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PHOSPHORUS SORPTION AND FLUX IN NORTHERN EVERGLADES SOIL
UNDER DRAINED AND FLOODED CONDITIONS
JENNIFER A. LEEDS
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
This research was made possible by the South Florida Water Management District
through financial, technical and facility support and conducted as part of the Downstream
Monitoring and Research Program in the Everglades Division. I would like to
acknowledge District staff that provided assistance in the field and nutrient analysis in the
water quality lab. In addition, I would like to thank Dr. Mark Clark, Dr. Jana Newman,
Dr. Susan Newman and Dr. Samira Daroub for the invaluable support, guidance and
contributions they made to this research and thesis.
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ......... .................................................................................... ii
LIST OF TABLES .............. ........ ....................... ............... ........... .. v
LIST OF FIGURES ......... ....... .................... .......... ....... ............ vi
ABSTRACT .............. ..................... .......... .............. vii
1 PHOSPHORUS MOVEMENT BETWEEN THE SOIL AND OVERLYING
W ATER COLUM N IN W ETLAND S ................................... ....................................1
Introduction ............... ........................ ............................... ...............
Equilibrium Phosphorus Concentration (EPC)................................... ............... 2
Factors Regulating P-sorption .............................................................................4
P hosphorus F lux ................................................ ......................... 5
N northern E verglades Field Site............................................................ ............... 7
N eed for R research .................................................. .. ....... ................ .9
2 PHOSPHORUS SORPTION AND DESORPTION CAPACITY ..................... 12
In tro d u ctio n ...................................... ................................................ 12
M eth o d s .............................................................................. 13
Isotherm calculation s ................................................................ ............. .... 15
R e su lts ...................................... .................................................... 1 6
D iscu ssio n ...................................... ................................................. 2 4
3 FLUX OF BIOAVAILABLE PHOSPHORUS........................................................27
In tro d u ctio n ............ ..... .... ..... ................. ................ ................ 2 7
M eth o d s ..............................................................................2 8
R esu lts ......... ...... ........... ...................................... ............................32
D iscu ssion ......... ....... ......... ..................................... ............................ 4 6
4 C O N C L U SIO N S ..................... .... .......................... .. .... ........ .... ......... ..5 1
Phosphorus Isotherm s ......................... .......... .............. ............... 52
P -Sorption F actors .............................................. .. .... .... ......... .. .... .. 54
P -flu x ................................................................5 5
C conclusion ...................................................................................................... ....... 56
A P-ADSORPTION RESULTS FROM THE LINEAR, FREUNDLICH AND
LANGMUIR ISOTHERM EQUATIONS AT EACH SITE AND P-
C O N C E N TR A T IO N ...................................................................... .....................58
B WATER QUALITY NITROGEN RESULTS FROM INTACT SOIL CORES
SAMPLED FROM THE P-FLUX EXPERIMENT. ............. ............... 60
L IT E R A T U R E C IT E D ............................................................................ ....................6 1
B IO G R A PH IC A L SK E TCH ..................................................................... ..................65
LIST OF TABLES
2-1 List of regression (R2) coefficients used to determine which isotherm equation,
linear, Freundlich or Langmuir, best fit the P-sorption data. ..................................20
2-2. P-sorption parameter results for the linear, Freundlich and Langmuir equations
for equilibrium phosphorus concentration (EPC). ................................................20
2-3 RWMA soil nutrient concentrations for each site in the RC transect .....................24
2-4 Comparison of oxalate extractable Fe and Al content is soils ..............................24
3-1 Water Quality standard testing methods according to Clescerl et al. 1999 and the
South Florida Water Management District water quality procedures....................31
3-2 Fluxing rates of SRP in mgL-1/hr for dry soil cores at each transect site.................34
3-3 Percentage of oP and iP fluxing from the soil into the water column over time......45
3-4 Algae TP concentration measurements in mg/kg................. ............................46
LIST OF FIGURES
1-1 Map showing the location of the Rotenberger Wildlife Management Area .............8
2-1 Linear isotherm result for the RC transect A) RC1 site. B) RC2 site C) RC3 site
D ) R C 4 site............ ................................................................ ......... 18
2-2 Freundlich and Langmuir isotherms for the RC transectA) RC1 site. B) RC2
site. C) R C3 site. D ) R C4 site. ............................................................................ 21
2-3 Oxalate-extractable Al composite samples. .................................. .................23
2-4 Oxalate-extractable Fe composite samples. .................................. .................23
3-1 SRP water column concentrations in mg/L at a)RC1 site. b)RC2 site. c)RC3 site.
d )R C 4 site ..........................................................................3 3
3-2 TP water column concentrations in mg/L at a)RC1 site. b)RC2 site. c)RC3 site.
d)R C 4 site ............... ................................................. ..... ........... 35
3-3 TDP water column concentrations in mg/L at a)RC1 site. b)RC2 site. c)RC3
site. d)R C 4 site. ........................................................................37
3-4 NH4 water column concentrations in mg/L at a)RC 1 site. b)RC2 site. c)RC3 site.
d)R C 4 site ............... ................................................ ..... ........... 38
3-5 SRP water column concentrations measured in mg/L at RC 1, RC2, RC3 and
R C 4 ............................................................................ 4 0
3-6 NH4 water column concentrations measured in mg/L at RC 1, RC2, RC3 and
R C 4 ............................................................................ 4 0
3-7 Dissolved organic phosphorus (DOP) water column concentrations in tg/L at
a)RC1 site b)RC2 site. c)RC3 site d)RC4 site. .............................. ......... ...... .41
3-8 Organic phosphorus (oP) water column concentrations in tg/L at a)RC1 site.
b)RC2 site c)RC3 site d)RC4site. ........................................ ........................ 43
3-9 Particulate phosphorus (PP) water column concentrations in tg/L at a)RC 1 site.
b)RC2 site c)RC3 site d)RC4 site. ........................................ ....................... 44
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of
the Requirements for the Degree of Master of Science
PHOSPHORUS SORPTION AND FLUX IN NORTHERN EVERGLADES SOIL
UNDER DRAINED AND FLOODED CONDITIONS
Jennifer A. Leeds
Chair: Mark W. Clark
Major Department: Soil and Water Science
Wetland soils have the potential to function as a phosphorus (P) source or sink
depending on antecedent soil conditions, P loading rates, water column P concentrations
and characteristics. Hydrology, water quality and soil chemistry are primary factors
affecting soil P sorption and desorption. The Rotenberger Wildlife Management Area
(RWMA), part of the northern Everglades, has undergone alterations in hydrology
resulting in shortened hydroperiods, severe soil oxidation and peat fires. These
alterations have subsequently elevated available P concentrations in the soil. In
accordance with the Everglades Forever Act (EFA), the South Florida Water
Management District began hydropattern restoration in RWMA in July 2001, utilizing
discharges from Storm Water Treatment Area 5 (STA-5). As part of the northern
Everglades, it is uncertain whether the RWMA soils will act as a source or sink for P. To
address this question, P-isotherms were run to determine the equilibrium phosphorus
concentration (EPC) of the soil, while soil P-flux differences between drained vs. flooded
soils were assessed using intact soil core microcosms collected from the field. Results
show EPC measurements across the marsh ranged from 73.6 utgL-1 to 94.5 utgL-1 at three
of the four sites, with one site measuring 18.8 tgL-1. The P-flux in intact soil cores
indicated drained soil treatments fluxed significantly higher concentrations of soluble
reactive phosphorus (SRP) (110 [tgL-1) versus continuously flooded soil treatments (7
[tgL-1). Water column SRP concentrations of intact soil cores peaked between 8 and 24
hours after re-flooding with ambient marsh water then water column concentrations
declined to SRP levels measured in flooded soil cores by 380 hours. Flooded soils, at all
sites, released low levels of SRP concentration upon re-flooding within one hour and then
appeared to reach equilibrium, as concentration levels remained constant. Additionally,
results measured concentrations of oxalate-extractable Fe (1039.5 g/m3) and Al (514.5
g/m3), normalized based on bulk density (0.21 g/cm3), and are significantly higher
compared to other Everglades soil, which may be regulating P sorption. EPC
measurements suggest P water column concentrations less than 90 utgL-1 will result in P-
release from most soils in RWMA. Furthermore, this system continues to experience soil
dry out and oxidation for several months (February June) during the dry season, which
can lead to rapid and elevated flux of SRP upon re-flooding, as compared to continuously
flooded soils. These results can be incorporated into adaptive management strategies to
modify operations of the system to achieve hydropattern restoration targets while slowing
the movement of existing soluble soil P further into the Everglades.
PHOSPHORUS MOVEMENT BETWEEN THE SOIL AND OVERLYING WATER
COLUMN IN WETLANDS
Wetland soils can function as a source or sink for phosphorus (P) depending on
antecedent soil conditions and interstitial porewater P concentration. Phosphorus flux
from soils is an important biogeochemical process occurring in all types of wetlands.
Because P cycles within the soil and does not have a significant gaseous phase such as
nitrogen (N), P within a wetland can be transferred between soil, water column, flora and
fauna, although is not easily released outside of the wetland system (White et al. 2000).
For this reason, P movement within a wetland is of major concern in systems such as the
Florida Everglades that initially developed under low P concentrations then received high
P inflows resulting in increased P loading. Over time, continuous loading of P to soils
can shift historically oligotrophic (low nutrient) wetlands, to eutrophic (high nutrient)
systems (South Florida Water Management District 1992; Newman et al. 1997). In
addition to external P loading, P dynamics and bioavailability can change as a result of
altered hydrology, biology or chemistry of a site (Reddy et al. 1998). Due to
anthropogenic alterations in hydrology, many areas in the Northern Everglades have
severely reduced hydropatterns, leading to excessive drying and oxidation of the soil,
thereby raising soil P concentrations and promoting soil loss. Determining environmental
conditions regulating P-sorption or flux in the soil, is largely dependant upon the P
concentration and composition of P, both the soil and in the water column, influencing
the mobilization of soluble P further into the Everglades marsh interior.
Equilibrium Phosphorus Concentration (EPC)
Phosphorus sorption/desorption studies primarily use P-isotherms to define the
equilibrium phosphorus concentration (EPC) of the soil and measure the maximum P-
sorptive capacity of a soil. Furthermore, determining the water column P concentration
where soil P-sorption and desorption are at equilibrium, EPC, aids in determining when a
wetland will act as a source or sink for P. This value, relative to source water P
concentrations, or known acceptable water column P concentrations, is an important
variable in restoration, and in predicting potential wetland impact and recovery times.
