THE ORGANIC R-MTER BUDGET AIvID ENERGY FLOW OF A TROPICAL LOWLAND AQUATIC ECOSYSTEM By MARK McCLELLAN BRINSON A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FL01?IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1973 Copyright by Mark McClellan Brinson 1973 10 m PARENTS ACICNOWLEDGMENTS 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 otlier members of the supervisory committee, Dr. David Anthony, Dr. Dana Griffin, 111, and Dr. Leland Shancr, for critically reviewing the manu- script. 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 to the field research. Dr. Hugh Popenoe's continned i-^t'^rest in '•"'"'e Izabal Witershed provided the fr?-me'"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 Leon, who was also res- ponsible for processing equipment through customs. Adolfo Orosco Pardo assisted in counting much of the plankton and Renato Zuniga 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 EXMIRAL mining company for supplying climatolog- ical data during the period of study. I am particularly grateful for my wife's assistance antl encouragement during our memorable experiences in the Guatemalan lowlands. IV TABLE OF CONTENTS Page ACKNOWLEDGMENTS ^^ LIST OF TABLES ^ LIST OF FIGURES ^^^ ABSTRACT '. • xvi INTRODUCTION ^ Terrestrial Ecosystem Exports 2 InfloKS to Aquatic Ecosystems 5 REGIONAL SETTING ^ Climate • ^ Lakeside Temperature, Rainfall and Solar Radiation 9 Precipitation for the Remainder of the Watershed 16 Geology and Soils ^^ The Lake ^^ 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 ^^ General Methodology--Field Stations, Logistics, and Schedule •^'"' TABLE OF CONTENTS - continued Page Hydrologic Measurements 36 Analyses of Water Characteristics 38 Results and Discussion 41 River Discharge and Runoff 41 Water Budget . 52 Water Characteristics 60 Thermal properties and circulation patterns of the lake 60 The seasonal pattern of water characteristics.... 66 Mineral analyses and tlie Na:Cl ratio 70 Brackish water movement into Lake Izabal 80 Summary Statement 99 NET PLANKTON AND BENTHIC COMMUNITIES 102 Methods 103 Results and Discussion 104 Phytoplankton and Zooplankton Communities 104 Diatoms 104 Green algae 108 Blue-green algae 115 Otlicr algae 116 Phycomycetes 116 VI TABLE OF CONTENTS - continued Page Copepods '■^' Cladocera ^^"^ Rotifers ^^^ Possible Controlling Factors 122 Benthic Community 1-^^ Summary Statement ^^-^ METABOLISM AND ORGANIC MATTER i38 Methods ^ "'^ Chemical Oxygen Demand ^^^ 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 ■'•^■^ Light and dark bottle method 147 Results and Discussion 148 Concentrations of Chemical Oxygen Demand 149 Swamp waters 149 Rio Polochic distributaries 152 Smal 1 rivers ■ 1 ^^ o VIX TABLE OF CONTENTS - continued Page 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 1 78 Efficiency of gross primary productivity 185 Light penetration and cc".pen:;aticn depth 1S9 Balance of the Organic Matter Budget 195 Summary Statement 202 CONCLUSIONS 204 Seasonal Pulse of Organic Detritus Movement 20S 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 VI 11 TABLE OF CONTENTS - continued Page LITERATURE CITED 244 BI0GR.-\P!1ICAL SKETCH 251 IX LIST OF TABLES Table Page 1 Sampling dates for chemical oxygen demand (COD) , bio- logical oxygen demand (BOD), and primary productivity experiments (Prod) 34 2 Monthly discharges (m^ x 10 ) of rivers and watersheds draining into Lake I zabal 50 3 Summary of hydrologic data for the major watersheds draining into Lake I zabal 51 4 Monthly free water surface evaporation measurements for selected warm climates and warm seasons 53 5 Summary of water budget for Lake 1 zabal (November 1971-October 1972) 55 6 Ranges of temperature, pH, total alkalinity (T.A.)j specific conductance, and dissolved oxygen satura- tion for all stations sampled at Lake Izabal 67 7 Sodium and chloride concentrations (mg/ liter) and Na:Cl 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/m 132 10 Characteristic oxygen equivalents (O.E.) in approxi- mate order of their limnological importance 141 11 Monthly COD inflows (g COD x 10^) 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 hukIs 1"73 LIST OF TABLES - continued Table P^ge 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) ISS 17 Calculation of light intensity at compensation depths as percent of surface intensity 1S4 18 Sunmiation of organic matter (CM) flows for Lake Izabal from March to October 1972 • 198 19 Daily and annual rates of gross primary productivity fPg) for some tropical lakes and comparative ranges for temperate lakes 201 XI LIST OF FIGURES 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 25 5 Calibration curves for the small and large probes used for specific conductance measurements between 100 and 30,000 ymho/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 8 The seasonal march of monthly discharge rates for some maj or rivers em.ptying into Lake 1 zabal 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 storages 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 pH for lake stations 69 XI 1 LIST OF FIGURES - continued Figure ^^^e 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 (Coinercio, Coban, 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 (Marrh 22-23, 1972) alnnp the Rio Dulce frc:u San Feliiie (Station 