
) + (r
o
bCor°
+ be r )
O O
0 (V-ll)
where ms
- amount of sediment resuspended (g).

132
If the concentration of resuspended sediment is known, the derived
Langmuir coefficients and this information can be used to predict the
extent of release and hence, the final (equilibrium) aqueous phase
concentration.
Sorption behavior observed in sediments from Lake Apopka
(Station A5) and Stations 010, Oil, and 013 from Lake Okeechobee agreed
extremely well with the Langmuir model (Table V-2); consequently, the
above model can be employed to predict equilibrium aqueous phase P
concentrations resulting from the resuspension of these sediments. The
poor fit of the Langmuir model to data for Stations 07 and 014, however,
necessitates the use of a different model. The curvilinear response of
sorption to increasing initial concentration for these sediments
precludes the application of linear models; consequently, a series of
nonlinear empirical models were evaluated through least squares analysis
for adequacy of fit. Sorption data for Station 014 sediments were found
to closely fit (r^ = .989; p <0.01) a logarithmic model:
— = -120.1 + 22.2 In C (V-17)
ms o
An equivalent model applied to sorption data for Station 07 sediments
yielded a poorer fit (r^ = .865), but these data were described
adequately by the following quadratic model (r^ = .976; p <0.01):
— = -43.6 + 0.221 C - 9.10 x 10”5 C 2
ms o o
(V-18)

133
Because of the characteristics of a quadratic curve, use of this model
at initial concentrations approaching and exceeding 1,200 ug P/L will
introduce significant error. A potential source of error in this
approach is that sorption is assumed to be independent of the solids/
solution ratio; however, Hope and Syers (1976) concluded that this error
is not significant if equilibrium is established.
Sediment Resuspension Model
The oscillatory motion of water under wave action generates a
stress, xq, at the sediment-water interface that will induce scour
or resuspension if a critical threshold stress, tc, is exceeded.
Once particle motion has been initiated from the sediment surface, the
rate of erosion is a function of the wave-induced stress in excess of
the critical shear stress (Sternberg 1972; Sheng and Lick 1979; Fukuda
1978); for example
J = fct(Ax)
(V-19)
where Ax = xQ ~ xc, and
J = the flux of sediment.
The magnitude of xQ depends on prevailing wind conditions and
associated wave parameters; xc depends on the sediment physical
characteristics. Development of a physical sediment resuspension model
thus can be viewed as comprising three distinct subunits: (1) deter¬
mination of wind-induced stress, (2) evaluation of the critical stress
for a particular substrate type, and (3) prediction of the sediment

134
flux. Once developed, Che model can be interfaced with a
nutrient-release submodel to predict water column phosphorus concentra¬
tions (Figure V-16). The following sections trace the sequential
development of the integrated sediment resuspension model for Lake
Okeechobee and Lake Apopka sediments. As a final phase of the
discussion, sediment resuspension rates and net nutrient release are
calculated for several different wind events.
Calculation of Wind-Induced Stress at the Sediment-Water Interface
Surface waves produce an oscillatory motion in the water column
that may extend to the bottom in shallow lakes to produce an oscillating
horizontal motion. The maximum horizontal velocity at the edge of the
bottom boundary layer generated by this periodic motion can be
approximated from linear (Airy) wave theory by
ttH
s
Um Ts sinh (2Ttd/L^)
where u^ - the maximum horizontal velocity;
Hs - the significant wave height;
Tg = the significant wave period;
d = the local depth; and
L^ = the wavelength for depth, d.
The wavelength L¿ may be solved iteratively from
La = L tanh (——)
Ld
wi th
(V-20)
(V— 21)

135
Figure V-16.
Schematic Diagram of Se
Integrated Sediment Dis
quential Applicat
persion-Nutrient
ion of
Release
the
Mode 1

136
(V-22)
where L = the wavelength in deep water, and
g = gravitational acceleration.
In the absence of empirical determinations, the significant wave height,
Hs, and significant wave period, Ts, at a particular location may be
estimated by the Sverdrup-Munk-Bretschneider (SMB) wave hindcasting
method (U.S. Army Coastal Engineering Research Center 1977) for shallow
water. Input parameters to the model include wind speed and duration,
fetch length, and average depth over the fetch. Sheng and Lick (1979)
recently compared the predicted results of this and several other
hindcasting models to measured wave data from Lake Erie and found the
SMB shallow water model to be superior to the other models.
The bottom shear stress generated by the periodic motion of the
waves can be expressed as
= 1/2 P f^2
(V-23)
where P = the density of the fluid medium, and
fT, = the dimensionless wave friction factor,
w
Riedel and Kamphuis (1973) have shown that fw is a function of the
wave Reynolds number, Rg, and bottom roughness. Similarly Jonsson
(1967) developed the following expression for f for laminar flow:
(V-24)

137
where dQ = the orbital amplitude, and
v = the kinematic viscosity of water.
Since the wave Reynolds number can be expressed as
(V-25)
Equation V-24 reduces to (Madsen and Grant 1975)
fw = 2 (Re)
-1/2
(V-26)
For fully developed turbulent flow, fw is independent of Re (Riedel
and Kamphuis 1973; Sheng and Lick 1979). The graphical method of Riedel
and Kamphuis (1973) was used in formulating the present model to
evaluate fw as a function of hydrodynamic conditions.
Determination of Critical Shear Stress
Sediment motion in response to a shear stress at the sediment-water
interface is not initiated until the stress reaches a critical threshold
level. Development of threshold scour criteria for unidirectional flow
conditions, which has been of interest for a considerable time, has
focused on the use of nondimensional Shields diagrams. The scour
criterion established by Shields (1936) expresses the relative
(dimensionless) threshold stress, 0t, as a function of the particle or
shear Reynolds number, viz.
u*D
©t = fct (■
v
)
( V—2 7)

where
i 38
0t (p - p)g D
(V-28)
where D = the particle diameter, and
u* = the critical shear velocity.
The relationship between 9t and the particle Reynolds number has
been determined empirically and is presented in Figure V-17. The
Shields curve closely resembles Hjulstrom's (1935) empirical model for
particle erosion and deposition in riverine systems as a function of
average flow, U, and particle size (Figure V-18), and has been verified
experimentally for a variety of inorganic sediments (cf. Graf 1971).
Furthermore, recent studies conducted by Fisher et al. (1979) demon¬
strated that organic detrital sediments also conform to the Shields
relationship for the initiation of motion.
Comparable studies of the threshold criteria for sediments under
wave-induced oscillatory flow are rather limited. Komar and Miller
(1973) contended that application of the Shields criterion to define
incipient motion of sediment under oscillatory flow can lead to
considerable error. Using experimental data of Bagnold (1946), these
authors demonstrated substantial scatter for a plot of a modified
Shields threshold criterion, 9¿, versus sediment grain diameter.
The threshold parameter 9¿ was defined by the following
dimensionless expression:
pu
ft' = m
(p - p )g D
(V-29)

