Group Title: organic matter budget and energy flow of a tropical lowland aquatic ecosystem /
Title: The Organic matter budget and energy flow of a tropical lowland aquatic ecosystem
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Title: The Organic matter budget and energy flow of a tropical lowland aquatic ecosystem
Physical Description: 251 leaves : ill. ; 28 cm.
Language: English
Creator: Brinson, Mark M.
Publication Date: 1973
Copyright Date: 1973
Subject: Aquatic ecology -- Guatemala   ( lcsh )
Water-supply -- Guatemala   ( lcsh )
Botany thesis Ph. D
Dissertations, Academic -- Botany -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis (Ph. D.)--University of Florida, 1973.
Bibliography: Includes bibliographical references (leaves 244-250).
Statement of Responsibility: by Mark McClellan Brinson.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00098368
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000566377
oclc - 37876216
notis - ACZ2808


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Mark McClellan Brinson


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The author wishes to express his appreciation to Dr. Ariel Lugo

for ecological insight and logistical support which helped to insure the

success and feasibility of this investigation. Sincere appreciation goes

to Dr. Frank Nordlie for helpful discussions on Lake izabal, and to the

other members of the supervisory committee, Dr. David Anthony, Dr. Dana

Griffin, III, and Dr. Leland Shanor, for critically reviewing the manu-


Financial support that was provided by the Foreign Area Fellowship

Program during the field study and write-up period is gratefully acknow-

ledged. The Center for Tropical Agriculture provided assistance during

a reconnaissance trip prior tc the field research. Dr. Hugh Popenoe's

continued i-;ntrest in the Izabil Watershed provided the frinme.'ork for

research on a regional level.

Successful affiliation was established with employees of the Depart-

ment of Wildlife, Ministry of Agriculture, Guatemala. This was possible

through the efforts of its Chief, Jose Ovidio de Le6n, who was also res-

ponsible for processing equipment through customs. Adolfo Orosco Pardo

assisted in counting much of the plankton and Renato Z6niga helped with

a phase of the field operations.

Thanks goes to the many people of El Estor who helped to provide day-

to-day needs, and to the EXMIBAL mining company for supplying climatolog-

ical data during the period of study. I am particularly grateful for my

wife's assistance and encouragement during our memorable experiences in the

Guatemalan lowlands.



ACKNOWLEDGMENTS................................................. iv

LIST OF TABLES .................................................. x

LIST OF FIGURES ................................................. xii

ABSTRACT...... ................................................. xvi

INTRODUCTION..................................................... 1

Terrestrial Ecosystem Exports............................. 2

Inflows to Aquatic Ecosystems ............................. 5

REGIONAL SETTING ................................................ 9

Climate................................................... 9

Lakeside Temperature, Rainfall and Solar Radiation.... 9

Precipitation for the Remainder of the Watershed...... 16

Geology and Soils......................................... 19

The Lake .................................................. 21

The Fisheries and People.................................. 24

Immigration and Labor Alternatives.................... 25

Fishing Regulations and Yields........................ 26

Fish Behavior and Fisheries Management................ 28

HYDROLOGY AND WATER CHARACTERISTICS............................. 32

Methods.................................................... 33

General Methodology--Field Stations, Logistics,
and Schedule....................................... 33


Hydrologic Measurements ..............................

Analyses of Water Characteristics.....................

Results and Discussion ...................................

River Discharge and Runoff...........................

Water Budget........................................

Water Characteristics................................

Thermal properties and circulation patterns
of the lake....................................

The seasonal pattern of water characteristics....

Mineral analyses and the Na:Cl ratio.............

Brackish water movement into Lake Izabal.

Summary Statement ................................


Methods ..........................................

Results and Discussion.............................

Phytoplankton and Zooplankton Communities....


Green algae..............................

Blue-gredialgae .........................

Other algae..............................













Copepods ......................................... 117

Cladocera......................................... 117

Rotifers................................*....... 122

Possible Controlling Factors......................... 122

Benthic Community .................................... 130

Summary Statement..........................*.... ......... 133

METABOLISM AND ORGANIC MATTER .................................. 138

Methods.............................**....** ........... .. 139

Chemical Oxygen Demand................................ 139

Fraction definitions and conversion criteria..... 139

Collection and treatment of samples .............. 140

Dissolved Oxygen Concentration Determinations........ 143

Biological oxygen demand......................... 144

Respiration of bottom muds and their organic
content ........................................ 145

Light and dark bottle method..................... 147

Results and Discussion................................... 148

Concentrations of Chemical Oxygen Demand.............. 149

Swamp waters ..................................... 149

Rio Polochic distributaries....................... 152

Small rivers............................ ......... 152



Lake stations and outlet......................... 153

Monthly Flows of COD................................. 154

Respiration Rates (BOD) ............................... 160

Respiration and Organic Content of the Bottom Muds... 168

Measurements of Primary Productivity................. 175

Seasonal rates of gross primary productivity
and respiration ................................ 178

Efficiency of gross primary productivity......... 185

Light penetration and ccm:ponation depth ......... 19

Balance of the Organic Matter Budget................. 193

Summary Statement........................................ 202

CONCLUSIONS.................................................... 204

Seasonal Pulse of Organic Detritus Movement.............. 208

Movement of Organic Matter to Site of Consumption.... 208

Mechanisms for Steady-State Balance.................. 210

Oscillations in Food Concentration for Consumers.......... 212

Consequences of the Connection to a Marine Ecosystem..... 213

Ecosystem Management ..................................... 213

Fishery Management ................................... 214

Modern Agricultural and Industrial Man............... 215

APPENDICES..................................................... 220




LITERATURE CITED.............................................. 244

BIOGRAPHICAL SKETCH............................................. 251


Table Page

1 Sampling dates for chemical oxygen demand (COD), bio-
logical oxygen demand (BOD), and primary productivity
experiments (Prod).......................................... 34

2 Monthly discharges (m3 x 106) of rivers and watersheds
draining into Lake Izabal ................................... 50

3 Summary of hydrologic data for the major watersheds
draining into Lake Izabal................................... 51

4 Monthly free water surface evaporation measurements
for selected warm climates and warm seasons................. 53

5 Summary of water budget for Lake Izabal (November
1971-October 1972).......................................... 55

6 Ranges of temperature, pH, total alkalinity (T.A.),
specific conductance, and dissolved oxygen satura-
tion for all stations sampled at Lake Izabal................ 67

7 Sodium and chloride concentrations (mg/liter) and
Na:C1 ratios for lake stations and distributaries
of the Rio Polochic .......................................... 79

8 Species list of net plankton frequently collected from
Lake Izabal................................................. 105

9 Bottom fauna of Lake Izabal expressed in numbers/m2......... 132

10 Characteristic oxygen equivalents (O.E.) in approxi-
mate order of their limnological importance................. 141

11 Monthly COD inflows (g COD x 106) for individual rivers
and watersheds, total monthly inflows to Lake Izabal,
and monthly and total outflow at San Felipe.................. 155

12 Relative contribution (percent) of organic matter run-
off into Lake Izabal from the Rio Polochic Valley and
from the minor watersheds................................... 159

13 Hourly and daily respiration rates of Lake Izabal
bottom muds ................................................. 173

LIST OF TABLES continued

Table Page

14 Metabolism calculations of light and dark bottle experi-
ments for Stations A, B, and C during 1972................... 181

15 Total daily incoming radiation averaged on a weekly and
monthly basis from October 1971 through October 1972........ 186

16 Daily efficiencies of energy conversion from visible
solar energy to energy fixed by gross primary pro-
ductivity (Pg) .............................................. 188

17 Calculation of light intensity at compensation depths
as percent of surface intensity ............................. 194

18 Summation of organic matter (OM) flows for Lake Izabal
from March to October 1972................................... 198

19 Daily and annual rates of gross primary productivity (Pg)
for some tropical lakes and comparative ranges for
temperate lakes............................................. 201


Figure Page

1 The Izabal Watershed ........................................ 11

2 Monthly averages of the maximum, minimum, and calculated
mean temperatures at Las Dantas for the period of study
(October 1971-1972)......................................... 15

3 Temperature-rainfall climate diagram for Las Dantas and
Mariscos during the year of study........................... 18

4 Bathymetric map of Lake Izabal illustrating sampling
stations .................................................... 23

5 Calibration curves for the small and large probes used
for specific conductance measurements between 100 and
30,000 imho/cm at 25 C...................................... 40

6 Monthly estimates of all inputs to the lake (runoff and
direct rainfall) and the monthly averages of lake
water levels................................................ 44

7 Linear relationship between monthly rainfall and monthly
discharge of rivers emptying into Lake Izabal............... 46

S The seasonal march of monthly discharge rates for some
major rivers emptying into Lake Izabal...................... 48

9 Relationship between velocities at the San Felipe outlet
from direct field measurements and those calculated by
balancing the water budget................................... 57

10 Summary diagram of water storage and annual flows........... 59

11 Vertical temperature profiles of Lake Stations A, A-B,
and B recorded at approximately monthly intervals........... 63

12 Vertical temperature profiles of Lake Stations B-C and C
recorded at approximately monthly intervals................. 65

13 Seasonal changes in dissolved oxygen concentration,
temperature, total alkalinity (T.A.), conductivity,
and pi! for lake stations .................................... 69


Figure Page

14 Seasonal changes in dissolved oxygen concentration,
temperature, total alkalinity (T.A.), conductivity,
and pH for swamp waters ...................................... 72

15 Seasonal changes in dissolved oxygen concentration,
temperature, total alkalinity (T.A.), conductivity and
pH for distributaries of the Rio Polochic (Comercio,
Cobin, and Bujajal).......................................... 74

16 Seasonal changes in dissolved oxygen concentration,
temperature, total alkalinity (T.A.), conductivity, and
pH for small rivers (Sauce, San Marcos, and Manacas Creek)... 76

17 Map of the Rio Dulce-El Golfete system....................... 82

18 Conductivity profiles (March 22-23, 1972) along the Rio
Pulse fro: San: Felipe (Station 1) to Amatique Pay
(Station 11) ................................................. 84

19 Temperature profiles at two stations illustrating a
slight temperature increase associated with the halocline.... 86

20 Observed diurnal change in the conductivity profile of
the waters at the lower reaches of El Golfete (Station 8).... 88

21 Conductivity of the ground water in a swamp forest at
Cuatro Cayos ................................................. 91

22 Conductivity profiles along the Rio Dulce from San Felipe
(Station 1) to the lower reaches of El Golfete
(Station 8).................................................. 93