1) to Ajp.?tique ^-'^y (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 Melosirg granulata 107 Xlll LIST OF FIGURES - continued Figure : Page 26 Seasonal changes in abundance of St aura strum 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 lli^ 30 Seasonal changes in abundance of cladocera 121 31 Seasonal changes in abundance of colonial rotifers 124 32 Seasonal changes ir\ 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 m- of surface area (g C0D/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 1"' -7 xiv LIST OF FIGURES - continued Figure Page 39 Organic composition of Ekman samples from Lake Izabal bottom muds 170 40 Respiration rates of mud sam.ples from Lake Izabal 172 41 Example of curves generated from light and dark bottle experiments from uiiich 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 Sccchi dislc transparency measurements recorded at StdtioiiS rv, L) , anci C 191 45 Simplified model of the principal organic matter flows and storages in Lake Izabal as averaged over the period of study 196 46 Summary diagram of energy and matter flows and storages that characterize the Izabal Watershed 206 47 Summary diagram of energy and matter flows and storages of the Izabal Watershed which includes some of the possible influences of development by modern agricul- tural and industrial man 217 XV 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 THE ORGANIC MA^TTER BUDGET AND ENERGY FLOW OF A TROPICAL LOWLAND AQUATIC ECOSYSTEM By Mark McClellan Brinson Decemberj 1973 Chairman: Ariel E. Lugo Major Department: Botany This study examines the influence of regional coupling mechanisms on the organic matter budget of a lovland ti-opical lake and documents the principal energy flows that contribute toward making the waterslied h coiiesivo ecologicaj. uiiit. TiiC uowrinili j-xOW ci organic mai^tei i.Oi>i^ 2 2 and water from the terrestrial (6,860 km ) to the aquatic (717 km 1 ecosystem was quantified and evaluated as to its effect on physical conditions and metabolic activity of Lake I::abal, Guatemala. The lake type is warm pol>Tnictic due to its shallow mean depth (11.6 m) . Its water mass has a short residence tin;e (6.6 months), and the annual gross 2 productivity is high (1,592 g OM/m ). The hydrologic regime exerted control on OM flow into the lake. Mean runoff for the watershed was calculated as 6S percent of mean annual rainfall (2,992 mm), most of which occurred during the 9-month wet i-eason (greater than 100-mm monthly rainfall). Organic matter runoff was char- acterized by an initial "flushing" of the watershed at tlie beginning of the wet season, when the particulate fraction (greater than O.SO microns XV 1 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 comjr.unity 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 v/hich steady- state conditions can be achieved on an annual basis. Daily rates of gross primary productivity ranged from 1.15-7.31 g 2 2 0 /m day and plaiiktonic 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 2 flows. The two OM sources were gross primary productivity (3.730 g/m 2 day) and OM imports (0.632 g/m day). The three OM losses were by OM exports (0.452 g/m day), planktonic respiration (3.875 g/m" day), and 2 respiration of the bottom muds (0.36 g/m" day). The mean residence time 2 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. XV ii The connection of the lake to the marine environment, via a 42-kir. 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 of regional coupling mechanisms to pre- vent fisheries deterioration and to ensure continued yields. XVI 11 INTRODUCTION Many years ago Torbes (1887) discussed lakes as microcosms, and in so doing emphasized their isolation from the terrestrial ecosystem. Much of the limnological u-ork 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 tJie 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 com.ponents 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 exiimine 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 do\\Tiliill processes (Bormann et al. 1969). However, organic matter fixed by terrestrial photosynthesis under- goes some leakage that eventually appears downstreiim. 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 autum.n 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 witli 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 dovmstream 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 do\mstream 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 otlier geological characteristics of watersheds are factors that should be considered in the regulation of organic matter runoff. Runoff waters from the steep mountain environm.ent in the Amazon Basin have higher concentrations of dissolved and sus- pended organic matter than those of the lower i'^jTiazon (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 therr. as a source of energy. This has been demonstrated for estuaries (Teal 1962; Darnell 1967; Heald 19 71; Cooper and Copeland 1972) and in flov/ing 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 lias 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 (1965) 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 epilim.nion. 