Fully developed
turbulent
velocity profile
Sym
Description
r, ,9/cm3
o
•
•
•
•
>
♦
•
Amber |
ST.
Bonte J
Sand (Cosey )
Sand (Kramer)
Sond (US WE S)
Sand (Gilbert)
t 06
1 27
2 7
4 25
2 65
2 65
2 65
2 65
Turbulent
boundary
layer
Sand (Vononi)
Glass beads (Vononi)
Sand (White )
Sand in air (White)
Steel shot (White I
2 65
2 49
2 61
2 tO
7 9
Figure V-17. Shields' Diagram Depicting Dimensionless Critical Shear Stress as a Function of
Shear Reynolds Number.
139

cm/sec
Erosion-Deposition Criteria for Uniform Particles.
Figure V-18.
From Hjulstrom (1935)
500

141
Variability was shown to be related to the wave period, T; i.e. for a
particular sediment grain, transport is initiated at a lower orbital
velocity for waves of short period than for waves of long period. This
relationship, which has been verified for fine- and medium-sized
sediments by Hammond and Collins (1979), can be anticipated from simple
wave mechanics which define the orbital velocity as an inverse function
of T:
(V-30)
For sediment particles less than 500 um in diameter, Komar and Miller
(1975) concluded that the threshold point is best related by the
following empirical equation:
©t = 0.21 (-jp)
1/2
(V-31)
A similar relationship has been developed by Sternberg and Larsen (1975)
for unconsolidated silt-sized sediment:
p u*
1/10
(P s “ P )g D
= 0.13
1/2
(V-32)
where uj/jq “ the higher 10 percent of the bottom orbital velocities.
Madsen and Grant (1975) argued that the scatter of Bagnold's (1946)
data observed by Komar and Miller (1973) does not necessarily imply the
failure of the Shields criterion to define threshold conditions under
oscillatory flow. An important difference between Komar and Miller's

142
0£ and the Shields' criterion (©t) is round in the numerator of the
two expressions: 0t is a linear function of the critical stress,
Tc (Equation V-28), while 0¿ is a function of fluid density and orbital
velocity squared (Equation V-29). For boundary layer flows, applied
stress at the bottom is proportional to the square of the near bottom
orbital velocity:
To = 1/2 fwP'^m2 (V-23)
Thus, the difference between 0t and 0¿ is simply the exclusion
of the wave friction factor in 0¿ (Komar and Miller 1975; Madsen
and Grant 1975). Solution of Equation V-30 for dQ and substitution
of the result into Equation V-24 shows that fw is inversely related to
the square root of the wave period (T). This explains the finding of
Komar and Miller that 0¿ varies as a function of T for any given
sediment. Correspondingly, Madsen and Grant (1975) obtained good
agreement between Bagnold's (1946) threshold data and the Shields
criterion using the wave friction diagram of Jonsson (1967) to determine
fw- As a result, the authors concluded that the Shields function is
quite adequate as a general criterion for the threshold of sediment
movement subjected to oscillatory flow. Komar and Miller (1975)
independently arrived at the same conclusion.
Calculation of Sediment Resuspension Rates
Several models have been developed to describe sediment
resuspension rates as a function of bottom orbital velocities or shear
forces. The following empirical expression was developed by Lam and
Jaquet (1976) f or Lake Erie sediments:

143
P
J = Kr (r |-r) AU (V-33)
M s M
where J = the upward (areal) flux of resuspended sediment,
Kr = a dimensionless proportionality constant,
A = ua/ucr with ua being a reference velocity equal to 1 cm/s
and ucr is the critical (orbital) velocity, and
u = “m “ Re¬
settling velocity and the regeneration coefficient (Kr) were selected
heuristical ly with J equal to 0.25 m/d and K^. equal to 6.4 x 10“^.
The structure of this model is quite similar to the expression for
bedload transport developed by Sternberg (1972).
Resuspension of western Lake Erie sediments was studied by Sheng
and Lick (1979) using an annular flume. From the results of their
study, Sheng and Lick developed an empirical physical resuspension model
that relates the erosion rate as a two-region linear function of the
bottom shear stress:
J=cl (tq - 0.5) T0 2 dynes/cm^
J = C2 (T0 " 1.515) tq > 2 dynes/cm^
where c^ - 1.33 x 10-^ s/cm,
C2 = 4.12 x 10~b s/cm,
tq = dynes/cm^ (g/cm/s), and
J = the sediment flux in g/cm^/s.
(V-34)
(V-35)
More extensive studies were conducted by Fukuda (1978) who studied
both critical shear stress and entrainment rates for several different
substrate types. Fukuda (1978) demonstrated that the rate of

144
entrainment was dependent not only on the applied stress, but on the
water content of the sediment as well; i.e., the rate of entrainment
increased both with increasing water content and applied stress. Using
sets of data for which tc> toJ percent water (W), and entrainment
rate determinations are available, Fukuda's (1978) results for the
central and western basins of Lake Erie can be fit through regression
analysis to a multiple linear model:
J = -1.30 x 10-5 + 1.66 x 10_6(At) + 1.87 x 1CT7 (W) (V-36)
This model is similar in nature to the empirical model developed by
Sheng and Lick (1979), but incorporates sedimentary water content to
describe the effect of compaction on resuspension rates (Migniot 1968).
The effects of sediment grain size and density are implicitly included
in the term At.
Sediment Dispersion Model
The distribution of sediment particles in suspension due to
turbulence is described by the equation for conservation of mass (Graf
1971; Sheng and Lick 1979). In three dimensions, the mass transport
equation for the concentration of suspended sediment is
3m 3(uM)
31 3 t
_3_
3x
(Dv
3M*
9 x
3 (vM) 3 Kw + ws) M
+ —: +■
3 dy
3 3M, 3 , 3M.
+ — (Dh —; + — (D„ —)
dy 11 dy
9 z
3z '^v 3z-
+ S
(V-37)
where M = the concentration of sediment particles;
x and y = the horizontal coordinates;