23 Conductivity profiles taken at several stations in Lake
Izabal and the upper reaches of Rio Dulce (Stations
1 and 2) ..................................................... 95

24 Bottom profile of the Lake Izabal-Rio Dulce system,
showing locations of sampling stations...................... 98

25 Seasonal changes in abundance of pennate diatoms and
Melosira granulata........................................... 107



Figure Page

26 Seasonal changes in abundance of Staurastrum leptocladum
and S. pingue................................... .. .......... 110

27 Seasonal changes in abundance of Pediastrum simplex
and Staurastrum tohopekaligense.............................. 112

28 Seasonal changes in abundance of phycomycetes and
Anacystis cyanea ............................................. 115

29 Seasonal changes in abundance of copepods.................... 119

30 Seasonal changes in abundance of cladocera................... 121

31 Seasonal changes in abundance of colonial rotifers........... 124

32 Seasonal changes in abundance of solitary rotifers .......... 126

33 Total net-plankton abundance (units or organisms per
liter) represented by phytoplankton, zooplankton,
and phycomycetes............................................. 136

34 Concentrations of particulate and dissolved COD (mg/liter)
during the sampling period for (a) swamp waters, (b) Rio
Polochic distributaries, (c) small rivers, and (d) lake
stations (A, B, C, and San Felipe) ........................... 151

35 Rates of organic matter inflows and outflows of Lake
Izabal for the lake as a whole (g COD x 109/month) and
for an average m2 of surface area (g COD/m2 day).............. 157

36 Respiration rates (1-day and 5-day BOD) for (a) swamp
waters, (b) Rio Polochic distributaries, (c) small
rivers, and (d) lake stations ................................ 162

37 Dissolved oxygen concentrations of water samples during
7-day incubation periods..................................... 165

38 Relationship between oxygen consumption rates (BOD) and
total COD concentrations for the four water types
characterized................................................ 167


Figure Page

39 Organic composition of Ekman samples from Lake Izabal
bottom muds ..................................... ............. 170

40 Respiration rates of mud samples from Lake Izabal............. 172

41 Example of curves generated from light and dark bottle
experiments from which metabolism is determined
planimetrically .............................................. 177

42 Method for calculating the number of hours of effective
light per day................................................ 180

43 Gross primary productivity (Pg), 24-hour respiration
(R24) and Pg/R24 ratios at Station A, B, and C............... 184

44 Scchi disk transparency measurements recorded at
Stations A, B, and C......................................... 191

45 Simplified model of the principal organic matter flows
and storage in Lake Izabal as averaged over the period
of study ..................................................... 196

46 Summary diagram of energy and matter flows and storage
that characterize the Izabal Watershed ....................... 206

47 Summary diagram of energy and matter flows and storage
of the 1zabal Watershed which includes some of the
possible influences of development by modern agricul-
tural and industrial man ..................................... 217

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



Mark McClellan Brinson

December, 1973

Chairman: Ariel E. Lugo
Major Department: Botany

This study examines the influence of regional coupling mechanisms

on the organic matter budget of a lowland tropical lake and documents

the principal energy flows that contribute toward making the watershed

a cohesive ecological huit. The downhill flow of organic matter (OM)
and water from the terrestrial (6,860 km ) to the aquatic (717 km)

ecosystem was quantified and evaluated as to its effect on physical

conditions and metabolic activity of Lake Izabal, Guatemala. The lake

type is warm polymictic due to its shallow mean depth (11.6 m). its

water mass has a short residence time (6.6 months), and the annual gross

productivity is high (1,592 g OM/m2).

The hydrologic regime exerted control on OM flow into the lake.

Mean runoff for the watershed was calculated as 65 percent of mean annual

rainfall (2,992 mm), most of which occurred during the 9-month wet season

(greater than 100-mnm monthly rainfall). Organic matter runoff was char-

acterized by an initial "flushing" of the watershed at the beginning of

the wet season, when the particulate fraction (greater than 0.S0 microns

diameter) constituted a large portion of the total organic flow. During

the remainder of the year, OM runoff was nearly all contained in the dis-

solved fraction (less than 0.80 microns diameter). Approximately 50 per-

cent of the total OM runoff occurred during the 3 wettest months of

the year.

During the dry season when OM inputs from the watershed were low,

the lake experienced a net gain in OM when gross primary productivity

exceeded community respiration. During the wet season, there was a net

loss of OM in spite of increased inputs of allochthonous organic detritus.

This loss increased as the wet season progressed, due to a combination of

decreased rates of gross primary productivity and increased rates of com-

munity respiration. The periodicity of OM accrual and loss provides a

mechanism, apparently controlled by hydrologic patterns, by which steady-

state conditions can be achieved on an annual basis.

Daily rates of gross primary productivity ranged from 1.15-7.31 g

0 /m2 day and planktonic respiration rates from 0.50-8.38 g 0 /m day.

The average daily values for the organic matter budget were calculated

for the 8-month sampling period and were represented by five principal

flows. The two OM sources were gross primary productivity (3.730 g/m2

day) and OM imports (0.632 g/m2 day). The three OM losses were by 0M

exports (0.452 g/m2 day), planktonic respiration (3.875 g/m day), and

respiration of the bottom muds (0.36 g/m2 day). The mean residence time

for the average OM content of the lake (71.08 g/m ) was 16.3 days.

Seasonal periodicity was expressed in the net plankton by a bimodal

pulse of abundance. Peaks in plankton density occurred at the end of the

dry season (April-May) and followed the initial period of heavy rainfall

(August-September). Causal factors for this response remain undetermined.


The connection of the lake to the marine environment, via a 42-km

long waterway, allows additional mechanisms for ecosystem coupling.

Evidence was collected to demonstrate the control of the Na:Cl ratio of

lake water by dry-season penetration of brackish water into the lake.

The waterway also provides marine vertebrates and invertebrates access

to a fresh-water environment. Periodicity of OM metabolism in the lake,

high productivity of surrounding lagoons and coves, and brackish water

penetration from the coastal marine ecosystem are discussed as factors

influencing consumer activity and seasonal migration. The lake's fish-

eries, dependent on the marine contingent of fishes, may best be managed

by utilizing the understanding ef regional coupling mechanisms to pre-

vent fisheries deterioration and to ensure continued yields.

xv ii


Many years ago Forbes (1887) discussed lakes as microcosms, and

in so doing emphasized their isolation from the terrestrial ecosystem.

Much of the limnological work since that time has taken this myopic

view of lakes with little regard to the activities beyond their boundaries.

It is these extra-lacustrine activities that give lakes their character-

istics, and they can be dealt with effectively only by expanding the

ecosystem boundaries to include the whole of the watershed. The co-

hesive nature of the watershed, and its well-defined boundaries, are

characteristics that make it a conceptually attractive unit for ecolo-

gical study.

Powered by the proper energy sources, water is the common denominator

that couples this ecological unit by virtue of its geological constraints

(downhill flow) and its biological indispensibility. The characteristics

of this water, its flux from the terrestrial to the aquatic subsystem,

and the effect of this flux on the lacustrine ecosystem are all impor-

tant components of the present study.

The study was conducted in the Izabal Watershed on the Caribbean

slope of Guatemala. Because of the high rainfall and seasonal climate

of the region and the suspected short residence time of the lake's

waters, I hypothesized that upstream activities in the terrestrial sub-

system would be reflected by short-term (<1 year) responses in the down-

stream subsystem of the lake. The main focus of the study was to examine

the metabolic activity of the lake in response to organic matter inflow

from the watershed.

Tailing (1969), in reference to the poorly understood seasonality

of shallow tropical lakes with no long-term thermal stratification,

pointed out that "any periodicity [discovered] acquires a new interest."

In the Izabal Watershed, this interest may necessarily extend beyond

the boundaries of the lake to determine the extent of ecosystem coupling

and matter exchange between subsystems. The connection of the lake to

a marine environment adds to the difficulty of evaluating the watershed

as an isolated unit or closed system. This adds new dimensions for

possible mechanisms for ecosystem coupling.

Depending on the degree to which these coupling mechanisms create

interdependency on a regional scale, schemes for ecosystem management

and land use planning should demonstrate awareness of both the potential

benefits and inherent dangers of manipulation of isolated subsystems.

Therefore, as Odum (1971) emphasized, "it is the whole drainage basin,

not just the body of water, that must be considered as the minimum eco-

system unit when it comes to man's interests." Thus, there is an urgent

need for understanding these interactions on a regional level.

Terrestrial Ecosystem Exports

Naturally forested areas are extremely effective in recycling

materials, thereby preventing losses to downhill processes (Bormann et al.

1969). However, organic matter fixed by terrestrial photosynthesis under-

goes some leakage that eventually appears downstream. The degree to

which this leakage occurs depends on factors which characterize the

ecosystem. Some of the factors that significantly affect organic matter

runoff are: (1) seasonal phenomena, (2) runoff intensity, (3) ecosystem

perturbation, and (4) topography and other geological characteristics

of the watershed.

In temperate regions, the organic matter inputs to streams located

in forested areas are seasonal, being highest in the autumn during the

seasonal leaf drop (Kaushik and Hynes 1968). In tropical latitudes where

rainfall is the seasonal control, a deciduous forest presumably could

experience a similar pulse during the dry season. However, since water

is the medium that transfers organic matter downhill, one might expect

the downhill transfer of organic matter to be seasonally coincident with

high rates of runoff.

Although there are no data from tropical regions to support the

assumption that water runoff and organic matter runoff are positively

correlated, there is some evidence for this in the temperate zone. In

a Piedmont stream in the southeastern USA,Nelson and Scott (1962) found

that,although the dissolved and colloidal organic matter fraction of

river water increased with discharge rate, the particulate fraction in-

creased at a much more rapid rate. At low to moderate flows the dis-

solved and colloidal organic matter concentrations were two to ten times

higher than the particulate, while,during high flow rates, the particulate

organic fraction increased to double the concentration of the dissolved.

Causal factors which increased the particulate portion so dramatically

during high discharge were: (1) greater surface runoff associated with

heavy rainfalls, and (2) the flushing effect of high water. Therefore,

with increased rates of river discharge, organic matter flows increase at

a proportionately greater rate than water flows. There is no reason to

expect that these factors would operate differently in tropical latitudes.

Therefore, measurements of the organic matter runoff from the Izabal

Watershed to Lake Izabal would necessarily include a range of runoff

intensities in order to characterize size fractions of organic runoff

and to achieve a good estimate of absolute quantities.