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 (1965), 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) determ.ine 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. REGIONAL SETTING 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 8S°41' W to 90°54' W and 15°03' N to 15°52' 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 2 Lake Izabal, but only the region (6,860 km ) that drains into the lake Cl-igure 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 Cahabon, 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. Climate Lakeside Temperature, Rainfall and Solar Radiation The seasons associated with rainfall and temperature changes near Lake Izabal were described by Snedaker (.1!^70J. The wet season begins abruptly ctf C o •H bO O ^ 4h O tn (U fH rs ■p nJ tw i-H ClJ o •H X PU rt M t>0 o o nS u 3: ci3 1— 1 (U rt O ^ C 03 03 N ■(-> 1 — 1 fn O (U CI, X 6 H • H 11 12 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 m.onthly 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 caniculas fdog 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, 45.5-0 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 horizonally across the lake. Snedaker (1970) recorded hourly values of net incident 13 radiation daring I9bi 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 kiii to the east-southeast. Average temperatures v%'ere cal- culated from the maximum and minimum temperatures by interpolation using the same relationship that Snedaker (1970) observed from his hourly readings at Finca Murcielagos. Figure 2 shows that the two warmest months were April and May during the dry season. Ihe 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 i\kiriscos 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 nm average from 1961-1967 at Finca Murcielagos (Snedaker 1970). The climate diagram, which uses the conventions of Walter and Lcith (1960-67), illustrates the monthly marcli of precipitation and average Figui'e 2.- Monthly averages of the maximum, minimum, and calculated mean temperatures at Las Dantas for the period of study (October 1971-1972). 15 Las Dantas season 20 J F M A M J J A MONTH S O N D 16 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 deficit. 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 nmi, 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 isopletlis (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 ana Mariscos during the year of study. Average monthly temperatures were calculated from a 6-year record at Finca Murcielagos. 18 30- 20^ 'C 10- 0- -600 -400 -200 mm 20 -0 T » 1 1 1 1 1 r- J fmamj j asond J Month 19 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 trencli (Bartlett trough) in the northern Caribbean (Walper 1960). The lake occurs in a block fault basin (Dengo and Bohnenberger 1969) . The Rio Folochic 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 Cahabon 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 (Ftoberts and Irving 1957). Nearby at Cahabon, beds of terrestrial conglomerate and sandstone predominate 20 and are believed to be of TeiLiary 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) nn his remarkable study of the response of soils of the Polochic Valley to shifting culti\'ation (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 tlic ionic comjiosition of the Iicadwaters of streams. However these local irregularities can be expected to cancel each otlier as tributaries converge, so tliat tlie doMistream parts of tlie rivers will tend to resemble one anotlier. 21 The Lake Lake Izabal is located between 15°24' N to 15°38' N and 88°58' W to 89°25' 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 bath>'Tiietric map (Figure 4) adapted from Brooks (1969) only 9.7% of the area of the 717 km" 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"^, giving it a mean depth (volume/area) of 11.6 m. Nearly SO 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 deptli) known as El Go 1 fete. Tsukada and Deevey (1967) suggested, on the basis of sediment cores that ended in sand and gravel, tliat lacustrine conditions were established, or reestablished, relatively late during the time of custatic 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 U X 1 Xi CO fl, " rt cQ e •> e CQ O 1 ^-1 < ^ "'O < • rt 0) +J ,irf w rt .— 1 W) C -P rt rt !h +^ j-> 10 1/1 3 M --I C 1— 1 -H • H >— 1 PU '-' S rt 3 ^ tn rt N t+H l-H O a> in M c rt o J -H 4-' 4-1 03 O O c fl,rH cii • 6 vO • H a> f-l (U .-1 •P J-l O rt I.U 10 X o •1-1 TJ o rt C !-i ca nJ 03 0) u 'J 23 24 find salt traces in t!ie 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 disco'/ery v;as the rela- tively high densities of a member of the Tanaidacae, a bottom-dwelling crustacean with marine affinities. Nordlie found the plaiiktonic 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 been, until recently, a region of only modest human activity. The lowlands near the lake were believed to have been settled sparsely durina Maya tinies (Voorhies 1969). Major ceremonial centers were completely absent from this area although there were cen- ters immediately to tlie south (Cop^n) and to the north scattered through- out the Peten. During the current century, the population did not in- crease substantially until malaria control was available and a road was completed from Coban to El Estor in 1948. This route replaced, at least during the dry season, the older loute up the Rio