145
z = the vertical coordinate (positive upwards);
t = time;
u and v = the velocities in the x and y directions, repectively;
w = the velocity of sediment (fluid) particles in the z
direct ion;
ws = the settling velocity of sediment particles relative
to the fluid;
and Dv = the horizontal and vertical eddy diffusivities,
respectively; and
S = a source term.
The left-hand portion of Equation V-37 represents unsteady and
convective components, while the right-hand side represents the
turbulent diffusion of suspended sediment and the presence of sources or
sinks in the water column.
Of primary interest in this analysis is the vertical distribution
of suspended sediment under either steady-state or nonsteady-state
condition. Assuming that the horizontal concentration gradients 3M/ax
and 3M/3y are zero reduces Equation V-37 to a one-dimensional problem.
Furthermore, the source term, S, for sediment generation from within the
water column (e.g., derived from algal production and decay) can be
assumed to be negligible within the timeframe of interest. Under
conditions of oscillatory flow, w has both positive and negative
components equating to a net velocity of zero. Finally, the vertical
eddy diffusivity, Dv, is assumed to be constant throughout the water
column. Under these conditions, Equation V-37 reduces to
wc
3M
3 z
(V-38)

146
Several boundary conditions must be imposed if Equation V-38 is to
be solvable. At the water surface (z = h), mass balance considerations
preclude the net vertical flux of sediment particles across the inter¬
face, i.e., the turbulent flux of material must balance the convective
flux due to settling (Sheng and Lick 1979):
(V-39)
A second boundary condition can be written at the sediment-water
interface (z = 0) as follows. Under conditions when sediment
entrainment occurs, the net vertical flux of sediment due to settling
and turbulent diffusion is balanced by the net erosion rate:
(V-40)
Dv T w_M = 6M - J
v dz s
where 3 = a proportionality constant with units of velocity; and
J = the resuspension rate of bottom sediments due to shear
stresses generated by waves and currents, as defined in the
previous section (Fukuda 1978; Sheng and Lick 1979).
Fukuda (1978) refers to 3 as the reflectivity parameter; 3 is inversely
related to the fraction of sediment particles reaching the sediment-
water interface that are reflected from the interface, and it can range
from zero to infinity. As 3 approaches infinity, an increasing fraction
of particles striking the interface are deposited.
A simplified solution for the time-dependent average suspended
sediment concentration in the water column can be formulated by
integrating both sides of Equation V-38 with respect to z:

147
h
(V-41)
which may be rewritten as
wsM
h
o
(V-42)
The average concentration, M, in the water column at a particular
time may be defined as
(V-43)
Substituting M(t) in Equation V-42 gives
, 3M „ 3M
h 3t °v 3z
wsM
(V-44)
The boundary condition at z = h requires that
3M
Dv 3z wsM
(V-39)
while at the sediment-water interface (z = 0)
Dv7T= (Mws) M - J
3M
9z
(V-45)

148
Application of the boundary conditions to Equation V-44 yields
(V-46)
In other words, the change in average concentration in the water column
at particular time t is simply the difference between the mass erosion
rate and the net output at the sediment-water interface corrected for
the volume defined by h. During steady-state conditions, the concen¬
tration at any location within the water column may be derived from
Equation V-38 to give
(V-47)
where MQ = concentration at the sediment-water interface (2 = 0).
Integrating with respect to z gives the average steady-state concentra¬
tion as a function of Mq
h
h
(V-48)
o
o
Equation V-48 can be simplified to
(V-49)
Substituting for MQ in Equation V-46 gives a first order differential
equation for M as a function of t.

149
(V-50)
where e = J/h, and
1
The time dependent solution for R is thus
(V-51)
It is important to realize that Equation V-51 represents a simplified
solution and is strictly valid only during steady-state conditions.
Under transient or unsteady-state conditions, the concentration profile
of suspended sediment deviates from the exponential relationship given
by Equation V-47. This error increases as the timeframe of interest
becomes increasingly smaller, i.e., as t -> 0. In other words, if the
time scale t is such that
(V-52)
and
(V—5 3)
the error is negligible.
Under conditions generally found in Lake Apopka and Lake Okeechobee,
however, the magnitude of this error is relatively small for timeframes
of the order of 24 hours.

150
Application of the Integrated Model to Lake Apopka and Lake Okeechobee
The contribution of sediment resuspension to nutrient release in
Lake Apopka and Lake Okeechobee can be evaluated by applying the
integrated sediment dispersion-nutrient release model developed in the
previous sections for a set of specified conditions. Sequentially this
process (Figure V-16) may be summarized as follows. From wind velocity
and direction data, the shear stress at the sediment-water interface can
be calculated for a particular locus as a function of both fetch and
depth of the water column. The critical shear stress required to
initiate sediment resuspension is calculated using the method of Komar
and Miller (1973) to determine the threshold velocity as a function of
sediment grain size and density (Equations V-29 and V-31), while the
graphical method of Riedel and Kamphuis (1973) is used to determine the
wave friction factor, fw, as a function of the predicted wave
characteristics. The resulting excess shear stress coupled with the
water content of the sediment defines the rate of sediment erosion or
upward flux, J (Equation V-36).
The time-dependent average suspended sediment concentration, M,
is then approximated using Equation V-51. The vertical eddy diffusivity
may be calculated from the following relationship between Dv and the
wind stress, tw (Sheng and Lick 1979):
Dv = 9.01 + 8.77|tw| (V-54)
where tw is given in dyne/cm^ and Dv is expressed in cm^/s.