Perturbation of terrestrial ecosystems by deforestation decreases

the ability of watersheds to prevent downstream losses by destroying the

mechanisms and adaptations for the recycling of matter. Regions of the

Izabal Watershed have received some alteration from deforestation and

agriculture. Again, specific data for estimating the magnitude of organic

runoff change by deforestation must be drawn from temperate-zone ecosystems.

The northern hardwoods of the Hubbard Brook Experimental Forest,

New Hampshire, provide the model on which to judge effects of perturba-

tion. There, drainage streams exported 5.3 grams of organic matter per
m of watershed area annually (Bormann et al. 1969). After clear-cutting

of the forest, organic matter losses doubled during the first two years.

These ecosystem exports represent inputs for downstream ecosystems

(Likens et al. 1970). If the downstream ecosystem were a lake, then

depending on its size, these can represent a significant source of organic

matter and illustrate the importance of one-way coupling between a ter-

restrial and aquatic ecosystem. The absolute values for tropical regions

may be different, but nevertheless deforestation could be expected to

result in increases in organic matter runoff. The only assumption neces-

sary for arriving at this conclusion is that the mechanisms for recycling

matter possessed by ecosystems of both latitudes would be lost or severely

damaged upon destruction of the forests.

Finally, the topography and other geological characteristics of

watersheds are factors that should be considered in the regulation of

organic matter runoff. Runoff waters from the steep mountain environment

in the Amazon Basin have higher concentrations of dissolved and sus-

pended organic matter than those of the lower Amazon (Gibbs 1967).

However, in large watersheds there may be a great deal of spatial varia-

tion in topography and geology as well as the previously discussed vari-

ables of runoff intensity and ecosystem perturbation. The larger the

watershed, the more these spatial variations tend to become integrated

by the confluence and mixing of tributaries by the time downstream

measurements are taken. Also the metabolic activities of riverine eco-

systems could be expected to modify both the quantity and quality of

organic matter after it is received upstream. In the Izabal Watershed,

the majority of the drainage areas become confined to one major river

before the waters discharge into the lake. Thus, any peculiarities in

organic matter sources will tend to cancel one another by the time the

water discharges into the lake.

Inflows to Aquatic Ecosystems

Now that the characteristics of organic matter runoff have been dis-

cussed and loosely established, it is essential to determine the quali-

tative and/or quantitative influence that this organic source may have

on downstream aquatic ecosystems. Showing that the energy fixed in the

form of organic matter in the terrestrial ecosystem can be utilized in

the aquatic ecosystem would establish that an energetic coupling or flow

occurs on a regional level.

There is good evidence that aquatic ecosystems adapt to organic

detritus inputs by utilizing them as a source of energy. This has been

demonstrated for estuaries (Teal 1962; Darnell 1967; Heald 1971; Cooper

and Copeland 1973) and in flowing waters (Odum 1956; Nelson and Scott

1962). That the Lake Izabal ecosystem should be an exception to this

concept would seem anomalous. Some workers have had the insight to com-

pare the relative contribution of organic matter import to the total

metabolism of coastal embayments. Of the total organic increment,

allochthonous inputs accounted for about one-half the total for the

Strait of Georgia (Seki et al. 1968), 7-21% in Moriches Bay on Long

Island (Barlow et al. 1963), and about 97% in the turbid and polluted

Chao Phya River estuary in Thailand (Pescod 1969). In addition to its

importance to the total metabolism of some areas, organic detritus has

been shown specifically to be a principal food item for many estuarine

vertebrates and invertebrates (Darnell 1961, 1967; Odum and de la Cruz

1967; W. Odum 1971). The basis for the nutritive and presumably the

energetic value of particulate detritus is the high quality of organic

matter (e.g. proteins) associated with the micro-organisms that colonize

the particles. Wherever ecosystems exist that have organic detritus

inputs, there is good evidence that the organisms are adapted to utilizing

the organic component as a source of energy.

Lakes, however, have apparently received the least attention of all

aquatic ecosystems that may derive some of their energy from organic

matter transported from outside their boundaries.

Again, specific examples that demonstrate this are drawn from studies

in temperate latitudes. Two independent approaches have been used to

measure allochthonous organic matter sources for lakes. One is to

directly monitor the organic matter runoff from a watershed--the ap-

proach used in the present study. This was done by McConnell (1963) in

small impoundments of the semi-arid southwestern USA. There, relatively

unleached oak litter entered a lake by surface runoff at an annual rate
of about 750 g dry weight per m of lake surface. This allochthonous

source of organic matter supplied the lake with approximately one-third

of the total organic matter increment; the remainder was supplied by

primary production.

The other approach is by indirect measurements and is feasible

only in lakes that thermally stratify. In eutrophic Lake Mikalajki,

Poland, Lawacz (1969) trapped seston as it sank from the trophogenic

zone to below the epilimnion. The total organic matter production by

this method was about half again as great as that of the plankton over

a year as measured by the oxygen method. Lawacz attributed the difference

in production to unmeasured dissolved organic substances that are pro-

duced in the littoral zone. These substances presumably move into the

pelagial zone and sink after transformation into particulate form.

Most attempts to quantify the organic sources of tropical fresh-

waters have been made by measuring the primary productivity of relatively

large lakes. The size of these lakes would tend to diminish the impor-

tance of an allochthonous organic source, and thus the "lake as a micro-

cosm" view is justified, at least for organic matter production.

As pointed out by Ruttner (1963), the quantity and composition of

allochthonous organic matter depends on the ratio of the surface area of

the lake to that of the watershed. In general, allochthonous organic

sources will be of greater importance to small lakes with large water-

sheds than to large lakes with small watersheds. Already discussed are

the important influencing factors such as climate, the morphological and

geological character of the watershed, and plant cover of the watershed.

Considering these factors, the humid tropics are a likely region

where lakes may receive flows of significant magnitude from outside their

boundaries. These flows can have a marked influence on the limnological

characteristics of these lakes. Lakes in which the residence time of the

water mass is relatively short are likely candidates for such tightly

coupled influences. In the context of ecosystem management, it follows

that alteration or perturbation of watersheds could have a profound

effect on the activities within such lakes. Similarly, these lakes may

be our most sensitive indicators of changes in upstream activity.

Thus, the objectives of this study are to (1) determine the

quantity and seasonal distribution of organic matter transfer from the

terrestrial to the lacustrine ecosystem, (2) quantitatively compare the

allochthonous organic matter source of the lake with the organic matter

derived by in situ primary production, (3) evaluate how this one-way

regional coupling mechanism influences metabolic activities in the lake,

and (4) document the principal energy flows that contribute toward

making the watershed a cohesive ecological unit.


The Izabal Watershed extends 205 km eastward from the interior of

Guatemala to the Caribbean coast. The orientation of this 50-km wide

strip of terrain is east-west, located at 88041' W to 90034' W and

15003' N to 15052' N. The drainage pattern is from the highlands in

the western sector, downward to Lake Izabal only a few meters above

sea level, and then through the Rio Dulce, finally reaching Amatique

Bay in the Gulf of Honduras on the Caribbean. However, for the purpose

of this study, the watershed will not include the area downstream from

Lake Izabal, but only the region (6,860 km2) that drains into the lake

(Figure 1).

Two mountain ranges delimit the Izabal hydrologic unit; the Sierra

de las Minas and the Montanas del Mico lay end-to-end and parallel the

southern boundary, while the Sierra de Santa Cruz and Sierra de Chama

create the divide to the north. The major rivers are the Rio Polochic

and the Rio Cahab6n, which converge into a massive river that is actively

building a delta across the end of the lake. Other rivers and small

streams, about 40 in all, drain less extensive areas and often are inter-

mittently dry during periods of low rainfall.


Lakeside Temperature, Rainfall and Solar Radiation

The seasons associated with rainfall and temperature changes near Lake

Izabal were described by Snedaker (1970). The wet season begins abruptly


Figure 1.- The Izabal Watershed. Rainfall stations and geographical features of regional
importance are labeled.

o R.O.nll

So~dy A,.e, loPdoay

--- S-11I WogIr.Ihd Bo-nd-.I.s

R.Infoll i.

0 10 20 30 40 50KM.

Amofilue Bay



S.err, a* C*om6

sometime in May and tapers off toward the end of the year; the dry sea-

son is from December through April. The daily temperature range is

greatest during the drier months, reflecting the ameliorating effect of

rainfall. The average monthly temperature range is 4.4 C over the year

and the annual mean is 25.2 C. The lowest temperatures generally occur

in December and January and are associated with cold air masses moving

in from the north during the winter.

Finca Murcielagos, located on the north-central shore of the lake,

received an annual average of 2,004 mm precipitation over a 6-year per-

iod (Snedaker 1970). Rainfall increases to the east and the west of

this location. No month is without rainfall, while July is the wettest

and February the driest month. August has less precipitation than

either of the adjacent months which coincides with the caniciilas (dog

days), a term that describes a week to 10-day period of dry and cloud-

less days. Snedaker reports that during his 6-year study, 45.5% of the

days experienced at least 1 mm of precipitation, and more precipitation

tends to fall at night (ca. 71% of the total).

Lake Izabal has a pronounced local effect on the climate, especially

the solar radiation. Mornings are generally cloudless in the lowlands,

but in the lake area, clouds begin to accumulate over the surrounding

terrestrial region from about 10 a.m. to noon. Differential solar heat-

ing over the land causes uplifting convective air currents and condensa-

tion of the water vapor while the sky above the relatively cooler lake

remains clear. About mid to late afternoon these clouds move horizontally

across the lake. Snedaker (1970) recorded hourly values of net incident

radiation during 1964 and calculated the total daily net radiation

received for May and June at 408.1 and 494.1 langleys/day, respectively.

The climate near the lake during the year of study (October 1971-

1972) was characterized by examining records maintained by the EXMIBAL

mining company on the north and south shores of the lake. The records

included daily rainfall and maximum and minimum temperatures from the

north shore at Las Dantas, and daily rainfall from the south shore at

Mariscos, 32 km to the east-southeast. Average temperatures were cal-

culated from the maximum and minimum temperatures by interpolation using

the same relationship that Snedaker (1970) observed from his hourly

readings at Finca Murci6lagos. Figure 2 shows that the two warmest

months were April and May during the dry season. The decrease in tem-

perature after May can be attributed to amelioration by rainfall during

the wet season. Temperatures increased until September and thereafter

decreased until February. The low November temperature, which interrupts

an otherwise continual decrease from September through February, can be

attributed to an unseasonably cool,week-long persistence of stormy

weather caused by a hurricane on the Atlantic coast.