Polochic to Panzos 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 nortliwestern region of the lake. All the exploratory work for mining the ore has been completed, but the construction of tlie jirocessing plruit for the extraction of tlic mineral is still pending 26 financial support and legal agreements. The already massive capital outlay, made possible through subsidization liy N'orth Ajnerican firms, has dwarfed other business interests in the valley. In tlie past 10 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. Altliough some families have been forced to leave, many remain with the hope that they wilJ 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 applv 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 emplo>'ment 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 IcH'.'tli nor Iiavc a mesh size ol" less than 7 iiiclics. However, gill nets 27 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 Pasion 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 subsistance 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 lakewide yield to be 9 ,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 Kcigiit. The>- are marketed bc\-onJ the Izabal Ba;;in, and only a small percentage of the total catch remains for local consumption. 28 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 only 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 region. 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 tlie 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, tliesc events coincide with the migration of fish to the open water of the lake where they be- come move dispersed and harder to catch. Fisli-cating birds also disperse. 29 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 (mojarra) and Brycon guatcmalensis (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 arc testimony of the seasonally brackish water in that area. Porpoises (Tursiops truncatus) apparently do not enter the lake, but do 30 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 tlie 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 disfunctional (Dickinson, in press). There is an urgent need for fisheries 31 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 m the Introduction. HYDROLOGY AND WATER CHARACTERISTICS 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 fu]ler 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 inliabitants. 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 ajnbient temperature. Direct measurements of runoff are few, and tl\e information presented in the following chapter should be valu- able for comoarison with studies that do exist. 32 33 Metliods 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- mouth stations v/ere sampled to detei'mine the quality 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 tlicsc collection trips temperature pro- files and Secchi disk transparencies were determined at tlic three sta- tions as well as at stations midway between them, lliese intermediate stations are referred to in the text as A-B and B-C. The large size of Lake 1-abai and llic ]t)iig distances between sainpiiiig stations required the use of a relatively fast and reliable mode of 34 Table 1.- Sampling dates for chemical oxygen demand (COD), biologscal oxygen demand (ROD), and primary productivity experiments (Prod) . Numbers represent day of month ■WET SEASON DRY SEASON Location J COD anuary BOD Prod February COD BOD Prod COD March BOD Prod COD April BOD Prod Station A 3 11. 29^ 2^ 28 4^ 21 10 24 Station B 3 28 21 Station C 28 21 San Felipe 3 8 28 21 Rio Oscuro 25 10 11 1 Ama t i 1 1 0 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 - - - Largartos - - - Primary productivity experiments were not duplicated. Date during month of September. 35 Table 1.- extended WET SEASON May COD BOD Prod COD June BOD P rod COD July BOD Prod August COD BOD Prod October COD BOD Prod 19 10,19 2S 9 20 29 21 9 5,23 8 27,^7 19 19 3 9 5 ,19 29 21 8 5 8 26^ 19 19 5,29 9 18 29 21 7 5 5 25^ 19 - 29 29 21 5 16 7 7 11 4 2 1,23 16 7 11 4 2 1 16 7 11 4 1 X 16 7 11 4 • 1 16 7 11 4 1,23 16 10 7 7 11 4 2 1 19 19 9 9 29 29 21 5 19 19 9 29 21 5 5 19 9 9 29 29 4 1 - - - 21 - 10 - - - - 36 transportation. For this a 25-hp outboard motor was brought from the USA and a 24-foot dugout cance (25 hundredweight capacity) vv-as purchased at the lake. Routine sample collection required approximately seven hours for stations at the western end of the lake in the Folochlc 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 tlie 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 montlily (occasionally twice-monthly) intervals from November 1971-Octobcr 1972. The f loat-and-dye method (Welch 1948) was used to determine the velocity by sprinkling fluorescein dye powder on 37 tlie 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 enougli 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. Discliarge 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 EXWIBAL 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. 