151
The wind stress tw is related to the wind velocity by the quadratic
stress law:
tw = cdp a |viv (V—55 )
where pa = the air density, and
c^ = the drag coefficient.
The value of c^ is rather variable; Csanady (1978), for example,
indicates that c¿ typically assumed a value of 1.6 x 10-^ while
Halfon and Lam (1978) reported an average value of 2.4 x lO-^ f0r the
Great Lakes. For the purposes of this model, a value of 1.6 x 10“3
was assumed for c¿. Calculated suspended sediment concentrations
are then input to the nutrient release submodel (Equation V — 11) to
determine the final phosphorus concentration in the water column.
Sediment resuspension was calculated for different locations in
Lake Apopka and Lake Okeechobee for which corresponding sorption
isotherm data were available for a series of several different wind
events. Resuspension and phosphorus release also were modeled at
Station A8 under the assumption that sorption characteristics of these
sediments are similar to Station A5 sediment. Three different wind
velocity events were considered: 6.7 m/s (15 mph), 8.9 m/s (20 mph),
and 11.2 m/s (25 mph). Wind intensity was assumed to be constant over a
period of 24 hours. Winds were considered to originate from the north
for Lake Apopka, corresponding to the prevailing direction generated by
sustained frontal activity during the winter months (Fernald 1981).
Winds in southern peninsular Florida are typically from the east

152
(Fernald 1981); consequently, an easterly wind was considered for Lake
Okeechobee in this analysis.
The counteracting influence of gravitational settling on
resuspension was calculated using settling velocities of 0.050 and
0.047 cra/s for Lakes Apopka and Okeechobee, respectively; these values
were determined from settling tube experiments for Station A5 and 014
sediments. The value of the reflectivity parameter, g, was assumed to
be 0.008 cm/s and corresponds to the value reported by Sheng and Lick
(1979) for a series of different sediment types and experimental
conditions.
Net release or uptake of phosphorus was calculated from the
predicted average suspended sediment concentration at t equals 24 hours.
Initial water column SRP concentrations imposed on the model correspond
to average values reported in the literature; for Lake Okeechobee a
value of 45 ug P/L was used, which corresponds to the average
concentration observed in 1979 (Federico et al. 1981), while Brezonik
et al. (1981) reported an average SRP concentration in Lake Apopka of
50 ug P/L for the period 1977 through 1980.
Predicted suspended sediment and equilibrium phosphorus concentra¬
tions are presented in Tables V-8 through V-10. In Lake Okeechobee,
sufficient wave energy to initiate sediment resuspension occurred at
Stations 010 and 013 during the lowest velocity wind event modeled
(Table V-8). Under these conditions an average suspended sediment
concentration of 0.381 g/L is predicted at Station 010; this equates to
a net release of only 2.8 ug P/L and a final phosphorus concentration of
48 ug P/L. Although the effective fetch at Station 013 is nearly a

153
Table V-8. Summary of Predicted Wave Statistics, Sediment Resuspension Rates,
and Final Concentration of SRP at Selected Stations in Lake
Okeechobee and Lake Apopka Assuming an Initial Concentration of
45 and 50 ug P/L, Respectively, in the Overlying Water. Wind
Velocity is 6.7 m/s and Originates from the East for Lake
Okeechobee and from the North for Lake Apopka.
Station
Parameter
07
010
Oil
013
014
A5
AS
Hs (cm)
27
33
38
41
11
24
27
Tg (S)
2.0
2.2
2.3
2.5
1.2
1.8
2.0
Ld (cm)
622
746
806
671
218
515
611
d (cm)
340
240
340
90
180
170
170
Ujjj (cm/s)
2.77
12.50
7.46
54.0
0.33
10.26
15.00
'Writ) (cm/s)
5.42
11.5
11.1
13.6
5.77
5.49
5.70
t 0 (dyne/cm2)
0.52
2.13
1.27
7.72
0.09
1.94
2.66
i (dyne/cm2)
1.00
1.97
1.88
2.18
1.39
1.05
1.04
J (g/cm2-s) x 106
—
4.30
—
1.21
—
6.47
7.82
3 (g/L)
—
0.381
—
0.133
—
0.628
0.759
^equilibrium (u8 P/L)
45
48
45
47
45
81
85

i 54
Table V-9. Summary of Predicted Wave Statistics, Sediment Resuspension Rates,
and Final Concentration of SRP at Selected Stations in Lake
Okeechobee and Lake Apopka Assuming an Initial Concentration of
45 and 50 ug P/L, Respectively, in the Overlying Water. Wind
Velocity is 8.9 m/s and Originates from the East for Lake
Okeechobee and from the North for Lake Apopka.
Station
Parameter
07
010
Oil
013
014
A5
A8
Hg (cm)
35
41
49
51
15
30
34
Ts (s)
2.3
2.5
2.7
2.9
1.4
2.1
2.3
L¿ (cm)
800
928
1,110
785
285
563
641
d (cm)
340
240
340
90
180
170
170
Ujjj (cra/s)
6.69
21.1
17.0
71.8
1.31
13.9
17.9
utnicrit) (cm/s)
5.66
12.0
11.8
14.2
6.03
5.73
5.95
T0 (dyne/cra^)
1.15
3.23
2.60
11.7
0.31
2.43
3.00
t (dyne/cm^)
0.97
1.92
1.81
2.12
1.35
1.03
1.01
J (g/cm^-s) x 10^
2.84
6.20
3.69
7.92
—
7.33
8.42
a (g/D
0.238
0.591
0.309
0.901
—
0.755
0.867
^equilibrium (u§ ?/L)
53
49
48
48
45
86
90

155
Table V-10. Summary of Predicted Wave Statistics, Sediment Resuspension Rates,
and Final Concentration of SRP at Selected Stations in Lake
Okeechobee and Lake Apopka Assuming an Initial Concentration of
45 and 50 ug P/L, Respectively, in the Overlying Water. Wind
Velocity is 11.2 m/s and Originates from the East for Lake
Okeechobee and from the North for Lake Apopka.
Station
Parameter 07 010 Oil 013 014 A5 A8
Hs (cm)
41
49
Tg (s)
2.5
2.8
L¿ (cm)
959
1,080
d (cm)
340
240
1% (cm/s)
11.3
28.9
UnKcrit) (cm/s)
5.85
12.4
tq (dyne/cm^)
1.82
3.97
T (dyne/cm^)
0.96
1.88
J (g/cm‘-s) x 10^
3.98
7.50
M (g/L)
0.357
0.755
^equilibrium (u§
57
50
59
60
19
36
39
3.0
3.1
1.5
2.3
2.5
1,310
878
351
641
721
340
90
180
170
170
24.9
86.8
3.13
19.2
23.2
12.2
14.7
6.25
5.92
6.13
3.36
15.4
0.68
3.17
3.49
1.78
2.13
1.33
1.01
1.00
5.00
14.1
—
8.58
9.26
0.448
1.642
—
0.924
0.997
50
48
45
93
96