Mariscos received 3,236 mm rainfall during the year of study, which

is considerably more than the 2,210 recorded at Las Dantas (Appendix,

Table A). The month of heaviest precipitation for both stations was

August (630 mm at Mariscos and 426 mm at Las Dantas). The rainfall at

Las Dantas during the year of study is in close agreement with a five-

year average (1963-1967) for the same station and slightly higher than

the 2,004 nmm average from 1961-1967 at Finca Murci6lagos (Snedaker 1970).

The climate diagram, which uses the conventions of Walter and Leith

(1960-67), illustrates the monthly march of precipitation and average

Figure 2.-

Monthly averages of the maximum, minimum, and calculated
mean temperatures at Las Dantas for the period of study
(October 1971-1972).


O 25




ambient temperature (Figure 3). Rainfall is the average of the records

at Mariscos and Las Dantas during the year of study and temperatures are

those of Snedaker (1970). The shaded area above 100 mm represents months

when rainfall was in excess of evapotranspiration, and is the period when

the most surface runoff can be expected. The stippled area represents

the period when evaporation is greater than rainfall, implying a water


Precipitation for the Remainder of the Watershed

The low topography near the eastern part of the lake allows trade

winds to pass unobstructed into the basin. Orographic rains occur as

the moisture-laden air masses sweep inland from the Caribbean and upward

over the Polochic Valley. There is considerable spatial variation in

rainfall as illustrated by the isopleths in Figure 1. The data on

which these isopleths are based appear in the Appendix, Table B and also

include information from the Instituto Geografico Nacional (1966).

Heaviest rainfall, averaging close to 4,000 mm, is concentrated in the

north-central region of the Polochic Valley. Radiating from that area,

the average decreases to values below 2,000 mm. San Juan received 6,128 mm

in 1969, the highest amount recorded for any station in a single year.

To arrive at an estimate of the average rainfall for the Izabal

Watershed exclusive of the lake, the areas between the estimated rain-

fall isopleths (Figure 1) were determined planimetrically. For the year

of study, average rainfall for the watershed as a whole was calculated at

2,992 mm.

Figure 3.- Temperature-rainfall climate diagram for Las Dantas and
Mariscos during the year of study. Average monthly
temperatures were calculated from a 6-year record at
Finca lurcielagos.

30 7 / 60

oC20- 0
1 O- -20

0- -0
Sf mamj j a s o nd


Geology and Soils

The Izabal Watershed lies in the physiographic province known as

the Central American Mountain System, just to the north of the volcan-

ically active Pacific Cordillera (Walper 1960). The orientation of the

rivers in the watershed is controlled by the east-west faulting and

folding which has produced a series of anticlinal mountains. This major

zone of faulting,in which the Polochic Valley lies, is postulated as

being tectonically related with the fault zone of the Cayman trench

(Bartlett trough) in the northern Caribbean (Walper 1960). The lake

occurs in a block fault basin (Dengo and Bohnenberger 1969).

The Rio Polochic originates some 2,100 m above sea level and passes

a distance of 100 km before reaching the lake. The headwaters lie be-

tween the Sierra de Pansal to the south and the Sierra de Xucaneb and

Sierra Tzalamila to the north (Popenoe 1960). To the north of these

two latter confluent ranges, the Rio Cahab6n begins its eastwardly flow,

finally connecting with the Rio Polochic 53 km downstream. These waters

become distributed in the multilobate Polochic Delta before they dis-

charge into the lake. As levees of the distributaries protrude into the

lake and deposit alluvium, shallow coves and lagoons become isolated in

the delta region, providing interesting ecosystems for the study of

seasonal succession and metabolism of the plankton (Brinson 1973).

The structure and stratigraphy of the watershed is complex and in-

completely understood (Walper 1960). West of the lake, and at higher

elevations, prominent cliffs of sedimentary limestone and interbedded

dolomite constitute massive beds of Permian age (Roberts and Irving 1957).

Nearby at Cahab6n, beds of terrestrial conglomerate and sandstone predominate

and are believed to be of Tertiary age. Igneous rocks occur in lenti-

cular arrangement along a fault which parallels the north shore of the

lake. This serpentine area is believed to be of late Paleozoic and

early Mezozoic age. The mountains that parallel the southern area of

the watershed are metamorphic pre-Cambrian rocks consisting of undif-

ferentiated schist, gneiss, phyllite, quartzite, and marble.

The soils of the Izabal Watershed, as all the soils of Guatemala,

have been classified and mapped by Simmons et al. 1959. Soils around

the north and east perimeter of the lake were examined by Tergas (1965)

in relation to the primary production of natural vegetation. Some of

these soils have high ratios of calcium to magnesium (1:1.2 to 1:6) as

a result of their derivation from serpentine rock. Popenoe (1960) in

his remarkable study of the response of soils of the Polochic Valley to

shifting cultivation (slash-and-burn), described many of the soil pro-

perties. He stated that "Erosion is very slight on the steep lands of

the Polochic Valley, probably due to excellent soil physical conditions"

imparted by the relatively low bulk densities of the topsoils. Consider-

ing the diversity of parent material and the extremes in climatologic

regimes of the watershed, a more complete description of the soils would

be inappropriate and would necessarily include inaccurate generalizations.

The lithology of the Izabal Watershed is a heterogeneous one,

ranging from old metamorphic to relatively young sedimentary rocks. With-

in this mosaic, one would expect to find significant spatial variation in

weathering and differences in the ionic composition of the headwaters of

streams. However these local irregularities can be expected to cancel

each other as tributaries converge, so that the downstream parts of the

rivers will tend to resemble one another.

The Lake

Lake Izabal is located between 15024' N to 15038' N and 88058' W to

89025' W and lies only a few meters above sea level. The central basin

of the lake is a broad, nearly flat plain reaching a maximum depth of

about 16 m near the center. Thus most of the volume of the lake is be-

low sea level. Most of the shallow areas are the coves and lagoons at

the west end of the lake, bordering the delta of the Rio Polochic. Accord-

ing to the bathymetric map (Figure 4) adapted from Brooks (1969) only

9.7% of the area of the 717 km2 lake is less than 4.6 m (15 feet) deep;

50% of this occurs in the shallow areas near the delta, and the remain-

der around the perimeter of the lake. The volume of the lake is 8,300 x

10 m 3 giving it a mean depth (volume/area) of 11.6 m.

Nearly 50 streams and rivers flow into the lake, and while the

greatest number flow into the eastern, northern, and southern edges, the

greatest volume is received through the Polochic Delta to the west. The

outlet to the lake is at San Felipe, where the lake water enters to Rio

Dulce on its flow to Amatique Bay in the Gulf of Honduras on the Caribbean

Sea. About midway between San Felipe and the coastal port of Livingston

(a distance of 42 km) the Rio Dulce broadens into a large shallow area

(4.5 m depth) known as El Golfete.

Tsukada and Deevey (1967) suggested, on the basis of sediment cores

that ended in sand and gravel, that lacustrine conditions were established,

or reestablished, relatively late during the time of ecstatic rise of sea

level. Brooks (1969) speculated that the Izabal Basin originated as long

ago as the Miocene, and pointed out, on the basis of his inability to

Figure 4.- Bathymetric map of Lake Izabal illustrating sampling stations. A, A-B, B, B-C,
and C are the locations of sampling stations on the lake. Adapted from map by
Brooks (1969).


A-B S -- 1

*- ^ LLake Izabal

0 2 4 km
Depth in Feet

find salt traces in the interstitual waters of sediment cores, that

marine conditions have been absent in the recent past.

The shallow littoral zone of the lake is narrow and subject to the

abrasive action of waves. In some of the protected areas, especially

along the north and east shores, the forest grows to the water's edge.

The shallow bottom consists of large pebbles where the submerged aquatic

macrophyte, Vallisneria, grows in sparce densities. Sandy beaches are

more common along the south shore where there is greater exposure to

wave action created by prevailing northeasterly winds. Occasionally

isolated individuals or aggregates of water lettuce (Pistia stratiotes)

washed in from the shallow lagoons and black-water creeks of the delta

region can be seen floating on the surface of the lake.

A limnological survey of the lake was made by Nordlie (1970) in

August 1969 and March 1970. His most surprising discovery was the rela-

tively high densities of a member of the Tanaidacae, a bottom-dwelling

crustacean with marine affinities. Nordlie found the planktonic community

to have only moderate primary production. On his March visit he observed

extremely low densities of net phytoplankton, whereas in August, they

were present in "bloom" densities. Since most of the present study con-

tains detailed seasonal descriptions of the same phenomena that Nordlie

observed on his visits, his findings will be discussed in more detail in

the chapters that follow.

The Fisheries and People

The Izabal Basin has boen,until recently, a region of only modest

human activity. The lowlands near the lake were believed to have been

settled sparsely during Maya times (Voorhies 1969). Major ceremonial

centers were completely absent from this area although there were cen-

ters immediately to the south (Copin) and to the north scattered through-

out the Pet6n. During the current century, the population did not in-

crease substantially until malaria control was available and a road was

completed from Cobdn to El Estor in 1948. This route replaced, at least

during the dry season, the older route up the Rio Polochic to Panz6s

where the Verapaz Railway connected with Pancajche. A road terminating

at Pancajche completed the journey to the central highlands of Guatemala.

Prior to this, 19th-century ships sailed in from the Caribbean to the colonial

town of Izabal on the south shore of the lake. From this location, mule

transportation provided an overland access to the central highlands. In

the last decade, a spur road was completed, connecting the village of

Mariscos on the south shore with the Atlantic Highway that couples Guate-

mala City with Puerto Barrios and Puerto Santo Tomas de Castilla on the

Caribbean. Now the people living around the perimeter of the lake who

have access to dugout canoes with outboard motors can reach the road

terminating at Mariscos. From there the bus trip to Guatemala's capital

city lasts only 4-5 hours. Likewise, the access provided by the lake to

its perimeter opened the slopes of the Izabal Basin to agriculture.

Immigration and Labor Alternatives

The influx of Kekchi Indians from the upper Polochic Valley, and of

people from other parts of Guatemala, was initiated by the discovery) of

a rich nickel deposit near the northwestern region of the lake. All the

exploratory work for mining the ore has been completed, but the construction

of the processing plant for the extraction of the miiner;al is still peddling

financial support and legal agreements. The already massive capital

outlay, made possible through subsidization by North American firms,

has dwarfed other business interests in the valley. In the past 0O

years the town of El Estor has grown from a sleepy Indian village to a

bustling community.