For example, in a study by Richardson and Vaithiyanathan (1995) in Water Conservation
Area 2A (WCA-2A) conducted along a P-gradient, reported EPC at inflow sites were
high in elevated soil P concentrations, and significantly greater than interior marsh EPC
and soil P. They concluded the inflow sites have the ability to serve as an internal source
for P, even if external P loading was reduced, given the current inflow concentration
levels. Additionally, Zhou and Li (2001) showed a positive correlation between EPC and
soil P saturation in a study in the southern Everglades.
Wetlands, natural or constructed, can be used to treat runoff or wastewater
removing nutrients prior to downstream discharge. The P-sorptive capacity of a soil is an
important parameter in determining the amount of time, and range of P concentrations,
natural wetlands receiving nutrient enriched inflows, or proposed constructed wetlands
designed for nutrient removal, will function. In the northern Everglades, and areas north
of Lake Okeechobee, reducing enriched runoff from urban and agricultural areas has
become a primary focus of the Comprehensive Everglades Restoration Plan (CERP).
Studies by Nair et al. (1998) and Reddy et al. (1998), established that soils containing
enriched P concentrations, such as agricultural soils that are targeted to become Storm
Water Treatment Areas (STAs), or enriched Everglades soils found in current inflow
areas, may not have the ability to retain sufficient amounts of P for sustained periods of
time (- 10 years or more) due to historic P loading. In fact, these soils could become a
potential source of P upon receiving low P water inputs. In addition, this research
hypothesized physico-chemical properties of the soil such as aluminum (Al), iron (Fe),
magnesium (Mg) and calcium (Ca) content, can contribute to regulating the P-sorption
capacity of the soil. However, their effectiveness at retaining P was dependant upon soil
P concentrations, as well as aerobic or anaerobic soil conditions (Gale et al. 1994).
Similar studies conducted outside of the Everglades support these results (Khalid et al.
1977; Sallade and Sims 1997).
In addition to defining EPC, P-isotherms describe the maximum P-sorptive
capacity of the soil using the linear, Freundlich and or Langmuir equations (Nair et al.
1998; Pant and Reddy 2001; Zhou and Li 2001). The linear equation is typically applied
to low P concentrations, the Freundlich to medium P concentrations and Langmuir to
high P concentrations (Zhou and Li 2001). A principal aspect to consider when applying
P-isotherms is they are run under laboratory conditions, in which the soil is isolated from
other biological factors, such as plants and algae that could affect the sorptive capacity
(Gale et al. 1994). Furthermore, the sample processing method for P-isotherms exposes
all available soil sorption sites and cannot be directly related to field conditions where a
minimal number of sites are exposed. In addition, pore water diffusion rates play a
significant role in the exchange of porewater P with the overlying floodwater. Results are
best utilized in conjunction with other soil characteristics specific to field sites such as
nutrient content, dominant P-species (organic or inorganic) in the soil and, oxidized or
reducing conditions, as well as P-flux experiments.
Factors Regulating P-sorption
In wetland systems, P-sorption and desorption are influenced by many factors. The
physico-chemical properties of a soil, coupled with historic P-loading of a system, can
significantly affect P-sorption (Gale et al. 1994; Reddy et al. 1998). Soil chemistry,
specifically Fe and Al content, can have a substantial influence on P-sorption as well
(Sallade and Sims 1997; Zhou et al. 1997). Additionally, Reddy et al. (1998) found Total
Organic Carbon (TOC) significantly reduced P-sorption by forming complexes with Fe
and Al. In a report by Richardson (1985) on the P retention capacity of various
freshwater wetlands, results suggested the sorption potential of a wetland may be
regulated by the extractable Al content of the soil. The Fe and Al concentrations in the
soil can sorb P, forming poorly crystalline and amorphous oxide and hydroxide forms
(Khalid et al. 1977; Pant and Reddy 2001; Pant et al. 2001). Under aerobic conditions,
ferrous iron in the soil oxidizes to form ferric iron, a precipitate which is insoluble,
although can increase P-sorption sites (Gale et al. 1994). In systems with high
concentrations of Ca in the soil, such as in WCA-2A (Reddy et al. 1991; Richardson and
Vaithiyanathan 1995) and Everglades National Park (ENP) (Zhou and Li 2001), P-
sorption is typically regulated by calcium carbonate (CaCO3) (Raven and Hossner 1993).
Soil chemistry, organic matter, oxidation, as well as the concentration of available cations
such as Ca, Mg, Fe and Al, can influence P binding to form inorganic, bioavailable P-
species. Phosphorus species such as Ca/Mg-P and Al/Fe-P, which are inorganic P forms,
are regulated by acidic and alkaline soils, as well as aerobic and anaerobic soil conditions
influencing P solubility (Pant et al. 2002).
Phosphorus flux is typically used to describe the process of soluble P moving from
the soil to the overlying water column, under conditions where water column P
concentrations are less than soil porewater P concentrations (Reddy et al. 1996).
Phosphorus flux studies incorporating intact soil cores have been conducted in the
Everglades, on surrounding agricultural lands north of the Everglades near Lake
Okeechobee, and in eutrophic lakes in Central Florida, as well as in wetlands and lakes in
other parts of the country (Reddy et al. 1996; Fisher and Reddy 2001; Pant and Reddy
External P loading can have a considerable affect on the level of P concentrations
fluxing from the soil to the overlying water column. For example, a P flux study in
WCA-2A by Fisher and Reddy (2001) using intact soil cores was performed along a P
gradient, extending from water inflow structures to approximately 10 km into the marsh
interior. Their results indicated soil P flux was greater near inflow points and decreased
linearly with distance into the marsh interior, and is directly correlated to the soil P
concentration gradient. In addition, agricultural lands which later may be used as
constructed wetlands or storm water treatment areas, have the potential to be a nutrient
source depending on antecedent P loading of the soil. An initial high P concentration
flux can occur when agricultural lands are first flooded. Depending on soil EPC and
water column P (Pant and Reddy, 2003), if the EPC of the soil is elevated, the soil could
act as a continuous source of P, until an equilibrium is achieved with the overlying water
Two types of soil conditions exist in wetlands, aerobic (drained) and anaerobic
(flooded), with varying degrees in between depending on hydroperiod. In studies by Pant
et al. (2002) and Malecki et al. (2004), P fluxed in significantly higher amounts in re-
flooded, aerobic soils than in continuously flooded, anaerobic soils. One reason for these
differences in P release is the different form of soil P found under aerobic and anaerobic
conditions. Under aerobic conditions, where soils become oxidized, inorganic P species
usually form based on existing soil chemistry such as the Fe, Al, Ca or Mg content. In
systems with high Fe content, oxidized forms of Fe can adsorb P forming ferric
phosphate. Alternatively, in anaerobic conditions where the soil is not oxidized, P-
species primarily remain in their reduced, predominately organic state. Typically, organic
bound P is an immobilized form and not immediately available for uptake by plants or
algae. Therefore, for organic P to become bioavailable, it must first undergo
mineralization, which breaks down into non-bioavailable dissolved organic compounds
and bioavailable inorganic P. However, anaerobic conditions slow the mineralization
process of organic compounds thereby preserving P that is bound to organic particles.
The type ofP, inorganic or organic, has significant ecological implications. There
are three forms of P present in both the soil and water column, dissolved inorganic P
(DIP) also known as soluble reactive phosphorus (SRP), dissolved organic P (DOP) and
particulate organic P (POP). Soluble reactive phosphorus is readily available for uptake,
whereas DOP and POP must be mineralized to an inorganic form prior to becoming
bioavailable. The P species, and their relative concentrations in the water column, as
well as drained or flooded soil conditions, dictate which species will flux (Eckert et al.
1997; Novak et al. 2004). These relative concentrations and composition in soil and
water, affect the soluble P front at the landscape scale, having significant implications
within the Everglades ecosystem. The importance of which, is soluble P (DIP) is
bioavailable, a major factor in driving the system to transition at inflow points from an
oligotrophic to eutrophic marsh.
Northern Everglades Field Site
The Rotenberger Wildlife Management Area (RWMA), part of the Northern
Florida Everglades, is a 29,120 acre marsh located west of the Miami Canal and north of
Water Conservation Area 3A (Figure 1-1). Once part of the historic sawgrass plains
receiving continuous overland flow from Lake Okeechobee (McVoy, personal
communication), the RWMA became a degraded wetland, cutoff from continuous water
inflow, resulting in shortened hydroperiods and extended marsh draw downs. The
continuous cycle of short hydroperiods, allowed the soil to undergo severe dry out and
oxidation, elevating P levels, which led to soil subsidence (Newman et al 1998).
Consequently, perpetually dry soils, and low (<65%) soil moisture content (Wade et al.
1980) often resulted in typical wildfire becoming muck fires. Muck fires can combust
organic matter within the soil, thus significantly altering soil chemistry, increasing P-
levels and volatilizing N (Newman et al. 1998; Smith et al. 2001). Loss of soil organic
matter can change P soil storage pools from organic to inorganic forms (Leeds et al. in
progress). The combination of slow, biologically mediated oxidation of organic matter
and rapid thermally mediated oxidation of organic matter through muck fires has
significantly increased soil P levels, which now average 640 mg/kg. This change in P
concentration has resulted in a shift in marsh trophic state from oligotrophic to a
eutrophic system. DeBusk et al. (1994) suggested that Everglades soil P concentrations
greater than 500 mg/kg are considered enriched. The RWMA soils may contain a lower
organic matter content, due to the history of muck fires, and a higher mineral content,
although may still be classified as an enriched system as evidenced by soil TP content
and vegetation composition, as a result of severely altered hydrology.
Figure 1-1. Map showing the location of the Rotenberger Wildlife Management Area
(RWMA) north of Water Conservation Area 3A (WCA-3A) and south of
Lake Okeechobee with inflow point (G-410) from Storm Water Treatment
Area 5 (STA-5), the study transect RC and relative outflow point (G402-C).