156
factor of two larger, an average suspended sediment concentration of
only 0.133 g/L is predicted. Decreased resuspension relative to
Station 010 is due to the sandy nature of sediments at Station 013 and
their correspondingly low water content (26.7 percent). Concentrations
of SRP in the water column at Station 013 are predicted to increase
slightly to 47 ug P/L.
When the wind velocity is increased to 8.9 m/s, sediment resuspen¬
sion is predicted at all stations in Lake Okeechobee except Station 014
(South 3ay). The protected features of South Bay (Figure II-l) limit
the effective fetch at Station 014 to approximately 0.5 km and generally
precludes sediment resuspension by easterly winds. Highest suspended
sediment concentrations are predicted at Station 013, which is
characterized by the largest effective fetch of the stations modeled.
Although a vertically averaged sediment concentration of 0.901 g/L is
predicted at Station 013, only 0.6 mm of the sediment surface is eroded,
because of the high bulk density (1.84 g/craJ) of the sediment.
Final SRP concentrations at Stations 010, Oil, and 013 show little
predicted changes over initial conditions. The greatest releases in
Lake Okeechobee (8 ug P/L) occur with Station 07 sediments, which are
relatively enriched with labile phosphorus (Table V-2). The net
increase in SRP levels at Station 07 is predicted to be 8 ug/L.
Increasing the wind velocity to 11.2 m/s generally has little
further influence on predicted in situ SRP concentrations, although the
mass flux of sediment to the water column is enhanced substantially
(Table V-10). Excluding Station 07, predicted SRP concentrations are
elevated by only 5 ug P/L, yielding a final SRP concentration of

157
50 ug P/L. Net entrainment of the sediment surface ranges from 1 mm at
Station 013 to nearly 2 cm at Station 010.
Although the effective fetch of Lake Apopka is generally much less
than that of Lake Okeechobee, wind-induced sediment resuspension has a
much more pronounced influence on its nutrient regime. Ease of
resuspension reflects both the bouyant nature of flocculent, organic
sediments and the shallowness of the lake basin. Even under relatively
mild conditions, sediment resuspension is substantial in mid-Lake
Apopka, resulting in significant release of SRP. For example, wind
velocities of 6.7 m/s are predicted to scour 28 mm of the sediment
surface at Station A5, resulting in a net release of 31 ug P/L
(Table V-8). Desorption from suspended sediments accounts for 18 ug P/L
of the predicted change in concentration, and entrained interstitial
water accounts for the remaining fraction.
For the same wind velocity conditions, sediment entrainment is only
slightly higher at Station A8. The resulting increase in SRP concen¬
tration is only 4 ug P/L in excess of the release predicted at
Station A5. Because of the high water content of Lake Apopka sediments,
sediment resuspension rates are relatively high once the t has been
exceeded. Equation V-36 predicts an initial flux of 5.0 x 10“^ g/Cm^-s
once bed failure occurs and may represent a weakness in the model at low
At and high water content. The high rates of resuspension predicted by
the model, however, agree qualitatively with empirical observations (see
Section IV).
Variability of average suspended sediment concentrations and final
SRP concentrations as a function of fetch length are shown in

158
Figure V-19. For this wind event, resuspension is restricted to regions
of the lake having an effective fetch greater than 2.1 km. Once the
threshold stress is exceeded, the depth-averaged suspended sediment
concentration, M, is predicted to increase essentially as a
step function from an initial condition of no suspended matter in the
water column to 0.460 g/L. Further increases in fetch are predicted to
generate relatively small increases in M. Correspondingly, final
SRP concentrations in the water column are predicted to increase
significantly once sediment suspension is induced; beyond the "threshold
fetch," however, final SRP concentrations increase slowly with fetch.
The influence of pH on phosphorus release by resuspended sediment
is illustrated by Table V-ll, which compares predicted final SRP
concentrations at Station A5 for several wind velocities as a function
of both pH and initial SRP levels in the water column. For example, the
net change in SRP at pH 8.3 is predicted to be 36 ug P/L for a sustained
wind velocity of 6.7 m/s and an initial SRP concentration of 20 ug P/L
in the water column; a shift in pH to 9.30 predicts further release of
18 ug P/L for a total change in SRP of 54 ug P/L. The consequence of pH
shift on nutrient release is important because highest pH levels are
generally observed in the summer when primary production is maximal;
similarly, wind-induced disturbance of the sediments generally is most
pronounced during the summer when convective storm activity is greatest.

SRP ( mg P/L)
0.100
EFFECTIVE FETCH ( Km)
Figure V-]9. Predicted Suspended Sediment Concentrations (g/L) (Lower curve) and Final
SkP Concentrations (mg P/L) (upper curve) in Lake Apopka as a Function of
Effective Fetch. Wind Velocity is 6.7 in/s.
SUSPENDED SEDIMENT, M( g/L)

160
Table V-ll. Comparison of Predicted Final SRP Concentrations at Station A5 for
Various Wind Velocities as a Function of 3oth pH and Initial SRP
Concentration in Water Column.
Initial SRP
Concentration pH 8.3 pH 9.3
(ug P/L) 6.7 m/s 8.9 m/s 11.2 m/s 6.7 m/s 8.9 m/s 11.2 m/s
20
56
61
68
74
83
94
50
81
86
93
100
108
119