38 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 0 determinations were collected with a 2- or 5-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 bicarbonatej was determined by titration of duplicate 100-ml aliquots to pH 4.5 with a 0.02N HCl 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 ymho/cm and a large probe, one- tenth as sensitive, was used in the range of 700-40,000 ymho/cm. Fig- ure 5 illustrates the calibration curves determined with dilutions of a standard KCl 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 pmho/cm, based on the calibration curve. Higher conductivity readings, accomplished with the large probe. o to o bO c C 03 ■H +-> ■a u to rt 3 Wl (D t3 C U ^ • C ' •H ■a O ' -a r-t CD o CO Q) +-> CD +-i u o fn o (U •H 6 f-l <4H 0 f-l •H 43 O o u O X o o fn ■(-> c p^ ft 3 w o t — 1 +-I ■p f-i r^i Mh o cd c o CD 12 to o > ■P c r^ (1 (U •H 3 ^ -d O c c w o rt c +-> o c -a o ■H (U (U LO +-> e •H rt (U M-l c U M •H o Si 3 C (D •H W M 3 . — 1 rt ca +-> w O e (U U E LO 0) 40 (UJD/omjjr1)o>i pjepue^s o o o CO (uuD/ogiuri) \o>\ pjepuE-^s 41 required less precision sJiice 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 KCl 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, (J.bO-y 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 tlame emission spectrophotometer (Beckman DU) , phosphorus by colorimetric determination with molybdate (Golterman 1969), NO and CI 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 42 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 moutlis of the Rio San Marcos and Rio Sauce were low compared to the other rivers measured (Figure 8) . 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 area of tlie lake began in June, which accounted for the July increase in Rio Oscuro "s discharge (Figure 8) . The wet season in some regions of the Polochic Valley began in May,wluch 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. 44 M J J A MONTH 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 correction. 46 600 ConnercioJ 400 E E 'ct a: 200 y:>67.99* 2.06 X Coban y =-75.23 *0.74x y =-31.17 ♦1.12 X 0 200 400 600 800 0 200 400 Discharge (m3 x lOS/month) 600 Oscuro y:--103.08*0.75x -1 1 I L. y =146.85* 3.92 X San Marcos •y:6.09* 1.81 X O 200 400 0 50 100 0 20 40 Discharge im^xlO^/month) Figure 8.- The seasonal march of monthly discharge rates for some major rivers emptying into Lake Izabal. 48 JFMAMJJASOND MONTH 49 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 tlie lake was greater than ten times the contribution of the remaining watersheds. The total runoff volimie of all watershed areas into Lake Izabal was es- timated to be 13,290.9 m"^ x 10 during the year of study. Instead of expressing runoff as a voKune, it can be compared directly to rainfall by conversion to equivalent units. This was accomplished by dividing the known runoff volume (m ) by the area of the watershed (m ) (Table 3). The annual runoff value of 8,253 mm calculated for the Rio Oscuro watershed v;as much too high 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 2 combining the Rio Polochic and the Rio Oscuro areas (5,480 km ) and their annual runoff volumes (12,112 m x 10 ), the combined runoff would be 2,210 mm, a more realistic value. Runoff for all watersheds averaged 1,957 mm. Since the average rainfall for the Izabal U'atershed was 2,992 mm, the portion of the precipitation lost as runoff was 65'o. 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 waterslieds receiving 50 •t-> o o nS T3 w (U f-l -O f— i •H o fH X I o •-< •H rt oi Q +J 1 O 'O .— ( rt o 1 ,0 %c h 'O c3 +J rC CTi h— t o rt W ^— ' w :s (L> (U .-^ fH ■M ^ CO O o cti 10 ^— ' +-> z S c •H o bO u C H •H rt ^ C S r^ •H rt c ^H rt nj CO w -o (U cu O /— X 3 vo w cj ^ ^1 CO a> bOOOLn^O(NOOOrMO-^CT>(M invocncsitor^t^rsiO'^Lnvrj r-i-yor^Ci.— iOt-osDoor^-— I N^r^co•^^o^OK■)0^-o■^J■LO *-> i o <-< E- ^ ■H LO 03 O U 3 O ui O 3 c O u 0) E O u '^ t^ CM v£) lO Kl n- K^ O CTl CO LO vOi-H"— icoocTii—ir^^t-OLoco vOOit^r-nrMt-o.— lLr)Or^^o^^^ ro (M 1— I r— I '*'^r~-'*CO(NCNCOOOtOCO (Mt^'^OOOOCr>l^l-0(NL0 I— I'S-r-joooocTiCNvor^.-H M- 1— I "^ o ■n- ^ c3 rt ci rt r-iorsi\Drjr--critNCT>ocri'^o vDcoLot-^LOLno^ovoor^vor^ ,—1 1—1 CM CM r-( I— I OCO'3-l-OOOOOr--t^jLOCT>CN OOoOi— lOOOOCNOt^CTivO (N .— ( CTl \0 CM CN CTlrHK30t^t-^VDCri'*LOi-Hr~- cc LO CO ■^^ r^; CO LO ^ LO o o r~~ ■^Lor-Lot^r--Loi^LOi— i^co CnI \D (^ "^f I-'J CM K) O^ ^3 uo to i^ ,—1 I— 4 CM 1— I t— I LO t^ vD LO to .-H •^ 00 LO r^ o o rH LO CM !— I cTirjLocNOtoi-HOOcj^rMvDr^ COOO'^LO •tTK) (Ntn^CTl og to LO t-o >— I •^00'— it-^ocrir--covDcMO<7it-o vO'S-CTlOOCO.— IOO"s}-OOCO"rftO (Mcor^"— icMOi^'-Htor^t-o^cM (NCMr— ICMi— li-H .— ItOCMLOi-HtO f— lOCM'J-OtOLO'g-OOOCl-OCM rjt^LO\Dtor^'^r--tor^LO':t'^ rNJCT)(M00'*vOCMC0l~^O'-HV0v0 '^^tOtOCM'-H>-Hr— ^'^LOCO'^tLO COt^tOOOCM-^-vTLOOOOr-^O (MCriO'*tOCM>— itOOtOOOOO t-or^'-H'— loovD'^Lor^criCTir^o r-Ht— lf-^r-l r-lf— l(M.— ICM o to CTi to O O to LO CM to o en CM CM a~, o . — I CM LO LO 'a 4-J (/) 5-1 C •H 03 U o 3 6 U O C O •H (/) 1/1 CD 13 U O OO 13 C~-0 oi ■»-' 03 3 3 3 O CJ O S "-J "-5 < W O H S O u O a. a u o u u o c CD 3 5-1 O X O CD DO 5-1 CO OS 13 03 U 3 oi C/j o Cj-i O r: 3 5-1 e o 5-1 13 CU 3 O r-H o! o CO o; 3 f— I > ^ to ^1 cu i-> 03 ■M 5^ O CO o o 5h 03 O o c 3 u o 5h Cw -u 13 O o ■p ci 3 4-> 3 O CO CM O +-> CJ o o 4-> O > o e o M '4H o +J +-> c o s I I-H 0) s CJ 13 51 Table 3.