Need for Research
The Everglades Forever Act (EFA) recognized the RWMA as part of the historic
northern Everglades requiring hydropattern restoration. To achieve this, the South
Florida Water Management District (SFWMD) began constructing Storm Water
Treatment Areas (STAs) to improve hydrology and water quality, from urban and
agricultural runoff, prior to discharge into downstream areas of the Everglades. In July
2001, STA-5 began discharging into RWMA. However, due to years of P enrichment
and hydrologic alterations, it is unknown if soils in the RWMA will serve as a P source or
sink, relative to STA-5 floodwater concentrations. Since STA-5 began discharging,
water column TP concentrations have averaged 50 70 ggL-1, with spikes in
concentration entering the marsh as high as 130 ggL-1. Because the RWMA is not
considered part of the Everglades Protection Area (EPA), inflow TP concentrations are
not required to meet the 10 ggL-1 P-limit criteria set by the Florida Department of
Environmental Protection. However, as dictated by the EFA permit for STA-5,
discharges are required to be at 50 ggL-1 or below, a annual average. Determining the
ability of the soil to adsorb P, as well as defining the EPC of the soil, will aid in
predicting P-flux and the affect of STA-5 floodwater on RWMA marsh recovery.
Existing soil nutrient data shows fairly uniform TP concentrations throughout the
RWMA marsh, as well as with depth, therefore it is hypothesized EPC values and P-
fluxing concentrations would be spatially homogeneous. However, spatial differences in
EPC and P flux across the RWMA needs to be verified to clarify differences between
sites and variances in soil P concentrations that may occur from inflow to outflow.
Moreover, determining the type of P-species flowing into and out of the marsh, in
addition to fluxing out of the soil, is critical information for management of the system.
For example, inorganic P is readily available to plants and microorganisms where as
dissolved organic P is not directly available for uptake resulting in less effect on plants
within the marsh, but a higher probability of transport downstream. Also, the type of P
available in the soil will influence what P species is available to flux (Novak et al. 2004).
Finally, the potential of the soil to sorb P, over time, will assist in marsh recovery
calculations and establish concentration levels, at which P will be a significant source.
As water flows from the RWMA into the central Everglades, high outflow P
concentrations have the potential to impact P-loading limits at main pumping structures,
specifically the S-8 structure located on the Miami Canal. This structure pumps water
into northern WCA-3A, part of the EPA, affecting marsh areas further downstream.
Currently, the RWMA soils still experience excessive, dry season soil drying and
oxidation despite receiving STA discharges. Therefore, in its current state and
operations, and based on soil nutrient data previously collected, the RWMA soils can
potentially act as a source of P to downstream areas. The purpose of this study is to
determine if the RWMA soils will act as a P source or sink to downstream areas as well
as ascertain the potential of the soil to sorb additional P flowing in from STA-5.
By establishing the amount of P flux under dry vs. flooded soil conditions, in
addition to the amount of P which can be sorbed by the soils, changes to system
operations can be evaluated. For instance, if significantly more P fluxes from dry soils
than flooded soils, retaining water within the system, by altering inflow and or outflow
operations, may suppress significant P-fluxing and allow for biological interactions.
Through this adaptive management approach, results from P-flux and P-isotherm
experiments will be utilized to evaluate modifications of hydrologic operations to provide
conditions that meet restoration goals. In addition, the P flux research will provide data
for the SFWMD recovery model, used in predicting ecological responses of impacted
areas of the Everglades to STA effluent, as outlined by the Long Term Plan (LTP) and
directed by the EFA.
PHOSPHORUS SORPTION AND DESORPTION CAPACITY
Soil in many parts of the Everglades, has become P enriched due to years of over
drainage and oxidation, or as a result of high P inflows from urban and agricultural runoff.
The ability of these soils to retain P and not become an internal source is an important factor
in Everglades restoration. The water column P concentration level at which no P-
sorption/desorption occurs is defined as the equilibrium phosphorus concentration (EPC)
(Richardson and Vaithiyanathan 1995; Nair et al. 1998; Pant et al. 2001). Ascertaining the
EPC of the soil can be accomplished through the use of P-isotherms run under laboratory
conditions from samples collected at desired field locations. To define the maximum P-
sorptive capacity of the soil, isotherms are performed utilizing various solution P
concentrations, based on site specific soil P data. Results can be used to determine whether a
soil will flux P into the overlying water column based on P concentrations of inflow water.
Phosphorus retention in the soil is affected by several factors such as historic P loading,
existing P levels of the soil and incoming water column P concentration levels. In addition,
several biological factors, algae and macrophytes, uptake P from the water column and soil,
respectively (Bostrom et al 1988; Gale et al 1994). The ability of the soil to release sorbed
P or retain P, is dependent upon antecedent soil conditions, soil chemistry and, P adsorption
capacity of the soil (Zhou and Li 2001), all of which directly effects ecosystem restoration.
For example, if water column P levels from inflows are reduced below EPC levels, P will
desorb from soil and flux into the water column.
Soil chemistry primarily affects the solubility of P and influences P-sorption. In
studies by Khalid et al. (1977), Richardson (1985) and Sallade and Sims (1997), aluminum
(Al) and iron (Fe) content in the soil significantly affected P solubility. Furthermore,
oxidation of the soil, promoted by prolonged marsh draw downs, affects the types of Fe
(ferrous to ferric) available to bond with P. Under oxidative conditions, soils containing high
Fe content can regulate P-sorption (Reddy et al. 1998). These inorganic crystalline forms of
Fe-P can become soluble upon flooding versus organic forms, thereby releasing inorganic
bioavailable P (Patrick and Khalid 1974; Pant et al. 2002). Specifically, high Al/Fe content in
the soil can affect inorganic phosphorus (iP) species formation. High concentrations of Al
and Fe oxides will bond with P forming inorganic Fe-P and Al-P bound compounds (Sallade
and Sims 1997).
Generally wetland soils assimilate iP in greater concentrations than organic P (oP)
(Pant and Reddy 2001) and depending on soil chemistry, hydrology and biological factors,
can store iP more readily than oP. In addition, P storage pools in the soil can affect P-
sorption and solubility, based on predominately inorganic or organic forms. Ascertaining P
species (bioavailable) retained or released from the soil is ecologically important to wetlands
with respect to eutrophication. Depending on P-species in the water column, antecedent soil
conditions and oxidation, oP or iP can be released.
The objectives of this study were to 1) establish the EPC of the soil spatially from
inflow point across the marsh to outflow and 2) measure the maximum P-sorptive capacity of
the soil when exposed to low P and high P treatments.
Phosphorus isotherms were run on composite soil samples, collected from the RWMA
along an existing representative transect (RC) of soil, vegetation and hydrologic conditions,
located in the northern portion of the marsh, to determine the EPC of the soil and maximum
P adsorptive capacity of the soil. The 0-2 cm depth was collected being the most reactive
soil layer with the overlying water column, based on current soil nutrient data, containing the
highest P concentration. Three replicates of the top 0-2 cm layer of soil were combined into
one composite sample per site, representing each of the four sites (RC-1, RC-2, RC-3, RC-4).
The RC transect runs from west to east, the direction of water flow, with RC-1 located near
the western side (inflow) of the RWMA adjacent to the inflow structure (G-410 pump), and
RC-4 located near the eastern side (outflow), adjacent to the outflow structure (G-402 C, a
gated box culvert).
Each sample was collected using a 2 cm deep, 10.16 cm diameter wide coring ring.
Prior to field sampling all plant material was removed. Samples were then placed in plastic
bags, stored in a cooler on ice and transported back to the lab. All soil was spread out in a
thin layer on individual trays and air dried at room temperature (25.50 C), for approximately
five weeks. Prior to drying the soil, all plant material and small stones were removed. Then,
the soil trays were covered with screen to prevent contamination and, as seedlings emerged
from the soil, they were removed. Once dry, the soil was ground, passed through two sieve
sizes (2 mm and 0.88 mm), then ground and sieved again. Any remaining material which
could not pass through the sieves was discarded.
Initially six P concentrations plus a control (0 control, 10, 30, 50, 100, 500 and 1000
[tgL-1) were used to produce P-isotherm curves for each site, to determine the EPC and the
potential maximum P-sorptive capacity of the soil. Therefore, each site consisted of seven
treatments, with three replicates per treatment, for a total of 84 initial samples along the RC
transect. Three grams of soil were weighed and placed into 50 ml centrifuge tubes then
spiked with 50 mM KCL solution. Each spike contained one of the seven P concentrations
prepared from 1000 ppm standard stock solution, derived from dissolving 1.0985 g of
potassium dihydrogen phosphate (KH2PO4) into 100 ml of D.I. water, contained in a 250 mL
class A volumetric flask, then diluting with D.I. to the mark and mixing. Two sub-stock
solutions, 5 mgL-1 and 50 mgL1, were prepared from an original 1000 mgL1 solution to
spike each of the seven treatment concentrations. The KCL-P solution treatments were
prepared by measuring out 1.863 g of KCL dissolved into 200 ml of D.I. water within a 500
ml class A volumetric flask, spiked with the appropriate P sub-stock solution, then filled to
the mark with D.I. and mixed, yielding a total of 500 ml each of 0 (control no P additions),
10, 30, 50, 100, 500 and 1000 tgL1 P-treatment solutions. Centrifuge tubes were then
placed on a shaker table set at 180 rpm for 24 hours, to ensure complete mixing of soil and
solution. After shaking, tubes were centrifuged for 15 minutes at 22 C at 4000 rpm, and
approximately 5 8 ml of the supernatant was extracted using a transfer pipette. The
supernatant was then filtered through a 0.45 [tm filter. Each sample was measured for
soluble reactive phosphorus (SRP) by the SFWMD Water Quality lab using standard
methods (SM4500PF, Clescerl et al 1999) and SFWMD methods (SFWMD 3080.1 Rev.
Ground soils used for P isotherms were sub-sampled and submitted to DB
Environmental Labs, Rockledge, Florida for analysis of oxalate extractable Iron (Fe) and
Aluminum (Al) analysis using standard methods EPA 236.1 and EPA 202.1, respectively.
Phosphorus sorption data was calculated and fit using the linear, Freundlich and
Langmuir isotherm equations (Reddy et al. 1998; Zhou and Li 2001). The linear equation
was used to fit low P concentration results, while the Freundlich equation fit low to medium
P concentrations, and the Langmuir equation is applied to high P concentrations (Zhou et al.
Linear isotherm: S = KC So
S = the amount ofP sorbed in the solid phase, [tg g-1
K = P-sorption coefficient
C = P concentration in solution after 24-h equilibration [agL-1
So = constant, amount of originally adsorbed P
In addition, the equilibrium phosphorus concentration (EPC), which is defined as the
concentration of P in a solution when neither sorption or desorption is occurring, is equal to
C when S = 0 (Pant et al. 2002). Therefore the EPC was calculated from regression statistics
data as the intercept (point at which a line intersects the y-x-axis) divided by the x-variable.