The labor force required for the initial clearing and exploration

of the mining area is now largely unemployed. Although some families

have been forced to leave, many remain with the hope that they will again

work for the attractive wages paid by the mining company. Some of these

desperate people have shifted their means of living to agriculture and

fishing. Carter (1969) in an anthropological monograph on the Kekchi

cultivators described the problems and successes of these Indians in

their efforts to apply highland methods of shifting cultivation to the

lowland areas near the lake.

The fishing, which became more effective with the introduction of

nylon gill nets in the early 1960's, has since become so unprofitable

that it now offers employment and income for only a few dozen individuals

around the lake. Holloway (1948) recognized long ago the potential food

source of the predominately marine fishes that inhabit the lake, but

made no prediction as to what the carrying capacity of this resource

might be.

Fishing Regulations and Yields

Current Guatemalan fishing regulations, passed into law in 1936,

apply to all inland waters regardless of size or location. One of the

restrictions of the law is that gill nets can be no greater than 36 meters

in length nor have a: mesh si-e of less than 7 inches. However, gill nets

as long as one kilometer with 2 1/2-inch mesh are frequently used during

the fishing season. Initially, excellent yields from the relatively

virgin fisheries were the incentive for people with capital to invest in

this equipment. However, the subsequent decline in yields possibly may

have been the result of exceeding the carrying capacity of the fisheries.

The fishermen react to the decreased yield per unit effort in

several ways. Some cease fishing altogether, while others send their equip-

ment and hired labor northward to fish in the Rio de la Pasi6n in the

Peten when the'demand preceding Easter forces prices upward. Most owners

of gill nets have several thousands of dollars invested in equipment, but

fishing is usually subsidiary to their main business interests.

The general disregard for the irrelevant and unrealistic laws

favors individuals capable of making large capital investments and pe-

nalizes to exclusion those individuals fishing at a subsistence level.

Dickinson (in press) has discussed in detail the sociological implica-

tions of the fisheries as well as thoroughly documenting the geographi-

cal features of the region.

Since there is no governmental agency to keep records of inland

fish catches, past yields from the lake cannot be estimated. Carr (1971)

estimated current yields during a month-long study by observing the

weight of catch per unit length of net. He calculated a daily mean fresh-

weight of 11.6 pounds per 100 yards (5.75 kg/100 m) of net, and based

on other assumptions and measurements, estimated the lakeside yield to be

2,808 kg dry weight per week during the 1970 season. Some fishermen

Most of the catch is salted and dried, accounting for 59% loss of fresh
weight. They arc marketed beyond the I;t abal BRasin, iad only a small
percentage of the total catch remains for local consumption.

fish year round while others may be active only when fish prices are

high (2 or 3 months of the year). If an average fisherman worked 5

months of the year, then the lake would yield 56,160 kg dry weight

annually (0.071 g/m" yr).

Fish Behavior and Fisheries Management

No data exist on the standing crop of fish, on growth increments

of the population or on recruitment from immigration. Since much of

the catch consists of euryhaline marine species, immigration is probably

an important factor to consider in fisheries management. If the fisher-

ies resource of the lake is being overexploited, as it presumably is

(Carr 1971), then the immigration route through the Rio Dulce--El Golfete

region is a logical area for control measures. Disgruntled fishermen

that fish ony n on the lake are aware of this migration route, and of the

gill nets set across these routes by their counterparts in the Rio Dulce


Fishermen as well as large flocks of cormorants, terns, gulls, and

scattered pelicans find another popular fishing region in the shallow

coves and black-water lagoons of the Polochic Delta. There the fishing

is seasonal for both man and birds. During the dry season, these waters

are stagnant and perpetually in bloom with high densities of phytoplankton

(Brinson 1973). The increased consumer activity, in response to the

highly concentrated food source, coincides with the season when fishing

is legal. As law dictates, the fishing ceases in June when heavy rains

mark the beginning of the wet season. Ironically, these events coincide

with the migration of fish to the open water of the lake where they be-

come more dispersed and harder to catch. Fish-eating birds also disperse,

and only a few pelicans remain. The fishing laws thus present a paradox

by allowing fish to be caught when they are easiest to catch, and by

timing the open season with the high prices preceding Easter which

provides incentive for economic gain.

The black anaerobic waters that flush the delta are not completely

devoid of fish. Gobies (Gobionellus sp.) which are particularly poor

swimmers, can be seen gulping air under the sudd vegetation bordering

the lagoons. Vultures and egrets prey upon these fish, but the main

predators are schools of large tarpon (Megalops atlantica) that travel

the backwaters of the delta. By being facultative air breathers, the

tarpon are well adapted to the anaerobic environment.

Marine fish, such as the tarpon, make up the majority of the fish

caught in the lake. In order of decreasing yield they include Chloroscom-

brus sp. (zapatero or leather-jacket jack), several species of catfish

including Bagre sp. and Arius spp. (vaca and chunte), and the prized

Centropomus undecimalis (robalo or snook). The presence of schools of

sardines, anchovies (Anchoviella), and other small herbivorous fish

apparently provide much of the food for the larger carnivores. The impor-

tant fresh-water fishes include Cichlasoma gutulatum mojarraa) and

Brycon g'uatemalensis (machaca).

Some other predominately marine animals, although not directly

important to the fishing economy, are conspicuous components of the

fauna. Blue crabs (Callinectes sp.) are occasionally caught in nets, and

the barnacle, Balanus improvisus, attached to pilings in the eastern end

of the lake are testimony of the seasonally brackish water in that area.

Porpoises (Tursiops truncatus) apparently do not enter the lake, but do

follow the front of high-salinity water as far as the upper Rio Dulce.

Man has noticably reduced the abundance of some of the large aquatic

vertebrates which probably has altered aquatic food chains. Both shark

(Carcharhinus leucas) and sawfish (Pristis perotteti and P. pectinatus)

are reported to inhabit the lake (Thorson et al. 1966) although their

presence has gone unnoticed for the past 8 to 10 years. Fishermen assert

that nets frighten sharks, and that they have been absent from the lake

since gill nets became prevalent. Crocodiles (Crocodylus acutus), once

conspicuous carnivores of the aquatic community, have suffered consider-

able reduction of their populations as a result of hunting pressure.

Hunting may have also been responsible for reducing the manatee, a large

herbivore, to its present day low population density. Two species of

turtles, Dermatemys mawi and Pseudemys scripta ornata, are occasionally

caught by baited hook or by gill nets. However, they seem to be rela-

tively abundant and could serve as a potential source of food for the

local human population (Carr 1971). Based on the consequences of exploita-

tion of other components of the aquatic community, the "potential food

source" offered by turtles would be short-lived, at best. Undoubtedly,

the boney fishes will continue to be the principal aquatic resource for

human exploitation.

Considering the migration routes and seasonal activity of the fishes,

it is doubtful if enforcement of the law during the veda, or prohibition

period, would significantly relax fishing below its present intensity.

Complete enforcement of the law, which would limit the length of gill nets

to 36 meters, would paralyze the fisheries completely and be socially

dysfunctional (Dickinson, in press). There is an urgent need for fisheries


management on Lake Izabal. Any management, however, must demonstrate

an understanding of the role of the fishes in the ecosystem as well as

the needs of the commercial and subsistance-level fishermen.

Undoubtedly some sacrifices in the habits of the fishermen would

be necessary before the fisheries could reach a steady-state level of

maximum sustained yield. Proper management of the fisheries may neces-

sarily extend beyond the boundaries of the aquatic ecosystem if regional

coupling mechanisms are operative as hypothesized in the Introduction.


The principal objective of monitoring the hydrological regime of

the Izabal Watershed was to enable the calculation of rates of inflow and

outflow of organic matter to and from the lake. Subsidiary to this pur-

pose was the need to characterize the hydrologic properties of a lake

that historically has received almost no limnological attention until

recently (Brooks 1969; Nordlie 1970). Even these recent studies lack

the perspective of a long-term study necessary for a fuller understand-

ing of the ecological implications of a seasonal hydrological regime.

For example, Brooks (1969) labeled the likelihood of brackish water

penetration into tne lake as a "common misconception" although its oc-

currence is common knowledge among the non-scientific local inhabitants.

Other "anomalies" of the hydrology and water characteristics might be

of ecological significance, and their occurrence, if undetected, would

unknowingly detract from a more complete understanding of the Izabal

Watershed ecosystem.

Information on runoff characteristics of watersheds in the humid

tropics has been approached mainly by calculating the excess of pre-

cipitation over evapotranspiration from empirical formulae applied to

rainfall and ambient temperature. Direct measurements of runoff are few,

and the information presented in the following chapter should be valu-

able for comparison with studies that do exist.


General Methodology--Field Stations, Logistics, and Schedule

The sampling stations are indicated in Figure 4 and encompass nine

river-mouth stations, three main lake stations, and the outflow station

at San Felipe. To achieve the objective of the study of estimating the

lake's organic matter budget as modified by seasonal changes, samples

were collected during both the dry and wet seasons. The dry-season data

were collected during the months of March, April, and May. The clear,

cloudless days during the warmer dry season helped to distinguish it

from the wet season. January, February, and June through October were

considered wet-season months because the average local precipitation was

greater than 100 mm per month.

The river-m south stations were sampled to determine the qu'aity and

quantity of water entering the lake and the outflow station was sampled

to determine the quality and quantity of exports. Lake stations A, B,

and C were sampled for chemical oxygen demand (COD) and biological oxygen

demand (BOD) as well as for light and dark bottle primary production

determinations (Table 1). Concurrently with COD collections, the follow-

ing data were recorded from surface and bottom samples: dissolved oxy-

gen concentration, pH, total alkalinity, and temperature.

Net plankton was collected from Stations A, B, and C at approximately

three-to four-week intervals. On these collection trips temperature pro-

files and Secchi disk transparencies were determined at the three sta-

tions as well as at stations midway between them. These intermediate

stations are referred to in the text as A-B and B-C.

The large size of Lake liablil and the lonI d istnc .s between s.ampi il.

stations required the use of a relatively fast and reliable mode of

Table 1.- Sampling dates for chemical oxygen demand (COD), biological
oxygen demand RBOD); ;Ind primary productivity experiments
(Prod). Numbers represent day of month

January February March April
Location COD BOD Prod COD BOD Prod COD BOD Prod COD BOD Prod

Station A 3 11,29a 2a 28 4a 21 10 24

Station B 3 28 21

Station C 28 21

San Felipe 3 8 28 21

Rio Oscuro 25 10 11 1

Amatillo 25 10 11 10

El Padre Cr. 17 10 11

Rio Polochic
Comercio 25 10 11 1
Coban 17 10 14 11
Bujajal 17 10 14 11 1

Rio San Marcos 28 21

Rio Manacas Cr. 28 21

Rio Sauce 28 21 10

Rio Tunico -


SPrimary productivity experiments were not duplicated.
Date during month of September.