- Sununary of hydrologic data for the major watersheds draining into Lake Izabal Voli ume of Area Annua 1 Runoff Annual Runoff Watershed (km2) (m^ . X 10<^') (mm) Rio Polochic 5,247 10 ,189 1,942 Rio Oscuro 233 1 ,923 8,253 Rio Sauce ■ 300 257 857 Rio San Marcos 170 144 847 North V/atershed 474 406 857 South Watershed 438 372 847 All Watersheds 6,862 13,291 1,937 52 less precipitation lose between 40-50-6 of the rainfall as runoff (Golley et al. 1971). Snedaker (1970) calculated runoff for Finca Murcielagos 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 Murcielagos. 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 llelenc, Anderson Cue, Lake Michie, Lake Chad, and the Caribbean Lowlands (Table 4). This 53 Table 4.- Monthly free water surface evaporation measurements for selected warm climates and warm seasons Region or Lake Monthly Evaporation (mm) Year Source Lake Helene, Florida Anderson Cue Lake, Florida' Lake Elsinore, Calif. Lake Tiberias, Israel Polish Lakes Lake Chad, Africa^ Caribbean Lowlands Lake Michie, N.C,^ Lake Colorado City, Texas^ East Africa (Nile Region) 128 1962 Pride et al. 1966 155 1966-68 Brezonik et al. 1969 231 - Szeicz ^ Endrodi 1969 193 1949 Reiser 1969 140-180 several Debski 1966 188 - Grove 1972 174-254 - Ray 1931 118 1962-64 Turner 1966 221-251 1955 Harbeck et al. 1959 90-120 - Tailing 1966 Based on an average of 5 warmest months (May- September) Based on an average of 4 warmest months (May-August) . Annual total divided by 12. 54 ft T is equivalent to 115 x 10 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 (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 precipitition, subtracting the evaporation, and subtracting the positive or negative change in volume resulted in the final value which estimates the monthly loss t'nrough 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 discliarge 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 194 8) . The ainiual water budget for the lake and the watcrslicd is summarized in Figure 10. The average residence time of the water in the lake was 55 CN u 6) O +J o o I 1—1 r^ U 0) ■i > o 2: C o VD to +-I O flj ^ — ^ i-H ^ u o c o •H •p rt t~^ o k-^ ni > tu n r-H ■p 1 — 1 o rt or-~ ^ooot^l-n^ocN^■^OvOOoo^^ •^ r, »\ ^ ^ r, I-H 1— I .— I to >— I r-H ^0 \0 ^ m ot t^ t^ ■^ VO 00 LO to (Nir-ii— icotoLOoot^rsitocr. -—I rsit^rHcor^ii— ifO^OLD'^i-O — i^H+ I++ i+t-ocMrNj,— I + 1 + + I I I-H r^ CM Lo tNi 00 vo-^tvO-i^r^CTior^iLoco^oo ^t^or-~>.0-*(Nf-HOr--crirH (N rH C-xI lO to .-H (M KJOOtOvOrslOOOtNO-NfOlCN '-H'^or^O'— ivot-ovooor^rH t-ot~^oo^tototoot-^0'a-LO to •-H .-H CM <^j r-i CM (Ni (^) r4 rvj (M rg + lOLOLOl-OLnLOLOLOl-OLOLOLr) o i-Ht — li — IrHi — li — ti — If-Ht-Hi — If— (i — I CO t-l .'H r-H f"< rH r-l r-i r-J r-H I-H i-4 .-I lO I I I I I I I I I I I I •> oo o CO \0 Lo r-^ LO CD O cn 03 0 (-3 -a o u w ■p a t/i C oj o o u", •H u t/i C o • H P 03 P t.0 -a •H w 03 JC r^ r-- r^ r-- r^ t^ (^ r-~ r-> t^ r- t-- < — 1 p oj c > u c J3 >H ^1 X c r-H GO C-. P p r\ o o 03 O C 03 Ci, EH O U Uh -o O P oJ a •H P 10 •H -a &, < 03 03 l4H O t/) (D fcO C 03 o E O ^-1 Mh "a o p 03 I — I u r-H 03 U c E i-H o o » — I 03 >

-6 - y O •*^ / u ^ / O / (^ >. / > .4 - / •n / o o 6 _ / / • o D .2 - / o u 9 0} _ / o n / / _L _i L_ 1 1 i 1 0 .2 .4 .6 .8 Measured Velocity (m /sec) o in 1/5 u w •p ■H c 3 o r-l 1 o --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 11.- Vertical temperature profiles of Lake Stations A, A-B, and B recorded at approximately monthly intervals. 63 Or 2 4 - Q - 10- 12- 14- o- 2 ^6 Q 8 Q 101- 12 14 STATION A O90O 0746 0620 0630 0656 0700 -SMjr BADT aiApr i3May UJun yjul ^ ■y ?7 ge ?9 g9 30 ?9 y> ?e 3027 STATION A-B 072O 07J0 0600 0847 0755 0830 23May 13Jun 9Jul 19 Aug 17Sep SOrt 30 31 29 31 28 30 29 3.3 28 30 28 30 ~1 0820 26 on 26 28 STATION B 0952 1210 1015 0945 0600 COX 3Jan 25Feb 16 Mar 8Apr 23May i3Jun ^t l^ 25__27^ 2926 28 28S30313031 0930 19 Aug 28 30 7 +J rt •o (U ^3 U O o 0 fH u -d C rt U 1 t/5 C o •H ■P rt ■p CO o ^ rt J M-i . o in r-H w nJ 1— 1 fH •H ID 4h P O c 5-( •H Oh X (D r-H U rC :3 P +-> c ctf o fn s (D fl, >% e t—i 0 p rt r-H ■§ o X •H o ■P !-. u f^ nJ 1 1—1 o u 3 M • H P, 65 ¥0m « — •— • — o— -•- ~~9 » • • O • •- — 9— •— • » — ♦ • •— o_ IPS -»— • — e — •— • — • — • — • « — « — ■►^ -• — • — • — ♦^ ■» — • — • — « — • — • — e — e — •- ^"^ .„ » — « — • — • — •— » — • — • — • — •- ■ I I I I I i I I I I 1 1 1 1 1 O C\j ■q^ (D 00 O C\J 1 1__1 1 I I J I I L. J 1 I i I C\J CT) C» O CNJ 66 vvater mass, the mininiiijT. 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, Tunico) . These groups are based not on water characteristics per se, but on the origin and locality of the water. The sarne 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 magniti'de of sers^rni rhTPges (Figure 13-16). T-ihle 6 summarizes the ranges of the extremes measured as well as the approximate ranges for m.ore 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 Pdo 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 sliowcd a trend toward increase after July. Temperatures increased during the dry season and began to decrease after June. Dissolved oxygen concentrations at rlie surface probably varied daily as much as tliey did seasonally. 