Freundlich isotherm: S = KCn
S as above
Langmuir isotherm: C/S = 1/(bSmax) + C/Smax
Smax = P sorption maximum atg g -1
b = a constant related to bonding energy
C/S = as defined earlier
All three isotherm equations (linear, Freundlich and Langmuir) were applied to the
initial P-sorption results measured from each site. For each site, P-sorption data remained
linear (Figure. 2-1 a-d), with no inflection point indicating the P-concentration in which the
maximum P sorptive capacity of the soil was reached. Therefore, the linear isotherm
equation best describes the low P-concentrations of all sites, supported by high regression
(R2) coefficients (Table 2-1). Additionally, EPC of the soil for each site was calculated and
results reported in Table 2-2. Sites RC1 (inflow) and RC4 (outflow) had similar EPC
measurements of 94.5 and 93.2 tgL-1 respectively. The RC3 site, which is in the marsh
interior, was lower at 73.6 tgL1, though not significantly. Conversely, the RC2 site was
significantly lower (P = 0.05) than all other sites measuring 18.8 ugL1.
To ascertain the potential maximum P-sorptive capacity of the RWMA soils, additional
P-isotherms of 2000, 5000 and 10,000 [tgL-1 were run and the data added to the initial results.
The Freundlich and Langmuir equations were then applied to determine the best fit according
to R2 values (Table 2-1). While both equations fit well, according to R2 values, the
Freundlich equation is a better fit for the higher P-concentrations (Figure 2-2 a-d). P-
sorption parameters measured from each treatment concentration at each site are reported in
Table 2-2. The isotherm inflection points indicate the P-concentration at which the soil is
reaching its maximum sorptive capacity. Because inflection points (line begins to curve) on
each graph for each site is not well defined, estimates of the P-sorptive capacity of the soil is
approximated as: RC1 40-60 ug g-l, RC2 40-60 ug g-l, RC3 ~ 40-60 ug g-l, RC4 ~ 40-60
ug g .
0 20 40 60 80 100 120 140
Equilibrium P Cone (pig/L)
0 5 10 15 20 25 30 35
Equilibrium P Conc. (pg/L)
Figure 2-1 Linear isotherm result for the RC transect depicting the equilibrium phosphorus
concentration (EPC) on the x-axis and the amount of P-sorbed on the y-axis. The
line is the best fit for all values which shows EPC is actually a range of values.
A) RC1 site. B) RC2 site C) RC3 site D) RC4 site.
20 40 60 80 100 120 140
Equilibrium P Conc. (iLg/L)
20 40 60 80./ 1t0 120 140 160 180 200
Equilibrium P Conc. (iLg/L)
Figure 2-1. Continued.
Current soil nutrient concentrations for each site are reported in Table 2-3. All P-isotherm P-
concentration measurement data used to calculate each isotherm equation and associated plot
Table 2-1. List of regression (R2) coefficients used to determine which isotherm equation,
linear, Freundlich or Langmuir, best fit the P-sorption data.
Linear,R2 Freundlich,R2 Langmuir,R2
RC1 0.930 0.990 0.880
RC2 0.900 0.980 0.830
RC3 0.980 0.990 0.980
RC4 0.920 0.980 0.840
Table 2-2. P-sorption parameter results for the linear, Freundlich and Langmuir equations for
equilibrium phosphorus concentration (EPC), Kf- a constant in the Freundlich
equation, Cn measure of P concentration in solution after 24 hours, Smax the P
sorption maximum and b a constant in the Langmuir equation.
Linear Freundlich Langmuir
EPC, [tgL-1 Kf Cn Smax u 1 b
RC1 94.5 0.469 0.858 397.7 0.0007
RC2 18.8 1.49 0.853 268.4 0.0041
RC3 73.6 0.506 0.78 206.4 0.0009
RC4 93.2 0.201 0.848 189.5 0.0006
concentration measurement data used to calculate each isotherm equation and associated plot
data is listed in Appendix 2-1.
Results of oxalate-extractable Al (Figure 2-3) and oxalate-extractable Fe (Figure 2-4)
were variable with no apparent spatial trends, therefore results are combined. The isotherm
soils were composite samples collected from the field and not replicates. Due to the single
measurement per site, statistical differences between sites could not be established, and are
used to characterize soil chemistry. Concentrations of Fe and Al were high compared to
other Everglades marsh areas. The average Fe content for the RC transect in RWMA is 4950
275 mg/kg compared to Fe content in WCA2A at 344 37 mg/kg (SFWMD, Everglades
Threshold data) and Loxahatchee National Wildlife Refuge (LNWR) of 339 57 mg/kg
(SFWMD, Everglades Threshold data). Average Al content is 2450 + 515 mg/kg for the RC
transect in RWMA, 248 13 mg/kg in WCA2A (SFWMD, Everglades Threshold data) and
100 200 300 400 500 600 71
Equilibrium P Cone (pg/L)
Equilibrium P Cone (pg/L)
Figure 2-2 Freundlich and Langmuir isotherms for the RC transect depicting which equation
best describes the P-sorption maximum. A) RC1 site. B) RC2 site. C) RC3 site.
D) RC4 site.
Figure 2-2. Continued.
Equilibrium P Cone (pg/L)
400 600 800 1000 1200 1400 1600 1E
Equilibrium P Cone (pg/L)
Figure 2-3. Oxalate-extractable Al composite samples collected at the sites along the RC
transect and therefore are not replicates so error bars are not shown.
Figure 2-4. Oxalate-extractable Fe composite samples collected at sites along the RC transect
and therefore are not replicates so error bars are not shown.
In LNWR, 353 + 51 mg/kg (SFWMD, Everglades Threshold data). For comparison
purposes, the results were corrected for bulk density and compared to soil from LNWR and
WCA-2A (Table 2-4).
Table 2-3. RWMA soil nutrient concentrations for each site in the RC transect. Data
collected by the SFWMD, Downstream Monitoring and Research Program.
mg/kg TN, mg/kg TC, mg/kg Bulk Density % Ash
RC1 741 53 34066 2140 465333 9135 0.16 0.015 16.46 2.22
RC2 740 79 33000 643 454333 2333 0.16 0.012 17.1 0.58
RC3 547 44 33300 1007 449666 5696 0.20 0.015 20.96 0.86
RC4 694 18 32000 557 468666 1764 0.17 0.003 14.63 0.26
Table 2-4. Comparison of oxalate extractable Fe and Al content is soils, corrected for bulk
density, from the RWMA, LNWR and WCA-2A. Data reported for LNWR and
WCA-2A from the SFWMD, Everglades Threshold Program.
Bulk density, Ox-Fe, Ox-Al,
g/cm3 g/m3 g/m3
RWMA 0.21 1039.5 514.5
LNWR 0.06 20.34 21.18
WCA-2A 0.068 23.39 16.86
The inflow (RC 1) and outflow (RC4) sites had similar EPC values, as did the RC3 site,
while the RC2 site was significantly lower from the other three sites. Topographic elevations
indicate the RWMA resembles a saucer, with the western and eastern edges of the marsh
higher in elevation than the interior, suggesting the two outer sites (RC1, RC4) are drier and
oxidize for longer periods of time than the interior sites (RC2, RC3). While soil TP data (~
640 mg/kg) of the top 2 cm of soil is consistent throughout the RWMA (SFWMD,
Downstream Monitoring and Research Program), hydrology may have a significant influence
on soil conditions with increased oxidation (Leeds et al. in progress).
Although the difference in EPC value at RC2 was unexpected, this site as well as the
RC3 site, is slightly lower in elevation equating to a longer hydroperiod. Additionally, the
RC2 site is closer to the STA-5 inflow point from than the other interior site, RC-3, and
maybe be affecting by increased water levels due to exposure to inflows. Therefore, the soils
do not dry out and oxidize as frequently, suppressing the formation of soluble P. The EPC
results of all transect sites reveals the heterogeneity of the soil composition. One hypothesis
for these results is, while the RC-2 site has a greater ability to sorb P, sites closer to the
inflow receiving water lower in P concentration than the soil, will flux P into the water
column. The fluxed soluble P is transported downstream to lower EPC sites, where P-
sorption will occur, thereby increasing the soil P concentrations. Over time the localized P-
flux and sorption cycle may eventually reach equilibrium. However, ecological impacts
could be significant if equalization is due to, high P inflows and continued oxidation and
In comparing the RWMA P-isotherm data to other Everglades areas, EPC is correlated
with P-enrichment of the soil (Richardson and Vayithianythan 1995; Zhou et al. 1997). In
WCA-2A, Richardson and Vayithianythan (1995), using both the linear and Langmuir
models, showed a linear decrease in EPC, from enriched to un-enriched soil EPC values
along an existing P gradient. Additionally, they had reported higher EPC values at inflow,
which contains an approximate soil TP concentration of 1500 mg/kg (SFWMD, Everglades
Marsh Ecology Program) versus the RWMA TP soil concentrations at 640 mg/kg.
Alternatively, they predicted maximum P-sorptive capacity to be 700 mg/kg, where as I
estimated the RWMA maximum P-sorptive capacity closer to 40 60 mg/kg, based on the
amount of P sorbed at 80 tgL-1. In WCA2A and other areas of the Everglades, P-sorption is
controlled by calcium (Ca) and may have a higher sorption potential than Fe as it is not
influenced by redox. Furthermore, the difference between these two studies could be the
higher TP soil enrichment in WCA-2A which, increases P-sorption, or could be due to a
difference in available cations in the soil. When RWMA isotherm results are compared to
un-enriched (< 650 mg/kg) southern Everglades soil (Zhou and Li, 2001) where EPC values
(0.002 0.010 tgml 1), maximum P-sorptive capacity ranges from 1780 2575 tg g-1 and
soil TP concentrations are low, the RWMA soil had a much lower maximum P-sorptive
capacity, 40 60 tg g1.
When corrected for bulk density, the Fe and Al content in the RWMA soil is high
compared to other Everglades marsh areas regardless of TP content. While both Fe and Al
concentrations are elevated, the Fe content is higher than Al concentrations, suggesting Fe
could be the primary regulator of P sorption and Al a secondary regulator (Zhou et al 1997;
Pant et al. 2002), as there are more available cations for P to bind to. Similar studies by
Reddy et al. 1998 and Nair et al. 1998, reported P retention was strongly correlated to Fe and
Al content present in the soil. Furthermore, Khalid 1977, stated under reduced conditions, Fe
and Al were primary factors in P retention in flooded soils, and anaerobic conditions will
increase P solubility (Gale et al. 1994).