CHAPTER VI
DISCUSSION AND SUMMARY
Discussion of Internal Loading
The relative significance of internal loading to the phosphorus
dynamics of a lake can be evaluated by constructing a budget quantifying
the various phosphorus fluxes through the system. Simplified input-
output or mass balance type budget calculations have been used
extensively to calculate the net flux of phosphorus to or from sediments
(e.g., Bengtsson 1975; Ryding and Forsberg 1977; Cooke et al. 1977).
Cooke et al. (1977), for example, used short-term budgets to demonstrate
that internal loading was the source of 65 to 105 percent of increases
in phosphorus content observed in the eutrophic Twin Lakes (Ohio)
following thermal stratification.
Detailed hydrologic and nutrient budgets have been constructed by
this author (Brezonik et al. 1978) and Federico et al. (1981) for Lake
Apopka and Lake Okeechobee, respectively. The hydrologic and nutrient
budgets for both systems were based on the continuity equation:
AS = II - 10 (VI-1)
In other words, the change in storage of a particular constituent in a
system (As) is simply the difference between the sum of the inputs (II)
and the sum of the outflows (10). Hydraulic income to a lake includes
surface inflows from streams, overland runoff, precipitation directly on
161

162
the lake's surface, and ground water seepage; hydraulic losses comprise
surface outflow, ground water seepage, and evaporation. The hydrologic
budget for a lake, therefore, can be expressed in detail by expanding
Equation VI-1:
AS = (P - E) A + + VQ _+ G (VI-2)
where S = change in water storage,
P = precipitation on lake surface,
E = evaporation from lake surface,
A = lake surface area,
V^_ = sum of surface inputs to lake,
VQ = sum of surface outputs, and
G = flux of ground water.
Nutrient budgets subsequently were calculated by determining the
nutrient flux asssociated with each particular component of the
hydrologic budget.
Hydrologic and nutrient budgets for Lake Apopka were prepared on a
monthly basis for the 1977 water year which was prior to the removal of
all point source wastewater discharges to the lake in 1978. Federico
et al. (1981) presented annual budgets for Lake Okeechobee for the
period 1973 through 1979. Table VI-1 summarizes the average hydrologic
budgets for Lake Okeechobee and Lake Apopka; while Table VI-2 summarizes
the respective phosphorus budgets. The methodology and assumptions
inherent in the construction of the monthly budgets for Lake Apopka have
been presented by Brezonik et al. (1978) in addition to the monthly
results. Federico et al. (1981) provided a more complete discussion of
the mass budgets specific to Lake Okeechobee.

163
Table VI-1. Summary Annual Hydrologic Budget for Lake Okeechobee
(1973-1979) and Lake Apopka (1977 Water Year)
(all fluxes in cP x 10°).
Lake Apopka*
Lake Okeechobeet
Inputs
Surface
55.6
2,626
Precipitat ion
131.7
1,666
Seepage
4.0
—
TOTAL
191.3
4,292
Outputs
Surface
29.2
1,246
Evaporation
163.8
2,541
Seepage
3.6
64
TOTAL
196.6
3,851
Change in Storage
5.3
257
Other Sinks
133
* From Brezonik et al. (1978).
t From Federico et al. (1981).

164
Table VI-2. Summary Annual Phosphorus 3udget for Lake Okeechobee
(1973-1979) and Lake Apopka (1977 WaCer Year)
(all fluxes in kg x 10-5).
Lake Apopka*
Lake Okeechobee!
Inputs
Surface
33.5
501.6
Precipitation
6.3
100.9
Seepage
1.7
—
TOTAL
41.5
602.5
Outputs
Surface
5.9
95.2
Seepage
0.8
4.0
TOTAL
6.7
99.2
Change in Storage
16.8
Other Sinks
34.8
486.5
* From Brezonik et al. (1978).
t From Federico et al. (1981).

165
Inspection of external loadings and export rates of phosphorus to
and from Lake Apopka and Lake Okeechobee indicates that both systems
function as net sinks of phosphorus; i.e., there is a net flux of
phosphorus in both lakes is to the sediments (Table VI-2). The
magnitude of this flux, which is estimated at 34,800 kg P/y in Lake
Apopka and 486,500 kg P/y in Lake Okeechobee, may be expressed as the
dimensionless phosphorus retention coefficient, Rp:
R
P
EP
out
EP
m
(VI-3)
where EP^n and ^ Pout = the total phosphorus mass flux imported to and
exported from the system, respectively.
Calculated Rp values for Lake Apopka and Lake Okeechobee are virtually
identical (0.84) (Table VI-3).
External loading rates, hydraulic flushing characteristics, and
Rp values may be input to simple input/output type modes to compare
predicted steady-state concentrations with observed annual average
concentrations. The adequacy of this type of model to predict steady-
state concentrations correlating with observed values may be taken as an
indication of the role of internal loading in controlling phosphorus
concentrations in the water column. Baker et al. (1981) recently
conducted an extensive review of phosphorus input/output models and
applied nutrient loading data for 101 Florida lakes to determine their
validity for subtropical lakes. From their analysis these authors
concluded that the best predictive equation was a modified version of a
model originally derived by Dillon and Rigler (1975):

166
Table VI-3. Summary of Nutrient Loading Model Input Parameters for
Lake Apopka and Lake Okeechobee.
Parameter
Lake Apopka
Lake Okeechobee*
Areal Phosphorus Loading
Rate, La (g P/m2-y)
0.334
0.347
Hydraulic Loading Ratet
(m/y)
0.48
1.52
Phosphorus Retention Coefficient,
Rp (dimensionless)
0.839
0.835
Average Depth, z (m)
1.70
2.64
Hydraulic Residence Time, xw
(year)
6.44
3.47
* Average value for 1973-1979 (Federico et al. 1981).
t Based on surface inflows and seepage (excluding rainfall).