67 Table 6,- Ranges of temperature, pH, total alkalinity (T.A.)j specific conductance, and dissolved oxygen saturation for all stations sampled at Lake Izabal Most Values Extremes Measured Lake Stations (A, B, C, & San Felipe) Temp. (C) pH T.A. (meq/ liter) Cond. (ymho/cm) O2 (% Sat.) 26.0-30.0 6.00-7.00 1.70-1.80 175-200 80-105 24.1-31.4 5.60-8.25 1.60-2.00 150-465^ 8-107 Swamp V/aters (Oscuro, Amatillo, El Padre Cr.) Temp. (C) pK T.A. (meq/liter) Cond. (limho/cm) 25.0-31.0 5.50-7.00 1.00-2.10 100-200 o-]no 23.8-31.3 5.20-7.30 0.82-2.37 65-230 0-113 Rio Polochic (Comercio, Coban, Bu j a j a 1 ) Temp, (C) pH T.A. (meq/liter) Cond. (ymlio/cm) O2 (% Sat.) 23.5-30.5 5.50-6.75 1.50-2.00 170-225 75-95 23.0-30.9 5.20-8.00 1.59-2.19 157-260 47-101 Small Rivers (Sauce, San Marcos Manacas Cr., Tunico) Temp. (C) pH T.A. (meq/liter) Cond. (ymho/cm) O2 ("6 Sat.) 25.0-29.0 6.00-7.00 1.00-2.75 100-250 90-100 24.2-32.8 5.75-8.20 0.80-2.82 79-268 84-105 The highest conductivity was recorded at San Felipe (5,000 ymho/cm) due to a localized brackish water m,ass. X +-> o •H O C cS •H U-( ^ u ca 3 r^ W S r— ( ctj 4->- u^ C rH r-l O ClJ 1/) -> +-> (D <— N O fH < ■IJ P^ fn O 0) -H u m -^ 3 0) oj ■P rH +J Cj U C/D fn !-i ^ O -H Ph U E 6 •H •H rt r-l •P • O rt w t/i PL, ^ S W) ■P o c c C -H -H 03 0 P 13 to (J ni rt V > r: p a> O > o C X U rCl o3 O O m 0) •V -o X / — s Ph ,-H C/) c O TJ 03 6 I/) -H a; (D 4-> ^ (N Wl o u rH c D 03 T3 T3 •\ X C C r-^ O O 03 CQ o ■-1 m C o3 - M i O C ^ C •H O • -H P W < 13 OJ rt • rt P (U H •H a) c o •H rt —1 f^-^ oi U ^ 3 >— 1 to rt ■P g ^ C rt 0) ^o +-> w o o - ■i-> U ■■-^ Ph o '■ o u •p T3 e •H •V -H LO c o O C/) +-> • H 03 ■P OS • w ?H W bO •P fn C C a; -H 0 P T3 O 03 03 C 5 0) O (-^ o a. C 3 O /- — \ CD S P ^ CO If) -P (D X o Ph t3 .— ( t/) oj o -a tu a, w C >-i W OJ o 1—1 •H M w tS •> -H > 1 X O C ^-' 6 •H -H U > 03 CM W -H O (D -P .-< T3 bO O O C C 3 03 03 -a T) ^ n c ». O O ci3 u O ,-1 w I— t o3 '. OO 1 — i G rs C •H O • -H ■P to < 13 03 03 • zi to ^-^ fn 0) 72 CO CD (J5)ii/6uu) N39AXO O O CO O ^, n c\) cvj c\J 0)dN3i CM C\J C\i ^ - - Vl AilAliDnaNOD r- U d) O o a, (D o ■H +-> ■P c o o o <4-l o (D •H Oj ■(-> -1 C +-> a> tn bO-H X! O ^1 o O (/I to Ph C 03 +-> > X o •p fH •H u C ■ H 00 r-H cd cd o A! cd 1—1 C Cti CTj s r— * Cd ■X3 +-1 c o cd ■p »\ •% m 0) o JH o p U +J cd rt S f-i (D c P^ ctf 6 CO 0) p »» a> •l o c :3 o cd •H on •P 03 M (fl +-> f-i C (U 0 > o •H f: M o o ! 1 f-H c Cd Os t— t o -O in c to cd • H -o •\ X c +-> •H •H > f> •H 0) +-> M O C 2 cti 'O X c (J o o —1 cd •4 C /-^ o • w < cd 0 H w ^.— ' vO 3 •H PL, 76 I/) o (J DC < < in O U iTi < < o a: UJ u < O q: _i I I J — I — i- -I I I l__l L. \ -\ J I I L. 2 ■o c o (/) E c "D C o to rd E E T3 C o E CO E CO CD O OD CD ^ CvJ V- O If) f^ CM CM CM (jsvi/Biu) (J»)l|/b3UJ) (LUD/OMUJH) •^o 0)dlAJ31 Vl ■QNOD N (D 2 o 77 that Mil, Fe, Zii, and Ni were detected only occasionally, and always at the lov;er 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 25) yielding suspiciously high values of 2.7 and S.5 mg/liter, respectively. Except for the Rio Agua Caldente 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. Dis?o1\'ed phosphate concentrations ranged between 0.04-0. 17E 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 loiv discliarge of this river wouJd 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 Lhe 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 t!ian the lake water as suggested by the consistantly 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. 78 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 CI 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 analysts of Na and CI provided a more sensitive method than conductivity measurements for detection of brackish water as it entered the lake from the Rio Dulce. The average Na:Cl ratio for the April 14 lake station samples (excluding San Felipe and Station C, 12.5) was 0.5?.. 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 0_, 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 CI concentrations were higher in the Rio Polochic, they were not as high as the more dilute lake water on October 23 79 Table 7.- Sodium and chloride concentrations (mg/liter) and Na:Cl ratios for lake stations and distributaries of the Rio Polochic. Averages are calculated for selected stations 14 April 1972 25 October 1972 mg/liter Na:Cl Na CI ratio Lake Stations Station A - Surf 4.5 7.8 .58 Station A - 11.5 m 4.3 8.2 .52 Station B - Surf 4.3 8.8 .49 Station B - 15 m 4.3 8.8 .49 Station C - Surf 4.3 8.2 .52 Station C - 12 m 8.2 10.5 - San Felipe ■ - Surf 5 10 m 7.7 15.6 - mg/1 iter Na:Cl Na CI ratio 3.3 4.1 .80 3.2 2.4 (1.33) 3.5 6.0 .58 2.7 1.6 (1.69) 3.6 5.5 .65 3.6 6.4 .56 3.7 4.6 .80 .68a Average .52 Rio Polochic Comercio Coban Bujajal Average .66 1.76 Numbers in parentheses not calculated in average. 3.1 4.9 .63 3.1 4.5 .69 3.1 1.9b - 1.9 1.0 1.90 2.2 1.0 2.20 2.0 1.7 1.18 b Accuracy of determination questionable. 80 (excluding Station A-li.S m, and Station B-IS m) . "By examining Table E of the Appendix, there is no evidence for possible sources of high Na or CI 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 CI 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 CI/ liter) were reported by Brooks for lake water samples. Brackisli 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-Hl 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 tliird 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 Fl 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 tidol forces (Figure 20). This (D £ W 4-> C O C -H O •(-> cti ,~s 4-1 ^ W 1— ( 1 (U 03 t-H +-> M W C rt W)-H C U •H -P I— 1 DhX e +-> cfl -H c/^ S 13 . c E 4-' C tu c ;5 4-) (D O IT) 10 j:i X / > ,-< 4-> !