Therefore, soils that are highly enriched, such as in WCA-2A, are not able to sorb as
much P from the water column, possibly because they have reached a P-saturation point.
These soils have the potential to act as an internal source of P, if inflow water to the marsh is
low (below EPC). In marsh areas similar to the RWMA, where EPC is variable and max P-
sorptive capacity is less than highly enriched areas, the potential exists to contain soluble P,
partially due to the availability of Fe and Al cations without negatively affecting marsh areas
downstream. In addition, soil chemistry conditions, in particular the high Fe and Al content,
which may be regulating P adsorption, are influenced in the RWMA by hydrology.
Restoring or adjusting the hydrology to prevent conditions conducive to oxidation and muck
fires is a critical component to RWMA restoration.
FLUX OF BIOAVAILABLE PHOSPHORUS
Wetland soils that have undergone nutrient enrichment can become internal sources
of Phosphorus (P) and Nitrogen (N) thus affecting restoration goals. Additionally,
hydrologic conditions can exhibit a significant affect on the flux rate of these nutrients
when hydroperiods are severely altered (shortened), resulting in elevated water column
concentrations (Olila et al. 1997; Pant and Reddy 2001). Furthermore, the inorganic or
bioavailable P and N species present, plays a major role in contributing to the
eutrophication of a wetland (Moore et al. 1991). Flux of P and N is dependant upon
antecedent soil conditions as well as, the various P storage pools in the soil, and P
concentrations in the overlying water column. Nitrogen can be lost to the system through
volatilization (Reddy et al. 1996), whereas P accumulates in the soil, often resulting in a
long-term mass increase in total P. If long-term P immobilization processes are altered,
or if antecedent soil conditions are anthropogenically enriched, marsh areas that once
provided a P sink can become a source of P, especially when receiving inflow containing
low P concentrations. The internal loading potential of these enriched or hydrologically
altered soils can perpetuate eutrophication and prolong marsh recovery efforts (Pant and
The Florida Everglades has undergone anthropogenic alterations to hydrology and
water quality for over 50 years. Changes in hydrology have extensively reduced
hydroperiods, particularly in areas of the marsh cutoff from continuous water inflows,
resulting in extended periods of dry out. Urban and agricultural runoff containing high P
concentrations have artificially elevated soil P concentrations in downstream areas of the
Everglades. When soil dries out for prolonged periods, moisture level is reduced,
resulting in aerobic conditions and increased rates of organic matter decomposition. In
addition, when soil moisture content reaches 65% (on a mass basis) and less, organic soil
becomes susceptible to burning (Wade et al. 1980). During organic soil burning (muck
or peat fires), P soil storage pools can shift from predominately organic to inorganic
forms (Smith et al. 2001; Newman et al. 2001), becoming available to flux out of the soil
The RWMA has been cutoff from overland flow for over 50 years, which
drastically altered the hydrology in this system. As a result, hydroperiods were reduced
to an average of three months and, during the remaining nine months, drained soil
conditions predominated. Prolonged marsh draw downs severely oxidized the soil,
leading to subsidence and muck fires. As a consequence of this altered hydrologic cycle,
soil P concentrations became elevated (Newman et al. 1998).
In July 2001, the SFWMD began utilizing STA-5 discharges to increase the
hydroperiod in RWMA, in accordance with hydropattern restoration as directed by the
Everglades Forever Act (EFA). The effect of re-flooding of these drained soils on P-
fluxing, and the movement of soluble P to downstream areas, was unknown. Therefore,
the objectives of this study were 1) determine the difference in P and N flux in dry versus
continuously flooded soils and 2) quantify differences in flux spatially.
Soil P concentrations, bulk density measurements and vegetation compositions, do
not vary significantly across the RWMA, therefore one representative transect (RC) was
chosen for the intact soil core P-flux study to characterize the soils throughout the marsh.
Four sites (RC-1, RC-2, RC-3 and RC-4) compose one transect running west to east with
the first, western most site RC-1, located adjacent to the inflow structure (G-410 pump)
and the eastern most site, RC-4, located adjacent to the outflow structure (G-402C gated
box culvert). The study was conducted during the dry season (December to May) for the
Everglades, hence air temperatures and day/night hours were identical to the required
environmental conditions. To alleviate the problem of soil within the intact soil core tube
pulling away from the side wall during drying and artificially increasing the surface area
in which the water column interacts, initial drying and flooding treatments were
performed in five gallon soil bucket cores. A standard five gallon bucket with the bottom
removed was used as an initial soil coring device. Two five gallon soil bucket cores were
collected from each site for dry and flooded treatments. Prior to inserting the bucket into
the soil, all vegetation was cleared then, a knife was used to severe all roots, minimizing
soil compaction. Cores were taken to a depth of 0-20/30 cm, then transferred to a
treatment bucket, by sliding the intact core out of the bottom of the soil core bucket and
into a treatment bucket. All soil cores were transferred to a SFWMD research facility
located in West Palm Beach, Fl and placed in enclosures to prevent incoming rain. Prior
to treatment, any remaining vegetation or algae was removed. One soil treatment bucket
from each site was continuously flooded with ambient marsh water, and maintained at an
approximate flooding depth of 5 cm, while the other experienced 45 days of dry down.
Seedlings were removed as they emerged during the treatment period.
During a pilot coring study it was concluded that three, 10 cm diameter soil cores
could be collected from each bucket core. Therefore, at day 45 which is an average
duration of draw down for this area of the Everglades, three replicates for each treatment,
dry and continuously flooded, were cored for a total of six intact soil cores per site and an
experiment total of 24 soil cores. The plastic coring tubes were placed on the soil surface
and any remaining root material was severed with a knife to minimize soil compaction.
The intact soil cores were removed, capped on the bottom and sealed to contain the core
and treatment. The coring tubes were then randomly placed on a table, within an open
enclosure containing a clear plastic roof to prevent rainwater from entering, but allowing
for full sunlight. Air temperature conditions at the time (February/March) of the study
ranged from daytime highs of 18.3 25.50 C, and nighttime lows of 7.2 18.3 C.
Ambient marsh water, collected from the RWMA marsh interior, was used to flood each
soil core tube from the top down in an effort to simulate the flooding processes in the
field, to a depth of 30 cm. No additional water was added to the tubes after the initial
flooding. Finally, all coring tubes were wrapped in aluminum foil to retard algal growth
with the top of each core left uncovered.
Water quality samples were extracted from each soil core tube with a syringe
inserted into the water column, and filtered as specified by lab analyte requirements, with
a 0.45 [tm filter, then placed on ice prior to submittal to the SFWMD Water Quality Lab.
Samples were analyzed for total phosphorus (TP), total dissolved phosphorus (TDP),
soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), total dissolved
Kjeldahl nitrogen (TDKN), nitrate/nitrite (NOx) and ammonium (NH4) at time zero, day
7 and day 21 according to standard (Clescerl et al. 1999) and SFWMD methods (Table 3-
1). In addition SRP and NF4 were analyzed at Ihour, 4hrs, 8hrs, 18hrs, 24hrs and days 2,
4 and 14.
Table 3-1. Water Quality standard testing methods according to Clescerl et al. 1999 and
the South Florida Water Management District water quality procedures.
Standard Methods, SFWMD, water quality
Water Quality Parameter Clescerl et al. 1999 procedures
SRP,(Orthophosphate, OPO4) SM4500 PF SFWMD 3080.1, Rev. 2.2
Total Phosphate, TP SM4500 PF SFWMD 3100.1, Rev. 3.0
Total Dissolved Phosphate, TDP SM4500 PF SFWMD 3100.1, Rev. 3.0
Ammonium, NH4 SM4500 NH3H SFWMD 3060.4, Rev. 1.0
Nitrate/Nitrite, NOx SM4500 NO3F SFWMD 3060.4, Rev. 1.0
Total Kjeldahl Nitrogen, TKN EPA 351.2 SFWMD 3070.1, Rev. 2.0
Total Dissolved Kjeldahl Nitrogen,
TDKN EPA 351.2 SFWMD 3070.1, Rev. 2.0
From the water quality results, the different forms of phosphorus P species present
in the water was calculated, then compared with field water quality data for similar
concentration percentages and P availability for algae and plants. The dissolved organic
phosphorus (DOP) concentration that fluxed into the overlying water column was
calculated by subtracting OPO4 from TDP. In addition, the organic phosphorus (oP)
concentration present in the water column was calculated by subtracting SRP from TP.
Finally water column particulate phosphorus (PP) was calculated as TDP subtracted from
Algal growth occurred in the dry soil core tubes after approximately 168 hours as a
floating mat, which gradually became thicker until conclusion of the experiment, at
which time the majority of the mat sank to the soil surface. The remaining floating mat
was skimmed off the top of the water, the soil core dropped out of the bottom of the tube,
and the remaining algae attached to the tube wall, scraped and added to the sample. No
soil was included in the algae samples, which were sent to DB Environmental Labs,
Rockledge, Florida for TP analysis.
Water quality data from the intact soil cores was analyzed using SAS JumpTM
version 5, SAS Institute Inc., Cary NC. A two-way, repeated measures ANOVA was run,
to determine significant differences for each analyte, temporally and spatially and
treatment, drained and flooded. A significance level of (a) 0.05 was used for all analyses.
Fluxing rates were calculated based on steepest curve at time 1, 4 and 8 hours.
Drained versus continuously flooded soils were compared for significant
differences in flux for all water quality parameters, at each site along the RC transect.
The dry soils exhibited higher flux concentrations across all sites and analytes compared
to flooded soils. In dry soil, SRP flux was significantly higher (P<0.05) than flooded soils
at all sites (Figure 3-1 a,b,c,d). At the RC1 (inflow) and RC4 (outflow) sites, dry soil
SRP concentrations were greater than the internal marsh sites RC2 and RC3.