Table 1.- extended


10,19 28

19 3

19 5,29









August October















- 10 -

transportation. For this a 25-hp outboard motor was brought from the USA

and a 24-foot dugout chance (25 hundredweight capacity) was purchased at

the lake. Routine sample collection required approximately seven hours

for stations at the western end of the lake in the Polochic delta. For

the remaining stations to the east of El Estor, the samples were col-

lected on another day as ten hours were required due to the longer dis-

tances involved.

Ice could be purchased for storing samples during collection trips

except during July and August when the ice plant closed for a minor re-

pair. Several weeks of research time were lost due to breakdowns of the

outboard motor and the car, which was used to transport equipment to the

dock. At least monthly trips to Guatemala City were necessary to pur-

chase repair parts and laboratory supplies.

A room of a concrete-floored house approximately ]/2 km from the

dock site was used for the laboratory. Electricity was available three

or four hours a day after dusk from a privately owned diesel generator

whose output ranged between 85-105 volts A.C. The low and variable

voltage was insufficient to operate the spectrophotometer in spite of

the constant voltage supply transformer. For this reason, chlorophyll

determinations were not made.

Hydrologic Measurements

Discharge rates of several of the major rivers entering the lake

were measured at monthly (occasionally twice-monthly) intervals from

November 1971-October 1972. The float-and-dye method (Welch 1948) was

used to determine the velocity by sprinkling fluorescein dye powder on

the water surface and recording the time elapsed in traveling between

two floats 50 m apart.

When the velocity was slow (less than 0.3 m/sec) a 25-m distance

was used. This procedure was repeated three times and the velocity was

estimated to be the mean of the three readings. Measurements were deter-

mined far enough upstream from the river's mouth to avoid backforce

from the river entering the lake. The cross-sectional areas of the

rivers were measured by determining the width and five depths (two near

the banks, one in the middle, and two halfway between these). A nail was

driven into a tree trunk on the river bank at each station to provide a

permanent point of reference for measuring changes of level in the river.

Discharge rates (cross-sectional area multiplied by velocity) for all

rivers, with the exception of the San Felipe outflow, were multiplied by

a factor of 0.8 in order to correct for frictional resistance of the

stream bed and banks (Welch 1948). At the San Felipe outlet where fric-

tional resistance is small because of the large cross-sectional area, a

factor of 0.9 was used.

Lake levels were recorded at frequent but variable intervals through-

out the study year on the municipal dock at El Estor. Two other stations

served as level markers on the lake: one on the western end at the mouth

of Rio El Padre Creek, and the other at the eastern outlet at San Felipe.

The EIXIBAL mining company also recorded lake levels at the plant site

2 km west of Las Dantas.

Temperature profiles were recorded with a YSI Model 51A oxygen meter

and a YSI 5419 oxygen/temperature pressure-compensated probe with a 50-

foot lead. Calibration was performed in the field for each profile, using

a mercury thermometer as the standard.

Analyses of Water Characteristics

Samples collected for analyses of pH, total alkalinity, and speci-

fic conductance were usually the same as those used for COD analyses.

Methods of collection are described in the section "Metabolism and

Organic Matter." Samples for 02 determinations were collected with a

2- or 3-liter Van Dorn bottle, transferred to 300-ml BOD bottles, and

fixed in the field. Temperature was recorded with a mercury thermom-

eter while the water was in the Van Dorn bottle. All other procedures

and determinations were made in the laboratory, usually within seven to ten

hours from the time the first sample was collected. Total alkalinity

and pH were determined first using a Beckman Model N2 pH meter and Beck-

man glass electrodes. Calibration was performed with factory-prepared

buffer solutions (pH 7.0 and pH 4.5). Total alkalinity (carbonate

plus bicarbonate) was determined by titration of duplicate 100-ml aliquots

to pH 4.5 with a 0.02N HC1 solution. The pH meter scale could be read

to an accuracy of 0.5 pH unit.

Specific conductance was determined with a Beckman Model RB3 Solu-

Bridge and readings were adjusted to 25 C. Two probes were necessary

for the range of conductivities encountered. A small, more sensitive

probe was used in the range of 50-800 pmho/cm and a large probe, one-

tenth as sensitive, was used in the range of 700-40,000 Imho/cm. Fig-

ure 5 illustrates the calibration curves determined with dilutions of a

standard KC1 solution of known specific conductance (Golterman 1969).

Readings of lake water, all within the sensitivity range of the small

probe, were corrected by adding 20 umho/cm, based on the calibration

curve. Higher conductivity readings, accomplished with the large probe,

Figure 5.- Calibration curves for the small and large probes used for specific conductance
measurements between 100 and 30,000 pmho/cm at 25 C. Small probe readings
(magnified on insert) were corrected by adding 20 pmho/cm to the meter readings
between 50 and 700 pmho/cm. Large probe readings were left uncorrected.


(wL/oqwrlj) O1 pJepueIls

0 00
8 o



o 0
,- E ,. 0

I "'

O 10

GO O ,
0 O"
) 0)

(w:/oNwH) 1:M pJepuelS

\ N

(w/o/oLurI) I3Y1 pjepue-4s

required less precision since detecting differences in the salinity

gradients was more important than acquiring absolute values. Neverthe-

less, the calibration curve is in close agreement with the standard KC1

solutions (Figure 5).

Samples for mineral analysis were collected April 14 and October 23,

1972. The samples were prepared by filtering the water through membrane

filters (47 mm-diameter, 0.80-p nominal pore size) to remove the seston.

The samples were transferred to 0.5-liter bottles and 1% formalin was

added as a preservative. The April samples were stored in polyethylene

bottles and the October samples in amber glass bottles. Both groups

were flown to Gainesville and mineral analyses were performed during

March 1973. An atomic absorption spectrophotometer (Perkin-Elmer 303)

was used for determination of Ca, Mn, Mg, Si, Fe, Ni, Zn, and Al.

Potassium and Na were analyzed with a flame emission spectrophotometer

(Beckman DU), phosphorus by colorimetric determination with molybdate

(Golterman 1969), NO, and C1 with specific ion electrodes in conjunction

with an Orion Potentiometer Model 801.

Results and Discussion

River Discharge and Runoff

Discharge rates from six rivers were used to estimate runoff from

the Izabal Watershed. All values were calculated from velocities and

cross-sectional area measurements of the rivers (Appendix, Table C)

except for the values recorded for the Rio Polochic distributaries and

the Rio San Marcos in August. These low August values are noteworthy

since they occurred during the month of heaviest rainfall. During this


period the level of the lake had risen to as high as 1.15 m (August 21)

above the lowest level recorded during the preceding dry season (Figure

6) and resulted in flooding of the delta areas. During flooding the

rivers were not confined to channel flow, but moved as a sheet across

the deltas. If August discharges had been calculated from velocity mea-

surements in the river channels, gross underestimates of runoff would

have resulted. To estimate the August runoff, linear regression formulae

were calculated from the other measurements using rainfall at Las Dantas

as the independent variable and monthly discharge as the dependent vari-

able. In this way August discharges could be calculated by extrapolation

from rainfall, assuming a linear relationship between discharge and rain-

fall. Values for other months were similarly calculated where data were

missing (e.g. February). These extrapolated values appear as open cir-

cles in Figure 7 and were used for the August values in Figure 8.

Discharge rates at the mouths of the Rio San Marcos and Rio Sauce

were low compared to the other rivers measured (Figure S). The similar

patterns of discharge for the Rio Polochic distributaries (Comercio,

Coban, and Bujajal) can be attributed to their common origin. Whereas

the Polochic distributaries demonstrated a sharp increase in discharge

during June, an increase of similar magnitude for Rio Oscuro did not

occur until July. This can be attributed to differences in local rain-

fall regimes. During the year of study the wet season in the arel of the

lake began in June,which accounted for the July increase in Rio Oscuro's

discharge (Figure S). The wet season in some regions of the Polochic

Valley began in May,which may have accounted for the June increases in

the discharges of the Polochic distributaries.

Figure 6.-

Monthly estimates of all inputs to the lake (runoff and
direct rainfall) and the monthly averages of lake water
levels. Lake levels are the number of meters above the
lowest recorded week which occurred during the second
week of March 1972.


Level (nm)

Figure 7.-

Linear relationship between monthly rainfall and monthly
discharge of rivers emptying into Lake Izabal. The open
circles represent extrapolated values for February and
August for which discharge data were missing or required

200 400 600 800 0
Discharge (m3 x 106/month)

Figure 8.- The seasonal march of monthly discharge rates for some
major rivers emptying into Lake Izabal.


S dry

Comerc io



0 1






Table 2 summarizes the discharge rates of all rivers and water-

sheds draining into Lake Izabal. Included is runoff from the water-

sheds to the north and south of the lake which was estimated from the

runoff of the Rio Sauce and Rio San Marcos watersheds. The annual dis-

charge from rivers emptying into the extreme western end of the lake

was greater than ten times the contribution of the remaining watersheds.

The total runoff volume of all watershed areas into Lake Izabal was es-

timated to be 13,290.9 m x 106 during the year of study.

Instead of expressing runoff as a volume, it can be compared directly

to rainfall by conversion to equivalent units. This was accomplished by

dividing the known runoff volume (m 3) by the area of the watershed (m 2)

(Table 3). The annual runoff value of 8,253 mm calculated for the Rio

Oscuro watershed was much toohigh since it is unlikely that rainfall

ever reached this value in any part of the watershed. The reason for

this overestimate was that at flood stage, the Rio Polochic apparently

overflowed into the southern branch of the Rio Oscuro headwaters

(Riachuelo Suncal). This resulted in high discharge rates for the Rio

Oscuro due partly to water originating from the Polochic watershed. By

combining the Rio Polochic and the Rio Oscuro areas (5,480 km ) and their

annual runoff volumes (12,112 m3 x 10), the combined runoff would be

2,210 mm, a more realistic value.