167
TP = 0.748 [La(l-Rp)/qs] °-862 (VI-4)
where TP = total phosphorus concentration (g/m3),
r\
La = areal surface loading rate of phosphorus (g/m -y), and
qs = hydraulic loading rate (m/y).
The two constants were derived through regression analysis to improve
the predictability of the original model (enclosed in brackets) for
Florida lakes.
The modified Dillon-Rigler model was applied to Lake Apopka and
Lake Okeechobee for comparative purposes. This analysis also was
extended to a modified Vollenweider (1976) model developed by Baker
et al. (1981) for the same data set and to a model recently developed by
Chapra (1982). Inclusion of the modified Vollenweider resulted from the
work of Federico et al. (1981), who demonstrated good agreement between
this model and the average total phosphorus concentration in Lake
Okeechobee between 1973 and 1979. In Vollenweider's (1976) model, the
apparent deposition or settling velocity of phosphorus to the sediments
is assumed to be inversely related to the square root of the hydraulic
residence time, Tw. The modified Vollenweider model has the
following form:
TP = 0.682 [La/qs(i+ tw)]0-934 (VI-5)
Chapra's (1982) model, on the other hand, accounts for the resuspension
effects by making the apparent settling velocity a function of lake
depth:

168
vp - vmax (1 - ^Z) (VI-6)
where vmax = the maximum apparent settling velocity (m/y), and
a = a parameter to describe the effect of depth on the decay
of vmaxâ– 
The resulting steady-state model is given by Chapra (1982) as
TP = TP¿ (l-fs) U+vmax (1 —e—otz) / qs ] -1 (VI-7 )
where TP¿ = total phosphorus concentration of the input, and
fs = the fraction of the input that does not affect mid-lake
quality.
Required model parameters including La and qs specific to Lake
Apopka and Lake Okeechobee are summarized in Table VI-3; results for
predicted and observed phosphorus concentrations are presented in
Table VI-4. Using the data of Schaffner and Oglesby (1978) and Oglesby
and Schaffner (1978), Chapra (1982) estimated values for vmax and a
as 24.5 m/y and 0.075 m/y, respectively. These values were subsequently
used in the analysis. The entire phosphorus input to each lake was
assumed to influence lake phosphorus concentrations (i.e., fg = 0).
Predicted and observed (average) values for Lake Okeechobee agree rather
well for all three models and this suggests that external loading
processes are controlling the lake's nutrient regime. These results are
also consistent with the relationship observed between the temporal
distribution of total phosphorus concentrations and annual external
loading rates (Figure VI-1). Total phosphorus levels have been
increasing steadily 0.049 mg P/L in 1973 to 0.097 mg P/L in 1979

169
Table VI-4.
Application of
Apopka and Lake
Total Phosphorus
Okeechobee.
Predictive Models to
Lake
Observed Average
Total Phosphorus
Concentration
(mg P/L)
Modified
Vollenweider*
(1976)
Modified
Dillon & Riglert
(1975)
Chapra**
(1982)
Apopka
0.221
0.149
0.113
0.098
Okeechobee
0.063
0.064
0.051
0.059
* TP = 0.682 [La/qg (1 + tw)]0-934 (Brezonik et al. 1982).
t TP = 0.748 [La (1-Rp)/qs]0-862 (Baker et al. 1982).
** TP = TPi (l-fs) [1 + vmax (l-e^z)/qs]-1.

AVERAGE CONCENTRATION ( mg P/L)
0.400
0.300
0.200
0.100
Figure VI-1.
Areal Phosphorus Loading Rates, La (g P/in^-yr), and Annual Average
SRP and Total Phosphorus (TP) Concentrations (mg P/L) in Lake Okeechobee
from 1973 through 1979. Data from Federico et al. (1981).
La ( g P/m -y)

171
(Federico et al. 1981), and chis trend generally corresponds to
increases in external loading rates. This general trend is not observed
in 1974 and 1975 when La increased sharply in 1974 to 0.437 g/m^-y
and then fell to 0.227 g/m^-y in 1975; total phosphorus concentrations
during the period increased slightly.
Application of the nutrient loading models to Lake Apopka shows
poor agreement between the average total phosphorus concentration
observed in 1977 and predicted concentrations (Table VI-4). All three
models predicted concentrations that are too low; the best approximation
was given by the modified Vollenweider model, which predicts a steady-
state concentration of 0.149 mg P/L compared with an average
concentration of 0.221 mg P/L. This discrepancy implies that internal
loading exerts a substantially greater influence than the model accounts
for. Further evidence of the relative importance of internal loading to
the nutrient dynamics is given by Figure VI-2 which shows the annual
average concentration of total phosphorus in Lake Apopka as a function
of time. Although nutrient loading data are not available beyond 1977,
nutrient loadings to Lake Apopka have undoubtedly decreased with the
cessation of all point-source wastewater discharges in 1978.
Despite reduced rates of nutrient import, average total phosphorus
concentrations were highest in 1978 and 1979 and probably are attribu¬
table to nutrient release from resuspended sediments. Such a mechanism
is also supported by the stochastic distribution of mean monthly
concentrations in Lake Apopka during the study period. No seasonal
trends are evident, and concentrations of both SRP and total phosphorus
increase and decrease quite markedly as a result of internal loading and
subsequent uptake by algae and settling processes, respectively.

CONCENTRATION (mg P/L)
172
0.300
0.200
0.100
1977 1978 1979 1980
DATE
Figure VI-2. Average Annual Total Phosphorus (TP) Concentrations
(mg ?/L) in Lake Apopka from 1977 through 1980. Data from
Brezonik et al. (1981).

173
An alternative approach to assessing the magnitude of internal
loading to the trophic status of a lake is to evaluate the nutritional
demands of primary producers and quantify the various sources satisfying
that demand. Implicit in this analysis is the recognition that the
recycling rate or turnover time of phosphorus in the tropnogenic zone
may be much more important that the steady-state concentration in
limiting primary production. Golterman (1973), for example, suggests
phosphorus turnover times of the order of 3 to 10 days.
Algal uptake requirements for phosphorus may be calculated
indirectly from the Redfield stoichiometric relationship (Redfield
et al. 1963):
P
106 C02 + 16 NO3 + HPO42 + 122 H20 + 18 H+ . '—-
R
c106 H263 °110 n16 P1 + 138 °2 (VI-8)
algal protoplasm
For Lake Apopka, with an average primary production rate of 911 g C/m -y
(Brezonik et al. 1978), this relationship implies a phosphorus uptake
rate of 22.2 g P/m -y. The lack of primary production data precludes
a similar analysis for Lake Okeechobee. Given an average concentration
of 0.221 g/m3 in Lake Apopka during the same period, an average
phosphorus turnover time of 6.2 days is calculated. If approximately
10 percent of the total phosphorus mass during each cycle is not
recycled but is lost to the sediments a total loss for the system of
about 2.2 g/m2-y is calculated. Of the required inputs necessary to
balance, this deficit, external loading contributes approximately