Additionally, time 0 results of dry and flooded soils were similar until the one hour time
mark, at which point concentrations between the two treatments were significantly
different until the 168 to 212 hour time mark. The exception is RC3 which peaked at
four hours, then water column P concentrations declined at the eight hour time mark to
initial P concentration levels at time 0 which, is also similar to flooded water column P
concentrations. Final concentrations measured at time 380 for the RC1 dry soils were
significantly lower (P = 0.03) than the beginning time 0 concentrations. Fluxing rates,
based on steepest curve, were greatest at the RC4 and RC1 sites respectively, than RC2
0.2 + RC1-D
0.15 RC -F
0 50 100 150 200
0.15 -a- RC2-F
0 50 100 150 200
0.05 --- RC3-D
0 50 100 150 200
Figure 3-1 SRP water column concentrations in mg/L measured in dry vs. flooded soil
core tube treatments at a)RC 1 site. b)RC2 site. c)RC3 site. d)RC4 site. All
scales are the same with the exception of (c) the RC3 site, which was re-
scaled to display the slope of the initial flux.
0.2 -- RC4-D
0.15 -- RC4-F
0 ,3------1,4 ------ n
0 50 100 150 200
Figure 3-1. Continued.
and RC3 (Table 3-2). Flooded RC1 soils exhibited no significant changes in SRP
concentrations measured in the water column throughout the experiment. Total
phosphorus (TP) measurements for dry and flooded soils at time 0 were not significantly
different, however final concentrations at time 380 hours were significantly different for
all sites (Figure 3-2 a,b,c,d). Total phosphorus trends for concentrations were different
between dry and flooded soil cores, as dry soil TP concentrations increased with time and
flooded TP concentrations decreased with time. Total dissolved phosphorus (TDP)
exhibited a similar trend at all sites for both dry and flooded soils, with the highest
concentration measurements at timel68 hours then decreasing in concentration levels at
time 380 to time 0 concentration levels (Figure 3-3 a,b,c,d).
Table 3-2. Fluxing rates of SRP in mgL^-/hr for dry soil cores at each transect site based
on steepest curve for time 1, 4 and 8 hours.
time, RC1-D, RC2-D, RC3-D, RC4-D,
hours mgL1 /hr mgL^-/hr mgL^/hr mgL^-/hr
1 0.090 0.076 0.053 0.106
4 0.056 0.043 0.022 0.057
8 0.052 0.033 0.013 0.046
For all sites Ammonium (NH4) fluxed out of the dry soil treatments at significantly
greater concentrations (P<0.05) than the continuously flooded soil from the 4-8 hour time
to 168 and 212 hour measurements (Figure 3-4 a,b,c,d). Additionally, final
concentrations measured at time 380 returned to similar concentrations measured at time
0. All flooded soils released a small amount NH4 into the water column within the first
eight hours then concentrations appeared to reach equilibrium between the soil and
overlying water NH4 concentrations at the final time of 380 hours. The RC1 inflow site
dry soil treatment NH4 concentrations were significantly less (P < 0.05) than the RC2,
0.25 -- RC -F
0 100 200 300 400
0.3 --- RC2-F
o 0.2 -
0 100 200 300 400
Figure 3-2 TP water column concentrations in mg/L measured in dry vs. flooded soil core
tube treatments at a)RC1 site. b)RC2 site. c)RC3 site. d)RC4 site.
0 100 200 300
Figure 3-2. Continued.
0 100 200 300
-- RC -F
0 100 200 300
0 100 200 300
0 100 200 300
0 100 200 300
Figure 3-3. TDP water column concentrations in mg/L measured in dry vs. flooded soil
core tube treatments at a)RC 1 site. b)RC2 site. c)RC3 site. d)RC4 site.
0 50 100 150
Figure 3-4 NH4 water column concentrations in mg/L measured in dry vs. flooded soil
core tube treatments at a)RC1 site. b)RC2 site. c)RC3 site. d)RC4 site.
n n -n-
4 -- RC3-F
0 n n n -
0 50 100 150 200
0 -- n n n 1 3 -
0 50 100 150 200
Figure 3-4. Continued.
RC3 and RC4 sites. The additional nitrogen parameters measured in the water
quality samples, Total Kjeldahl Nitrogen (TKN), Total Dissolved Kjeldahl Nitrogen
(TDKN) and Nitrite/Nitrate (NOx) results are listed in Appendix 3-1.
Finally, for each parameter and soil treatment, results of all drained treatments and
flooded treatments were compared between sites, within the transect, for significant
differences in concentration. Significantly higher concentrations of SRP (P < 0.05)
occurred during peak flux (time 1 hr to time 8 hr) in the drained soils between RC1 and
RC4 (inflow and outflow sites respectively) versus RC2 and RC3 (interior transect sites)
(Figure 3-5). However, there were no significant differences between sites in dry or
flooded soils, for TP or TDP and SRP. Conversely, results in dry soils for NH4, at the
RC1 site had significantly (P < 0.05) lower concentrations (P < 0.05) compared to RC2, 3
and 4 (Figure 3-6). However, there were no significant differences between sites in either
dry or flooded soils for TKN, TDKN and NOx as well as NH4 for flooded soil.
0.1 ---- RC4-D
0 50 100 150 200
Figure 3-5. SRP water column concentrations measured in mg/L in drained soil core
treatments all sites RC1, RC2, RC3 and RC4. The RC1 and RC4 sites fluxed
at higher rates and greater concentrations of SRP than the interior marsh sites,
RC2 and RC3.
0 100 200 300 400
Figure 3-6. NH4 water column concentrations measured in mg/L in drained soil core
treatments all sites RC1, RC2, RC3 and RC4.
Dissolved organic phosphorus, which fluxed from the soil into the overlying water
column, exhibited a similar increase in concentration with time in the flooded soils
(Figure 3-7 a,b,c,d). Flooded soil water column DOP measured at the RC1 site was
significantly (P=0.05) higher than the dry soil treatment. Drained soil DOP
concentrations in the water column remained constant throughout the experiment.
Alternatively, water column oP (Figure 3-8) and PP (Figure 3-9) concentrations within
drained soil significantly (P=0.05) increased over time, while flooded soil water column
oP and PP concentrations steadily decreased over time. When calculating percentages of
oP and iP fluxing concentrations from the soil to the water column spatially and
temporally (Table 3-3), two separate patterns emerge. At time 0, in both dry and flooded
soil cores, the percentage of oP fluxing from the soil measured in the water column
samples was greater than iP. Over the course of the study this trend remained, becoming
significant (P = 0.05) at time 168 and 380 hours, with a majority of the P measured in the
a 2000 RC1-D
W 1500 -
0 100 200 300 400
Figure 3-7 Dissolved organic phosphorus (DOP) water column concentrations in ig/L,
calculated in dry vs. flooded soil core treatments for a)RC1 site b)RC2 site.
c)RC3 site d)RC4 site.
b + RC2-D
0 100 200 300 400
0 100 200 300 400
0 100 200 300 400
Figure 3-7. Continued.
0 100 200 300 400
0 100 200 30(
0 100 200 30
Figure 3-8 Organic phosphorus (oP) water column concentrations in ig/L, calculated in
dry vs. flooded soil core treatments for a)RC1 site. b)RC2 site. c)RC3 site
0 0.06 1
0 100 200 300 4C
Figure 3-9 Particulate phosphorus (PP) water column concentrations in tg/L, calculated
in dry vs. flooded soil core treatments for a)RC 1 site. b)RC2 site. c)RC3 site
t7 ( 1I
water column as organic. Inorganic P percentages decreased over time in dry soil cores
and either remained the same in flooded cores or increased, though not significantly.
Algae began growing in the drained soil treatment core tubes at time 168 (-1 week)
and continued through out the remainder of the experiment, while no algae was present in
the continuously flooded treatment core tubes. The algae samples that were sent off for
TP analysis, exhibited sufficient variability among replicates and between cores, so
results were not significantly different (Table 3-4), and therefore were combined into one
average concentration. The average algal TP tissue concentration from the soil core tubes
(1183 195 mg/kg) were compared to average algal TP tissue concentrations reported at
RWMA field sites (999 + 100 mg/kg), representing an average of samples collected prior
to and after STA-5 discharges. The differences in TP concentrations from the field sites
versus the soil cores were not significant.
Table 3-3. Percentage of oP and iP fluxing from the soil into the water column over time
Time 0 hours
RC1-D RC1-F RC2-D RC2-F RC3-D RC3-F RC4-D RC4-F
%oP 48.13 79.63 56.98 85.06 80.14 86.73 72.18 70.00
%iP 51.87 20.37 43.02 14.94 19.86 13.27 27.82 30.00
Time 168 hours
RC1-D RC1-F RC2-D RC2-F RC3-D RC3-F RC4-D RC4-F
%oP 89.11 85.44 87.48 85.29 86.96 81.54 76.22 67.80
%iP 10.89 14.56 12.52 14.71 13.04 18.46 23.78 32.20
Time 380 hours
RC1-D RC1-F RC2-D RC2-F RC3-D RC3-F RC4-D RC4-F
%oP 90.12 76.47 90.71 75.76 93.22 54.76 90.51 60.00
%iP 9.88 23.53 9.29 24.24 6.78 45.24 9.49 40.00
Table 3-4. Algae TP concentration measurements in mg/kg collected from drained soil
core tubes at the conclusion of the study.
Site TP, mg/kg
RC1 1116.3 157.8
RC2 1230 101.4
RC3 1015.6 207.3
RC4 1370 316.6
In the RWMA two primary soil conditions exist, drained and flooded. Although
hydropattern restoration has been implemented (July 2001), hydroperiods had not been of
sufficient length to prevent the soil from drying in the outer and northern portions of the
marsh, therefore extended dry soil conditions continued to exist at the end of the wet
season, and throughout the dry season. Soil oxidation continued to occur during these
dry times, altering the soil chemistry.
As soil oxidizes, the Fe-P forms are soluble upon re-flooding (Pant et al. 2002)
especially when high Fe concentrations are present (Gunnars and Blomqvist 1997) in the
soil such as the RWMA soils. Additionally, upon re-flooding, iP species (SRP) will flux
from the soil to the overlying water column. Those species of P that are now soluble, and
bioavailable, can be transported further downstream to areas of the marsh containing
lower soil P, resulting in sorption, thereby moving the soluble P front further downstream
into previously un-impacted areas. While Al-P is not soluble upon re-flooding, the high
concentrations of Al ions are available to bind with P.
Results from the RWMA intact soil core experiment suggest that, drained soil
conditions significantly increase the flux of SRP, TP and TDP to the water column
compared to continuously flooded soils. Total phosphorus of drained soils, both
inorganic and organic forms, began at time 0 with similar concentration levels. Despite
some transect sites having different flux patterns, each drained soil core had significantly
increased P concentrations at the final sample time compared to the flooded soil. In fact,
the flooded soils appeared to release released very little phosphorus into the water
column and quickly reaching an equilibrium.