Runoff for all watersheds averaged 1,937 mm. Since the average

rainfall for the Izabal Watershed was 2,992 mm, the portion of the

precipitation lost as runoff was 65%. This is in close agreement with

the watershed of Lake Lanao, Phillipines (Frey 1969) which loses 67% of

the 2,873 mm rainfall it receives. Other tropical watersheds receiving

o'i -o o o'i o

t ro t )O o

N t LO 0

V b,- Ll

- G t- 00 C'

301 N

ts 00 L -

r moo l

c"J -4 1-4

CN co 0

cc r-
1-I Ln

oG ri-Ln CIA Lfn

01 C -O 4 -
-o4 1-1

n C \o r \
1-1 CIAi --1 -1




U u


CD u

o d
*i l

'-' (

m T

4-) 1- C)'

4-) C 00




U /-'

3 \
CT -


r-lCD CO
00 o


L V)


r 1


N O o -1 in

t i Ln co t -T

i)n 00

C r
--4 -
r~ O'

Co r~'

CT] r~

co ~ om
cO- 01 co 00

N 11r i
. . ..~N

) Cq) i- n-1

SI o o
\D ft L

04J > C- C C4J
Ozoo <'20m<'EO

00 LO \

- o 00 -
c o3

I 01



; C








O 3




(1) *T

n N C

\oD CI C'

CO L\h I

* * CD

OO Ct0




r- r~
. Co

in cr-O '

Lo i^

'T Lfl

CO0 1








Cl N-

co t- a o

cq C) C:) 'T
l-I I- I r-f 1-

Table 3.- Summary of hydrologic data for the major watersheds
draining into Lake Izabal


Rio Polochic

Rio Oscuro

Rio Sauce

Rio San Marcos

North Watershed

South Watershed

All Watersheds








Volume of
Annual Runoff
(nm x 106)







6,862 13,291

Annual Runoff








less precipitation lose between 40-50% of the rainfall as runoff (Golley

et al. 1971).

Snedaker (1970) calculated runoff for Finca Murci6lagos by a sim-

ple method devised by Holdridge (1967) which requires knowledge of only

mean annual biotemperature and annual rainfall. Using this method,

Snedaker's runoff estimate was 902 mm or 45% of the rainfall. This is

in close agreement with my value for the Rio Sauce watershed (Table 3)

which is located on the north shore of the lake near Murci6lagos. Dur-

ing the period of study, runoff was 857 mm or 39% of the rainfall (Las

Dantas records).

Water Budget

The water budget of the lake was calculated at monthly intervals

from inputs by river runoff (already discussed) and by direct precipi-

tation, in addition to the outputs by evaporation and losses though the

San Felipe outlet. The monthly contribution by direct precipitation was

calculated from the average rainfall at Las Dantas and Mariscos.

Since evaporation from the free water surface of the lake was not

measured, a literature search was made to arrive at a reasonable esti-

mate. Free surface evaporation estimates from tropical latitudes appear

to be few and the data presented in Table 4 include values for more

northern latitudes. However, summer climate regimes, especially in

Florida, approximate the year-round climate of the Lake Izabal region.

The value of 161 mm/month was chosen as the representative evaporation

by averaging the estimates from the humid areas of Lake lelene, Anderson

Cue, Lake Michie, Lake Chad, and the Caribbean Lowlands (Table 4). This

Table 4.- Monthly free water surface evaporation measurements for
selected warm climates and warm seasons

Region or Lake


Lake Helene, Floridaa

Anderson Cue Lake, Floridaa

Lake Elsinore, Calif.a

Lake Tiberias, Israela

Polish Lakesb

Lake Chad, Africac

Caribbean Lowlands

Lake Michie, N.C.a

Lake Colorado City, Texasa

East Africa (Nile Region)











1962 Pride et al. 1966

1966-68 Brezonik et al. 1969

Szeicz & Endr6di 1969

1949 Reiser 1969

several Debski 1966

Grove 1972

Ray 1931

1962-64 Turner 1966

1955 Harbeck et al. 1959

Talling 1966

asked on an average of 5 warmest months (y-September
Based on an average of 5 warmest months (May-AuguSeptember).
nuBased on an average of 4 warmest months (12.ay-August).
Annual total divided by 12.



is equivalent to 115 x 106 m /month for a surface the size of Lake Izabal

and will be considered constant throughout the year.

Changes in lake level (Appendix, Table D, see also Figure 6) re-

presented integrated results of both inputs and outputs. The volume of

the lake was calculated from the bathymetric map prepared by Brooks
6 3
(1969) and was estimated to be 8,300 x 10 m .Changes in volume were

calculated by multiplying changes between mean monthly lake levels by

the surface area of the lake.

Table 5 is a summary of the monthly contributions and losses for

Lake Izabal. Adding the inputs from runoff and direct precipitation,

subtracting the evaporation, and subtracting the positive or negative

change in volume resulted in the final value which estimates the monthly

loss through the San Felipe outlet. Direct measurement of discharge

from the San Felipe outlet was inadequate due to extreme variations in

surface velocity. On one day I even observed that the flow had reversed,

and further inquiry led to the conclusion that this was a common, if

not daily phenomenon. Apparently prevailing northeasterly winds are

capable of shifting the leeward level of El Golfete above the level of

the lake, thus generating the variation in discharge or reversal of

flow at San Felipe. In spite of this difficulty, there was sufficient

agreement between the velocities measured in the field and those cal-

culated (Figure 9) to regard the latter values as good estimates of

discharge at San Felipe. The direct field measurements were multiplied

by a factor of 0.9 to correct for frictional resistance of the banks and

bottom (Welch 1948).

The annual water budget for the lake and the watershed is summarized

in Figure 10. The average residence time of the water in the lake was

Table 5.- Summary of water budget for Lake Izabal (November 1971-October 1972)




m3 x 10 /month




Change Lake



Loss at Outlet
(San Feline1e




aAll watershed areas.
bCalculated from the average rainfall of two lakeside stations, Mariscos and Las Dantas.

SEstimated from pan evaporation from other localities (Table 4).
Calculated from level changes of lake (Table D, Appendix).
e The sum of co s a, b, and c, and the opposite sign of values in column d.
The sum of columns a, b, and c, and the opposite sign of values in column d.







Figure 9.-

Relationship between velocities at the San Felipe outlet
from direct field measurements and those calculated by
balancing the water budget. Perfect agreement between
the two estimates would fall on the dashed line. Direct
field measurements were multiplied by a factor of 0.9 to
correct for frictional resistance of the banks and bottom.

U /

E ,

_.6- / 0

0 /
O /
:>A- /
> "

0 / 0

M / o

U /

0 .2 ,4 .6 .8
Measured Velocity (m/sec)














Cl .


20,531 Ra
Rainfall Rain




8,300 1
Lake Evap


calculated by dividing its volume (8,300 x 10 m ) by the 12-month loss

by direct evaporation from the surface and the outflow at san Felipe

15,143.5 x 10 m ). This estimated the residence time of the water to

be 6.6 months or 0.55 )ear.

Water Characteristics

Lake Izabal, like other lakes that lack sufficient depth to develop

a hypolimnion, would be classified as a third-class lake (Hutchinson

1957) and could fall, functionally at least, into the class of polymictic

which is usually reserved for lakes of high mountain regions in equatorial

latitudes. Because of Lake Izabal's large area, shallow depth, and high

influx of relatively colder waters from rivers, some anomalous and in-

teresting temperature patterns develop.

The Tzabal Watershed lies in an area of diverse geological forma-

tions, and as a result, receives a variety of water types. Partly be-

cause of the large drainage area of a single influent river, the Rio

Polochic, much of the water from the various rock types has already

mixed before discharging into the lake. Regardless, the large deltaic

swamp at the western end of the lake has distinct modifying effects on

some of these waters. The proximity of the lake to the sea coast and

its connection with a marine environment cannot be overlooked as a poten-

tial factor that could influence the ionic composition of this fresh-

water lake.

Thermal properties and circulation patterns in the lake

Temperature profiles were recorded at approximately monthly intervals

at five equidistantly spaced stations along the length of the lake (Figure

4). Figures 11 and 12 illustrate the temperature profiles for each of

the five stations (A, A-B, B, B-C, and C). For most stations and dates

of sampling, the profiles were isothermal for the first 10-12 m, with

the exception of the upper 2-3 m which were heated directly by solar

radiation. This surface stratification was only temporary and dis-

appeared rapidly either at sundown or with windy conditions during the


More persistent thermal stratification often occurred in the

bottom 1-2 m. This was most pronounced at Station A after June when

the colder waters from the nearby Rio Polochic created a density current

resulting in a cooler layer along the bottom. At Station A-B this sea-

sonal change was more noticable, as the profile is isothermal until

June, the beginning of the wet season. Stations B and B-C showed the

same characteristics but the thermoclines were not as sharp as the more

westerly stations.

Station C was located 30 km east of the Rio Polochic and little

influence from the density layers was expected. The stratified layer

found on April 8 can only be explained if the 2 C increase in tempera-

ture of the upper column (since March 16) failed to circulate with the

bottom meter. More interesting was the noticeable increase in bottom

water temperature on June 13, due to the arrival of a warmer but denser

water mass of high specific conductance (465 mho/cm at 13 m and only

248 umho/cm at the surface). This warmer water originated from the

brackish conditions that developed at the San Felipe outlet during the

proceeding dry season (p. 89).

A dry-season increase in temperatures was recorded for all sta-

tions from a low in February to a maximum in June. For most of the

Figure 1].- Vertical temperature profiles of Lake Stations A, A-B,
and B recorded at approximately monthly intervals.

Depth (m)
Sr O OO ro
-I I 0 I ii N) 0

~ 8


Zi R

Depth (m)
ar 0 ( ) .t ro
r i 1 ; I I I i r i r ,

\ 1





-\ -^
\ ''os
-i* * i^ -
\ -
0-.-. M

^ --------- --- ^

-v -g1'
\- 'o

Depth (m)
o 0 0) Z, M) o

-t -

]2: Y



?--- r









t4-4 4-)

A -4H



*d 0


0 25 27
2 7







'345 1140 1024 (8503
25Feb 16ar OApr 23May
26 28 26 28 28 30 30 31
S I r1 '

i i
: ; T '

1429 1218
25 Feb 16 Mar
26 28 26 2

30 31

1059 0925 0945
8Apr 23May 13Jun
27 23 0 31 30 32


27 29


29 30

1050 0930
19Aug 17Sep
29 30 29 31


28 30
(-- -

28 30


water mass, the minimum was approximately 25.5 C and the maximum 30.4 C,

a difference of 4.9 C.

The seasonal pattern of water characteristics

The water samples were arbitrarily divided into four groups--

lake stations (A, B, C and San Felipe), swamp waters (Oscuro, Amatillo,

El Padre), Polochic distributaries (Comercio, CobAn, Bujajal), and

small rivers (Sauce, San Marcos, Manacas Creek, TAnico). These groups

are based not on water characteristics per se, but on the origin and

locality of the water. The same divisions will be recognized in the

treatment of the organic matter data. The results of measurements of

pH, total alkalinity, specific conductance, dissolved oxygen concentra-

tion, and temperature are presented graphically by station to illustrate

the magnitude of sepsnnil changes (Figure 13-36). Table 6 summarizes

the ranges of the extremes measured as well as the approximate ranges

for more representative values.