174
15 percent (Figure VI-3). Diffusion from the sediments, which was
estimated to range between 1.6 to 4.3 g P/m^-d generates an annual
flux of 0.6 to 1.6 g P/m^. The remaining fraction required to
balance the phosphorus uptake flux by primary producers may be accounted
for by resuspension (0.3 to 1.3 g P/m^-d). Assuming a net concen¬
tration increase of 30 ug P/L or greater characterizes the flux of
phosphorus from resuspended sediments during sustained wind events of
6.7 m/s or greater, between 6 and 25 wind events are required annually
to generate the flux required by the simplified uptake model. When
compared to the annual average number of thunderstorm days in Orlando
(74.4) (Davis and Sakamoto 1976), 25 km southeast of Lake Apopka, it is
apparent that wind-induced resuspension may assume an even more
substantial role in controlling the nutrient dynamics of Lake Apopka.
Summary
The shallow nature and broad fetch characteristic of Lake Apopka
(mean depth 1.7m) and Lake Okeechobee (mean depth 2.8 m) suggest that
sediment resuspension and the concomitant release of sorbed phosphorus
may be important in the cycling of phosphorus. As a result, a model was
developed incorporating physical, chemical, and hydrodynamic components
to predict internal loading of phosphorus in shallow lakes. Phosphorus
releases are predicted on a single event basis in response to wind-
driven turbulent mixing in the water column. The model synthesizes a
wave-hindcasting submodel to determine sediment resuspension rates and a
phosphorus release submodel derived from Langmuir sorption theory.
Langmuir constants used as model inputs were derived from adsorption-
desorption isotherms conducted at controlled pH.

^1
Un
Figure VI-3. Allochthonous Phosphorus Inputs and the Phosphorus Cycle in Lake Apopka. All
Fluxes are Civen as g P/rn^-y.

176
Under ambient conditions in the water column, both Lake Apopka and
Lake Okeechobee sediments are predicted to release phosphorus upon
resuspension. Model results indicate that a moderate wind event
(approximately 8.9 m/s) can more than double mid-lake concentrations of
orthophosphorus in Lake Apopka from 20 to 61 ug P/L at pH 8.3.
Predicted phosphorus release from Lake Okeechobee sediments under the
same wind conditions is substantially lower (approximately 3 to
8 ug P/L). Release is primarily from desorptive processes, although in
Lake Apopka entrained pore fluid contributes approximately 30 percent of
the net release. Release in both lakes is predicted to be maximal
during the summer when high rates of photosynthesis shift lakewater pH
upward, thus enhancing the magnitude of desorption. The cumulative
effect of cyclonic and convective disturbances during the course of a
year results in internal loading in Lake Apopka easily exceeding
external loading rates. Sediment resuspension is less significant in
Lake Okeechobee, and the nutrient regime appears to be controlled
primarily by external loading.
Turbulent mixing and nutrient release studies conducted on fresh
and dessicated sediments indicate lake drawdown will effectively
stabilize bottom sediments in Lake Apopka and greatly diminish wind-
induced sediment resuspension. Upon refill of the lake, however, a
large release of phosphorus is predicted. After a stabilization period
of several months, overlying water SRP concentrations are expected to
return to lower levels.
The integrated nutrient release-sediment resuspension model should
be viewed as semi-quantitative. The conjunctive influence of applied

177
shear stress, critical shear stress, and water content on sediment
erosion rates is not adequately described for lacustrine sediments
beyond the Lake Erie basin. Ease of resuspension is affected by the
amount of elapsed time between periods of disturbance which may confound
attempts to accurately quantify sediment resuspension rates in Lake
Apopka and Lake Okeechobee. In addition, the sediment resuspension
model is sensitive to changes in the reflectivity parameter (8);
additional research is required to determine this parameter specific to
both lakes. Despite these limitations, results from sorption studies
demonstrate that under current conditions in both Lake Apopka and Lake
Okeechobee, net release of phosphorus occurs upon sediment resuspension.
This mechanism is undoubtedly a major contributing factor accounting for
the stochastic nature of phosphorus dynamics in Lake Apopka and Lake
Okeechobee.

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BIOGRAPHICAL SKETCH
Curtis D. Pollman was born 2 August 1951 in the small coastal town
of Lewes, Delaware. Raised in Seaford, Delaware, he graduated from
Seaford Senior High School in June 1969. The following fall he entered
the University of Delaware, majoring in biology. During this period
Mr. Pollman embarked on a career as a tennis professional, an occupation
he pursued in earnest following his graduation with honors in June 1973
with a Bachelor of Arts degree. After a period as a corporate co-owner
and head professional of a tennis professional shop in Wilmington,
Delaware, he returned to academics and entered the Graduate School of
the University of Florida in September 1974, as a student in the
Department of Environmental Engineering Sciences. After receipt of a
master's degree in early 1977, he continued his graduate career at the
University of Florida as a doctoral candidate in the Department of
Environmental Engineering Services. He was married to the former
Kathleen Doughty in September 1981 and is the doting father of two cats
and two golden retriever dogs.
191

I certify that I have read this study and that in ray opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
fried?.
Chairman
Patrick L. Brezonik,
Professor of Environmental
Engineering Sciences
I certify that I have read this study and that in ray opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
/ â– 
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7x
T(^ ...
Wayhé C. Huber
Professor of Environmental
Engineering Sciences
I certify that I have read this study and that in ray opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a disseration for the degree of
Doctor of Philosophy.
Edward S. Deevey
Graduate Research Professor of
Zoology

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Associate Professor of Soil Science
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Tííomas L. Crisman
Associate Professor of
Engineering Sciences
Environmental
This dissertation was submitted to the Graduate Faculty of the College
of Engineering and to the Graduate Council, and was accepted as partial
fulfillment of the requirements for the degree of Doctor of Philosophy.
April 1983
lLU (Xj\
Dean, College of Engineering
Dean for Graduate Studies and
Research

UNIVERSITY OF FLORIDA
262 08666 914 9

UNIVERSITY OF FLORIDA
262 08666 914 9