For all sites, the drained soil cores, with the exception of the RC3 site which is
subject to slightly longer hydroperiods, fluxed significant concentrations of bioavailable
P (SRP) into the water column. In the initial hour of the study, the RC1 and RC4 sites
had higher flux rates compared to the RC2 and RC3 sites, suggesting the soils may
experience longer drying and oxidation, as the sites are located near the edge of the marsh
based on the difference in elevation reported on the RWMA 2005 topographic survey and
measured water depths at each site. However, by the final sample event, drained soil
water P concentrations were similar to those measured in the flooded soils. One
explanation for the decrease in bioavailable P concentrations in the drained treatment soil
cores could be from the algal growth, which began approximately one week after re-
flooding. By the conclusion of the study, all drained soil cores contained substantial
amounts of filamentous green algae. However, there was no algal growth in the
continuously flooded soil cores. Algae uptakes P directly from the water column, which
appeared to influence the SRP concentrations measured in the water quality samples,
after peaking at day 8 when the algae first appeared. Analysis of the P concentration of
the algae tissue samples were representative of P concentrations measured at the RC
transect sites and indicative of high nutrient soils. Under field conditions, soluble P
fluxing from the soil into the water column may be taken up by the algae, prior to low P
soil areas adsorbing the P into the soil. Similar results by Newman et al. 2004, reported a
rapid uptake of P by periphyton prior to soil adsorption. At the conclusion of the P flux
study, SRP levels in dry soil cores were similar to flooded soils.
Calculations of the iP and oP percentages that fluxed showed a greater percentage
of oP fluxed out of the soil throughout the study. However, when comparing drained vs.
flooded percentages over time, flooded oP is initially higher then decreases over time,
with increasing percentages of iP measured in the water column. The opposite trend was
observed in the dry soil treatments, in which initial iP percentages measured in the water
column were higher then decreased over time, until oP percentages were significantly
greater at the conclusion of the study. P-fractionation data in the 0-2 cm layer collected
by the SFWMD Downstream Monitoring and Research Program shows an equal amount
of P in the soil is stored as organic P and inorganic P (Leeds et al. in progress). However
in drained soils, such as those present in the drained soil core treatment, there may be a
certain amount of mineralization occurring. A majority of the P is stored in the soil in
inorganic forms, possibly explaining the large flux in SRP as more forms of iP are
available to flux. In addition, water entering the RWMA from STA-5 is approximately
44 % oP and 56 % iP (SFWMD, Downstream Monitoring and Research Program Water
Quality data). This P fractionation characteristic of inflow water may be providing an iP
source to bond with the potentially available Fe and Al content in the soil although, this
was not specifically addressed by this study. Results by Eckert et al. (1997) and Novak et
al. (2004) reported the soil in these studies that were high in iP (Fe-P and Al-P), will flux
high concentrations of iP species when water column P is less than soil porewater P.
Spatially, P flux in the intact soil cores in both the drained and flooded treatments
was fairly uniform in concentration across the transect, with the exception of the RC3
site, which is located in the interior of the marsh. The RC3 site experiences a longer
hydroperiod, possibly due to its location near the center of the marsh, and lower
elevation, (RWMA Topographic elevation survey 2004, SFWMD) relative to the other
transect sites. According to previous fires mapped by the FFWCC and SFWMD, the
RC3 site has not experienced the frequent muck fires compared to other transect sites,
therefore soil conditions may have been more stable, relative to sites experiencing shorter
hydroperiods and more frequent fire disturbance. However, soil TP concentrations are
similar to the other transect sites, suggesting hydrology may be the driving factor in soil
Nitrogen flux trends were similar to P flux results. Total Kjeldahl Nitrogen and
TDKN concentrations were significantly higher in water samples collected at the final
time in drained soils compared to flooded soil, which maintained a relatively constant
concentration. Furthermore, bioavailable N in the form of NH4, showed significantly
greater initial flux in dry soils, although by the end of the study, water column
concentrations were similar to the flooded water samples. The RC1 site, near the inflow
point, fluxed the lowest concentration ofNH4 relative to other sites across the transect.
This is maybe due to the vegetation composition of the site taking up the available N,
where sawgrass and cattail in this area are very tall and dense when compared to sites
with lower soil nutrient concentrations (Miao and Sklar 1998). Moreover, this site has
experienced a higher frequency of muck fires, resulting in an increased volatilization of
NH4 in the soil (Smith and Newman 2001). Also, RC1 under goes longer periods of
oxidation, due to the slight increase in elevation (RWMA Topographic elevation survey
2004, SFWMD) when compared to the other sites, allowing the soil to dry for longer
periods of time.
Intact soil cores are routinely used to quantify approximate P fluxing in the field
without conducting in situ measurements which may not be feasible due to transportation,
sampling and budget constraints. Though the soil cores contain a small surface area, no
continuous water flow or plants, they give a good indication of the potential flux rate
occurring in the field at the soil water column interface. In addition, sampling can be
conducted at predetermined time intervals, capturing initial flux upon re-flooding of the
soil versus the logistics of timing re-flooding in the field. Furthermore, flux data can be
incorporated in calculating marsh recovery and determining the affects altered hydrology
has on the potential of soluble P movement further into the marsh interior.
The Rotenberger Wildlife Management Area, part of the northern Everglades, can
be monitored for phosphorus (P) inflow and outflow concentrations, as well as controlled
hydrologically at inflow point, by a point source discharge pump, and outflow through
weir structures. Therefore, determining the potential of P to flux from the soil to the
overlying water column, thus moving P further downstream in the RWMA and
Everglades system, is a critical factor in determining the operations of the system. For
example, results from the P-isotherm experiment suggest water column P concentrations
below 90 pgL'1 may cause P to flux out of the soil. In addition, the intact soil core study
indicates antecedent drained soil conditions upon re-flooding, will potentially flux
significantly greater concentrations ofP to the water column, versus soil that is
continuously flooded. Based on this, I recommend adjusting hydrologic operations of the
outflow structures, to retain water in the system upon re-flooding. This provides
conditions to increase the hydroperiod, thus reducing the potential for over draining and
oxidation of the soil and suppress the flux of soluble, inorganic P into the water column.
Furthermore, maintenance of flooded soils allows for significantly lower concentrations
of inorganic P flux based on calculated iP and oP percentages in the drained and flooded
water column samples, thereby reducing the potential for RWMA soils to be a source of
soluble P to downstream Everglades marshes.
The P-isotherms, conducted on RWMA soil, displayed some spatial variability in
EPC and P-sorption, not evident in soil TP concentrations. RWMA soils ability to sorb P
is much less when compared to the max P sorption of soil along the P-gradient in WCA-
2A (Richardson and Vaythianythan 1995) despite using similar methods analytical
methods. In addition, the systems are different with respect to historic P-loading. WCA-
2A became enriched due to high P inflows instead of altered hydrology, oxidation, and
muck fires, which were responsible for elevating P concentrations in RWMA soils. This
difference may explain the isolated transect site in this study, where EPC results were
lower than other transect sites. For example, the high nutrient inflows in WCA-2A were
fairly uniform versus RWMA where geographic location of muck fires and topographic
variability of soils that dry and oxidize are primary factors. Consequently, P-sorption is
uniquely dependant upon soil characteristics representative of each system, as well as
historic P-loading. Several studies support this conclusion (Reddy et al. 1998; Nair et al.
1998; Pant et al 2002) suggesting Fe and Al are primary regulators of P-retention when
elevated concentrations are measured. In the RWMA, Fe and Al concentrations are high
compared to other Northern Everglades marshes such as, the Loxahatchee National
Wildlife Refuge and Water Conservation Area 2A, and may be regulating P-retention in
RWMA, although this was not specifically addressed in this study. Similar P-retention
regulation, related to Ca levels, has been documented in WCA-2A soil (Richardson and
Vayithianythan 1995) and from soil located in Everglades National Park (Zhou and Li
2001) suggesting soil chemistry composition should be included in determining the soils
ability for P-sorption.
Phosphorus isotherms are applied to define soil characteristics of constructed
wetlands and in proposed constructed wetlands sites, to determine the soils suitability to
remove P. Two questions typically addressed are 1) will the soil act as a source or sink?
and 2) how much P can be retained and for how long? Furthermore, P-retention can be
influenced by the biological component present, as vegetation and the algal community
will uptake P from the soil and water column respectively. Studies by Gale et al. (1994),
Nair et al. (1998) and Reddy et al. (1998) used P-isotherms to address these questions,
concluding the EPC of the soil can predict water column concentrations in which soil will
uptake or flux P. An example of utilizing such data in the management of a system is in
the RWMA. Water inflow and outflow to the marsh is regulated by pumping structures
(inflow) and gated weirs (outflow). Therefore, upon determination that inflow TP may
cause a significant flux of P, resulting in impacts to downstream areas, outflow water can
be retained by adjusting the stage schedule controlling the outflow structures. The data
collected during this study from the P-isotherms and P-fluxing experiments can aid in
answering these questions.
There are limitations to using isotherm results and their applicability to field
conditions. P-isotherms are run under controlled laboratory conditions, eliminating much
of the inherent variability present in field conditions that would affect P-sorption or flux
i.e., the influence of the microbial and algal community, plants and changes in soil
conditions (aerobic and anaerobic). The soil used in the RWMA isotherm study was
dried, representing aerobic conditions only, in addition to being shaken for 24 hours thus
exposing all available bonding sites. Under field conditions, P would only interact with
the exposed soil layer and the zone of porewater diffusion. A study by Schlichting and
Leinweber (2002) on the effect of drying soil, prior to a sequential P fractionation scheme
compared to fresh, moist soil, reported a significant variability between the two pre-
treatment methods on the P fractionation results. Additionally, the linear, Langmuir and
Freundlich equations were initially developed for use with mineral soils rather than
organic soils, which may explain some of the discrepancy between the EPC and P-
sorption results. Consequently, isotherm data should be incorporated with other soil data
(nutrient concentrations and soil chemistry) to predict P-sorption or flux when
characterizing the wetland.