Compared with the other groups, the lake stations (Figure 13) were

least variable seasonally for all parameters except specific conductance.

This exception was due to the upstream movement of brackish water from

the Rio Dulce, through the San Felipe outlet, and into the lake. Total

alkalinity showed little seasonal change. Some stratification was noted

at Station A due to the density current created by the Rio Polochic

waters of slightly higher total alkalinity. A decrease occurred from

above pH 8 in January to below pH 7 in March, and then showed a trend

toward increase after July. Temperatures increased during the dry season

and began to decrease after June. Dissolved oxygen concentrations at

the surface probably varied daily as much as they did seasonally.

Table 6.- Ranges of temperature, pH, total alkalinity (T.A.), specific
conductance, and dissolved oxygen saturation for all stations
sampled at Lake Izabal

Lake Stations
(A, B, C, &
San Felipe)

Swamp Waters
(Oscuro, Amatillo,
El Padre Cr.)

Rio Polochic
(Comercio, Cobin,

Small Rivers
(Sauce, San Marcos
Manacas Cr., Tinico)

Temp. (C)
T.A. (meq/liter)
Cond. (pmho/cm)
02 (% Sat.)

Temp. (C)
T.A. (meq/liter)
Cond. (iimho/cm)
02 (% Sat.)

Temp. (C)
T.A. (meq/liter)
Cond. (Qmho/cm)
02 (% Sat.)

Temp. (C)
T.A. (meq/liter)
Cond. (pmho/cm)
02 (% Sat.)

Most Values










a The highest conductivity was recorded at
due to a localized brackish water mass.

San Felipe (5,000 mmho/cm)

II~ _~ _

Figure 13.-

Seasonal changes in dissolved oxygen concentration, temperature, total alkalinity
(T.A.), conductivity, and pH for lake stations. Solid circles represent surface
readings and clear circles the bottom readings at 11.5 m (Station A), 15 m
(Station B), 12 m (Station C), and 10 m (San Felipe).

Sf mam j a s o nd j


5,000 A 355

mamjj ason



Stratification of oxygen is highly variable depending on the frequency

and the depth to which vertical circulation occurred.

The swamp waters (Figure 14) became diluted at the beginning of

the wet season as reflected by the decreases below dry-season values in

conductivity and total alkalinity after June. The pH was variable, but

tended to decrease during the dry season. Both temperature and dissolved

oxygen concentration decreased dramatically at the beginning of the wet

season. The flushing of these rivers by the colder and poorly oxygenated

waters originating in the surrounding swamp forest also destroyed the

stratification that had been established during the dry season.

The water of the Rio Polochic distributaries (Figure 15) was

characterized by dry-season decreases in pH, increases in conductivity

and temperature, little change in total alkalinity, and absence of low

dissolved oxygen concentrations.

All the small rivers (Figure 16), except for the Rio San Marcos,

ceased flowing during the dry season. Thus changes at the beginning of

the wet season are less marked in the San Marcos water than in the other

small rivers. The seasonal trends were similar to those of the Rio

Polochic distributaries described above.

The implications of these seasonal changes in water quality for

metabolic activity of the lake and plankton abundance will be discussed

in the following sections.

Mineral analyses and the Na:Cl ratio

Samples of lake and river water that were analyzed for dissolved

minerals were collected during the dry season (April 14, 1972) and the

wet season (October 23, 1972). The results (Appendix, Table E) show

Figure 14.- Seasonal changes in dissolved oxygen concentration, temperature, total alkalinity
(T.A.), conductivity, and pH for swamp waters. Solid circles represent surface
readings and clear circles the bottom readings at 5 m (Rio Oscuro), 3 m
(Amatillo), and 2 m (El Padre Creek).


Figure 15.- Seasonal changes in dissolved oxygen concentration, temperature, total alkalinity
(T.A.), conductivity and pH for distributaries of the Rio Polochic (Comercio,
Coban and Bujajal).


j f ma m j j a s o n

mamj j ason



Figure 16.- Seasonal changes in dissolved oxygen concentration, temperature, total alkalinity
(T.A.), conductivity, and pH for small rivers (Sauce, San Marcos, and Manacas Creek).


j fmam j as ond j f mamj J as on




that Mn, Fe, Zn, and Ni were detected only occasionally, and always at

the lower range of the sensitivity of the tests. Attributing signifi-

cance to these results would be unwarranted.

The presence of chloride apparently interfered with the measure-

ment of nitrate for the brackish waters of the Rio Dulce Salt Spring

and Amatique Bay (October 23) yielding suspiciously high values of 2.7

and 8.5 mg/liter, respectively. Except for the Rio Agua Caliente and

Rio Sauce during the wet season, nitrate was near the limit of detection

(0.62 mg/liter) of the specific ion electrode. Dry-season concentra-

tions of nitrate were greater than 1 mg/liter in some of the swamp

waters (Oscuro Bay, Amatillo, El Padre Creek, and Ensenada El Padre)

as well as the Comercio and Coban distributaries of the Rio Polochic.

Discolved phosphate concentrations ranged between 0.04-0.17: mg/liter

and in this range of sensitivity the significance of the results is

subject to question.

The high concentrations of Ca, Mg, and Si in the Rio Agua Caliente

can be attributed to the hot spring at its origin. However, the low

discharge of this river would have resulted in less contribution to the

lake of these ions than less concentrated rivers with higher discharges.

The Rio Sauce drained a limestone area (probably dolomite) and differed

from the lake water by its higher Mg and Si concentration and lower Ca.

Rio Manacas Creek during the dry season had a Mg:Ca ratio greater than

one. The Rio San Marcos was notably more dilute than the lake water

as suggested by the consistently lower specific conductance of the river

throughout the year (Figure J6). Silica concentrations for the October

23 samples may be high due to storage for three months in glass bottles.

The analyses for sodium and chloride in the lake and Rio Polochic

waters, however, have some interesting implications for understanding

circulatory patterns of the lake water. These data and the ratios of

sodium to chloride are presented in Table 7. On April 14 both Na and

C1 were higher at San Felipe and Station C at 12.5-m depth than in the

rest of the lake. The appearance of slightly brackish water was noted

also with conductivity determinations at San Felipe in April but not

until June at Station C (Figure 13). Thus the mineral analyses of Na

and C1 provided a more sensitive method than conductivity measurements

for detection of brackish water as it entered the lake from the Rio


The average Na:C1 ratio for the April 14 lake station samples

excludingg San Felipe :ind Station C, 12.5) was 0.52. This value is

slightly lower than the ratio 0.56 for sea water (Remane and Schlieper

1971). On October 23 the high ratios of 1.33 at Station A (11.5 m)

and 1.69 at Station B (15 m) distinguished them from the ratios of the

remaining lake stations. The origin of these high Na:Cl ratios is

apparent by comparing them with the October 23 average of the Polochic

samples which was 1.76. This provides a conclusive check for the exis-

tance of the density current implied by the 02, alkalinity and specific

conductivity stratification at Station A (Figure 13) after the initia-

tion of the wet season.

If the Rio Polochic had been the only source of water for the lake,

then the lake water would have been more dilute than it was. Even on

April 14, when Na and C1 concentrations were higher in the Rio Polochic,

they were not as high as the more dilute lake water on October 23

Table 7.- Sodium and chloride concentrations (mg/liter) and Na:C1 ratios
for lake stations and distributaries of the Rio Polochic.
Averages are calculated for selected stations

14 April 1972 23 October 1972
mg/liter Na:C1 mg/liter Na:C1
Na C1 ratio Na C1 ratio
Lake Stations

Station A Surf 4.5 7.8 .58 3.3 4.1 .80
Station A 11.5 m 4.3 8.2 .52 3.2 2.4 (1.33)
Station B Surf 4.3 8.8 .49 3.5 6.0 .58
Station B 15 m 4.3 8.8 .49 2.7 1.6 (1.69)
Station C Surf 4.3 8.2 .52 3.6 5.5 .65
Station C 12 m 8.2 10.5 3.6 6.4 .56
San Felipe -
Surf & 10 m 7.7 15.6 3.7 4.6 .80
Average .52 .68a

Rio Polochic
Comercio 3.1 4.9 .63 1.9 1.0 1.90
Coban 3.1 4.5 .69 2.2 1.0 2.20
Bujajal 3.1 1.9b 2.0 1.7 1.18
Average .66 1.76

SNumbers in parentheses not calculated in average.
Accuracy of determination questionable.

(excluding Station A-11.5 m, and Station B-15 m). -By examining Table E

of the Appendix, there is no evidence for possible sources of high Na

or C1 from the rivers sampled, except Rio Agua Caliente. However, its

wet-season discharge was low and the river did not flow at all through-

out most of the dry season. Further confirmation is available from

August 1969, when Brooks (1969) detected Na and C1 concentrations in

the Rio Polochic to be 3.2 and 2.2 mg/liter respectively. Higher con-

centrations (4.8 mg Na/liter and 7.5 mg Cl/liter) were reported by

Brooks for lake water samples.

Brackish water movement into Lake Izabal

To determine the seasonality and extent of movement of the saline-

fresh water interface between Lake Izabal and Amatique Bay, several

sampling trips were made into the Rio Dulce-Fl Golfete region (Figure

17). Two of these trips traversed the area between the San Felipe out-

let and the coastal port of Livingston; one was made during the dry

season (March 22-23, 1972) and the other during the wet season (October

26, 1972). A third transect (May 13) included only Stations 1-8. The

sampling stations are numbered from 1 to 11 in Figure 17.

On the first transect (March 22-23) a salt-water wedge was observed

extending from Station 11 at Livingston to El Golfete between Stations 5

and 6 (Figure 18). The deep high-salinity water was nearly isothermal

(29.4-29.6 C) and slightly warmer than the surface waters (Figure 19).

A strong (ca. 0.6 m/sec) outgoing current in the upper 1-2 m of the Rio

Dulce below El Golfete marked the interface with the relatively motion-

less deep layers. An increase in conductivity at Station 8 over a 17-

hour period was attributed to downstream tidal forces (Figure 20). This




C 0



Ln~ Ln

*H j 4 -


0 C V )

tR 0 0

4- -4
Pi-L 0 0

0 c
0~c 0





p- 2 Kilostsrm
I I, M 0ile$



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