Iron (Fe) and Aluminum (Al) content in RWMA soils, when corrected for bulk
density and compared to other systems in the northern Everglades, Loxahatchee National
Wildlife Refuge and Water Conservation Area 2, are significantly higher, and may be a
significant factor regulating P-sorption in this system. Additionally, the RWMA has
experienced severely altered hydrology resulting in draw downs leading to soil oxidation.
Iron in the soil, when oxidized, forms iron oxide which is available for sorption reactions
with P in the soil. More importantly, Fe-P, a soluble inorganic P, upon re-flooding of the
soil is released into the water column and may be available for uptake. Although Ca can
also regulate P-sorption, as was determined in WCA-2A and in Everglades National Park,
both marsh areas contain higher concentrations of Ca versus Fe and Al.
In support of the role of Fe and Al in P sorption, results from Gale et al. (1994),
Nair et al. (1998) and Reddy et al. (1998) in soil studies outside of the Everglades,
concluded increased concentrations of Al and Fe are directly correlated with P-sorption.
Moreover, each found soils with elevated concentrations of Fe will oxidize upon soil
drying conditions, then upon re-flooding, form soluble forms of P. Similar results were
reported in two studies conducted (Khalid et al. 1977; Pant et al. 2001) in marshes
outside of Florida. Additionally, the RWMA has experienced severe and widespread
muck fires that may have been a factor in concentrating Fe in the soil. At the conclusion
of the intact soil core study, Fe stains were present on the sides of the cores, and during
soil cores collection in the field, oxidized rhizospheres were present. While Al is
available to bond with P, Fe has the greater potential to interact with P.
As evidenced by the results in the intact soil cores, marsh hydrology can have a
significant influence on the P-flux rate. The concentration and duration of P fluxing out
of impacted soils, despite receiving low P water, creates conditions conducive to
maintaining a eutrophic system until equilibrium is reached. Results from this study
showed bioavailable P (SRP) fluxed in significantly higher concentrations in dry soil
versus flooded soils, at each site throughout the transect. Therefore maintaining flooded
soils, through retention, would be desirable to minimize large fluxes of inorganic P
concentrations into the water column.
The hydrology in RWMA prior to receiving STA-5 discharges was severely
altered, allowing for extensive soil oxidation, soil loss and muck fires due to low soil
moisture content, all of which contributed to elevated soil P-concentrations. On a smaller
scale within the marsh, the outer edges are higher in elevation by approximately 15 cm
compared to the marsh interior. The differences in elevation allow water to pool in the
interior and dry out for longer periods of time on the edges. The differential in soil
drying is reflected in the P-flux results at the RC 1 and RC4 sites that are adjacent to the
outer edges. In dry soil treatments, the outer edge marsh sites (RC1, RC4) fluxed
significantly higher concentrations of SRP when compared to sites located closer to the
interior (RC2, RC3). Alternatively, when all sites are continuously flooded, SRP
measurements showed an initial, small concentration increase, and then reached
equilibrium for the remainder of the experiment.
Percentages of iP and oP in the water column of drained and flooded soil cores flux
upon initial re-flooding were varied. In the drained soil cores over time, iP percentages
decreased while oP percentages increased until the conclusion of the study. However, in
flooded soil cores, iP percentages and oP percentages remained relatively constant over
time with insignificant increases and decreases in composition. Organic P percentages
did not significantly change between dry and flooded treatment cores over time, but when
oP and iP percentages are compared for each treatment at all sites, the measured water
column TP contains a greater percentage of oP. One explanation for the difference in iP
versus oP percentages in the water column over time in the drained soil treatment cores
would be the algal uptake of iP, which did not occur in the flooded soil core treatments.
The same scenario is likely occurring in the natural system after drained and oxidized
soils are re-flooded. There is an initial flux of iP into the water column that is assimilated
by algal blooms. However the oP is not taken up and therefore over time, a greater
percentage of the water column TP is organic. While many studies have addressed the
affects of iP to the system ecologically, little is known how oP will impact the marsh.
While P may be fluxing out of the soil near inflow sites, it is potentially undergoing
sorption in sites located further towards the interior of the marsh, as evident by EPC
results, as well as being affected by algal and plant uptake. The outflow sites, upon re-
flooding of drained soil, are a potential source of P to downstream areas. For example,
RC1 near the inflow has an EPC approximately 94.5 [gL-1. Based on water quality data
recorded at the G-410 inflow pumping structure, P will flux from the soil near the inflow
sites. In contrast the RC2 site, which is downstream ofRC1 and measured an EPC of
18.8 [gL1, will potentially sorb P. When allowed to dry out, soil from all sites have the
potential to release bioavailable P for seven to ten days, allowing soluble P to travel
further downstream and, in the case of RC 4, out of RWMA through the outflow
structures into downstream Central Everglades areas. However, over time, if dry soils
become continuously flooded, P-flux is reduced and P release shifts from inorganic P to
organic forms that are not readily available, and must be mineralized to become
bioavailable. Therefore, in a marsh system such as the RWMA, where water inflow and
outflow is controlled by pumps and gated weir structures respectively, modifying the
operations of the system to retain water for longer periods of time, is a viable option to,
minimize drained and oxidizing conditions which would otherwise advance the soluble P
front. Finally, there are many important factors such as hydrology, historic P-loading,
soil chemistry and a biological component, influencing P-sorption and flux, all of which
must be taken into account when applying adaptive management strategies to a natural
system. In this study each of these factors, in addition to the data collected from the P-
isotherms and P-fluxing experiments, suggests RWMA in its current state of operation
has the potential to act as a source of P. Therefore, utilizing the data from this study and
the factors outlined above, in conjunction with an adaptive management approach, can
allow for flexible operations of the system improving hydrology, providing conditions to
inhibit P-fluxing and retain soluble P, thus preventing impacts to downstream areas.
P-ADSORPTION RESULTS FROM THE LINEAR, FREUNDLICH AND LANGMUIR
ISOTHERM EQUATIONS AT EACH SITE AND P-CONCENTRATION.
Linear Freundlich Langmuir
C,ig/L S,[tg/g S+So C/(S+So)
0 88.4 + 4.7 -0.88 22.3 4.0
10 88.8 + 3.5 -0.79 22.4 4.0
30 90.0 + 6.3 -0.60 22.6 4.0
50 95.3 + 4.8 -0.45 22.7 4.2
100 102.0 + 6.9 -0.02 23.2 4.4
500 110.0 1.0 3.90 27.1 4.1
1000 127.3 + 6.7 8.73 31.9 4.0
2000 187 5.6 18.13 41.3 4.5
5000 333 3.1 46.67 69.9 4.8
10000 626 25.0 93.74 116.9 5.4
0 18.7 0.7 -0.19 17.5 1.1
10 20.4 0.2 -0.10 17.6 1.2
30 18.9 0.6 0.11 17.8 1.1
50 18.8 1.9 0.31 18.0 1.0
100 20.5 +1.5 0.79 18.5 1.1
500 21.5 + 2.1 4.78 22.5 1.0
1000 29.4 + 1.0 9.71 27.4 1.1
2000 36.1 + 0.3 19.64 37.3 1.0
5000 80.1 + 2.3 49.20 66.9 1.2
10000 182.0 2.0 98.18 115.9 1.6
69.3 + 0.6
76.2 + 3.9
94.6 + 2.6
197.7 + 2.5
421.3 + 6.1
1002.7 + 4.6
0 83.2 + 4.2 -0.83 8.3 10.0
10 98.1 17.4 -0.88 8.3 11.8
30 77.2+ 10.2 -0.47 8.7 8.9
50 99.5 30.2 -0.49 8.7 11.5
100 92.4 + 3.8 0.08 9.2 10.0
500 121.4 + 30.9 3.79 13.0 9.4
1000 176.3 3.1 8.24 17.4 10.1
2000 258.3 + 5.5 17.42 26.6 9.7
5000 644.0 + 9.2 43.56 52.7 12.2
10000 32.7 83.92 93.1 17.3
WATER QUALITY NITROGEN RESULTS FROM INTACT SOIL CORES
SAMPLED FROM THE P-FLUX EXPERIMENT.
hours RC1-D RC2-D RC3-D RC4-D
0 2.15 0.08 2.93 0.38 8.34 0.38 4.161.8
168 4.18 0.19 3.64 0.32 2.86 0.02 3.05 0.28
380 5.81 0.90 6.07 0.31 5.09 0.31 6.09 + 1.09
RC1-F RC2-F RC3-F RC4-F
0 2.94 0.63 3.37 0.32 3.61 0.11 2.48 0.15
168 2.57 0.15 2.78 0.31 2.36 0.03 2.63 0.02
380 3.2 + .014 3.95 + 0.53 2.74 + 0.02 3.03 + 0.02
hours RC1-D RC2-D RC3-D RC4-D
0 2.07 + 0.07 2.37 + 0.20 2.51 + 0.43 2.32 0.30
168 3.68 0.14 6.78 0.49 5.73 0.11 5.86 0.38
380 4.40 + 0.25 5.07 + 0.07 3.26 + 0.74 4.61 0.24
RC1-F RC2-F RC3-F RC4-F
0 1.85 + 0.03 1.81 + 0.01 1.79 + 0.003 1.82 + 0.02
168 2.67 0.15 2.84 0.27 2.47 0.03 2.75 0.01
380 3.17 0.12 3.67 0.29 2.68 0.03 3.07 0.004
hours RC1-D RC2-D RC3-D RC4-D
0 1.52 0.4 1.73 +0.32 2.40+ 1.13 2.80 1.60
168 0.04 + 0.02 0.009 + 0.02 0.077 + 0.02 0.018 + 0.004
380 0.005 + 0.001 0.008 + 0.002 0.007 + 0.005 0.005 0.003
RC1-F RC2-F RC3-F RC4-F
0 0.02 + 0.003 0.014 + 0.002 0.016 + 0.003 0.076 + 0.01
168 0.007 + 0 0.018 + 0.008 0.014 + 0.004 0.009 + 0.001
380 0.004 + 0 0.005 + 0.001 0.0007 0.002 0.002 + 0.001
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I received my B.S. in biology from San Diego State University in June of 1997,
where I worked as an undergraduate research assistant in the Pacific Estuarine Research
Lab on tidal wetland restoration projects. In July of 1997, I began working at the South
Florida Water Management District as an Environmental Scientist on Everglades
restoration projects and wetland assessments. I completed the University of Florida's
graduate certification in environmental policy and management in spring 2003, and then
was accepted into the Soil and Water Science Department's distance education program
for a master's in environmental science in which I will graduate spring 2006.