-i w O 1 — 1 1 — I t4H 3 u O Q (D 2= Pk O O rt -H 1 — i S Di V 1 > ^ (D !h a m 82 0 U-, C CO 6 o >-■ o o 1—1 3 Q O • H «; (1) ,c +j C o CM ' 1— 1 r^ ' .-1 Oi ,— , C o •* ■H t^ ■P (N nJ 1 +J fS C/D CM v_^ X X o rt M CQ r3 s: 0 Cv 3 cr w • H 0 ■P i-H rt •H <4H ^ O f-l o fl, -M X ■(-> ,- — s • H 1— 1 > •H c +-> o O •H 3 ■p -a rt c p o W u V > 1 • 00 t-H <1> ^H 3 bO (li 84 UJ 'Hid3a Figure 19.- Temperature profiles at two stations illustrating a slight temperature increase associated with the haio- cline. On Station S, the halocline occurs at 5 m, on Station 9, at 5.5 m. 86 TEMP °C 29 30 31 32 C 2- 3. 4 6- LU Q 9- 10- II- ,2- 13- 14 .J MAR. 23. 1972 STA. 9 \ o 1 o ^3> o I 1 o I O 1 o liiiiiliipill«ili iW u (I) s o 1— 1 r-H o rt X T3 +-> -H ■M +J rt o ■p w ^H Id rt S tn •H 0 X c +J o •H <4H -P o a •H (D U r-l 03 •H > •H (D 4-" X tJ H :3 XI C • O rs O 00 0) CI M X 6 ^ (D o O ^^ e • O CM (U (H a • H IX, 88 o o o o" o CM h- O) , ro (M CM CM CO r c o o k_ ^— o o ^ U) =8=8: o o Q, O E o \ o o Q 2! O .8 ^ n — r— in — r- U5 — r- 1^ 00 — t— — r— o ro — I— o uu 'Hld3a 89 resulted in an upward displacement of the halocline by approximately 2 Til, demonstrating the mechanism for upstream movement of the high- salinity water mass. On March 22 the specific conductance of the ground water (ca. 30-cm depth) of the swamp forest at Cuatro Cayos in the southwestern end of El Golfete was measured. The conductivity increased from the edge of the island to 5-m distance from the shore, and then decreased gradually to a distance of 55 m where the ground-water salinity was still higher than the surrounding El Golfete water (Figure 21) . The presence of this higher salinity water indicated that salt-water intru- sion had occurred during dry seasons prior to 1972 and that the swamp forest, where som.e red mangroves were present, maintained the brackish condition throu.'^bont the wet season. On May 13 brackish water was detected as far upstream as Station 1 at San Felipe (Figure 22) on a transect which was conducted downstream to the upper end of the lower Rio Dulce at Station 8. The main mass of the salt-water wedge, determined at ca, 14,000 ymho/cm on March 22 at Station 7, had moved 7.6 km across the Golfete to Station 4 in 22 days, or an average of 345 m/day. The last dry-season measurements on June 13 included lake stations whose conductivities were measurably higher than previously recorded. At Station C the reading of 465 pmho/cm at the bottom (Figure 13) and other measurements above 200ijmho/cm were detected between San Felipe and Station C (Figure 23). 1 failed to detect a continuously declining gradient of conductivity between San Felipe and Station C on June 13. The presence of higli conductivity water at San Felipe (Station 1, 10 m) U) W) c •H 13 a (^ • in o X rt u o u +-> 03 :3 u •M nJ +J w . U o • ■p e as o S o tJ t^ c 3 Mh o o f-l box +-> -O Uh rt o ■p >^ a ■p •H c > (D •H ^ +-> oi O ■P 3 TJ C) C M o 0) CJ 3: CM •H 91 0001 iuo/o^mn 'AiiAiionaNOO o ■p 1— 1 c o •H ■P rt +J w Vw^ fH •H O P ^ O O P .-< 'O c o o ^ u p 1 (N CM Ki 0) o E o • ■H 1— 1 P bO O c O •H o a »l o CN ^ W ' t3 CD r\ P 6 03 0 U •p H) m ba X M w C3 X, 0 (D o i-H W 3 •H Q 0) O rH •H rt Di u 1 CO 1—1 rt r— 1 ^ OJ rt o t^ •H 1— ( ■P !h 0 O r^ > rt J P^ p o CQ t/) (U g) .H 98 uoisSuTAT'] :;b jcg puss SDina "H a3M0T O^ISJTOD IH -- adTiaj UBS 3 uoTaB:^s a uoiaBr^S V uot:;b:is sJtpBd IH BpBuasug- o z. c o • H 4-> 0) +•> CO (m) qtldoQ 99 explain ^Tiedra ulna (Nitzsch) Ehrenberg Blue-green Algae Anacystis cyanea (Kiitzing) Drouet 5 Daily Anabaena flos-aquae (Lyngby) Brebisson L>nigbya sp. Green Algae Staurastrum pingue Telling • S. leptocladum Nordst. var. denticu latum S. tohopekaligense Wolle Cosmarium sp. Pediastrum simplex var. duodenarium (Bailey) Rabenhorst Eudorina sp. Coelastrum sp. Pyrrophyta Ceratium hirundinella [O-F. Mueller) Schrank Cladocera Bosmina longirostris (O.F. Mueller) Eubosmina tubicen (Brehm) Moina micrura Kurz Ceriodaphnia lacustris Birge Diaphanosoma brach)airum Copepoda Diaptomus dorsalis Marsh Pscudodiaptomus culebrensis Marsh Mesocyclops edax (E.A. Forbes) Mesocyclops (Thermocyclops) inversus Kiefer Rotif era Brachionus falcatus Zacharias B. liavanaensis Rousselet Yeratella cochlcaris (Gosse) Filinia pej Icri Hutchinson Conochilus unicornis Rousselet Conochiloides dossuarius (Hudson) Sinantherina sp. Hcxarthra sp. Platyias sp. Figure 2S.- Seasonal changes in abundance of pennate diatoms and Melosira granulata. 107 C\J o X I/) c Melosira granuiata C Q _»__»_i_, .J — i — -U i— ^4=A--e L-^ .B 0 2 1 O £±— ,* L-e Jt^-'^-T--^ l_t ai m—i • ^ _A k- ix season -* — »- F M A M J J A MONTH S O 108 M. granulata filaments were present in high densities only at Sta- tion A but occasionally appeared in smaller numbers at Stations B and C. The origin of the Melosira at Station A was from horizontal displace- ment rather than in situ growth. It is a common dry-season occurrence for a steady breeze to blow every morning from the Polochic Valley, over the delta, and across the lake. On one morning I observed this phenome- non from a hill at Las Dantas overlooking the lake. A large plume of water whose color differed from lake water, moved out from Ensenada Los Lagartos and into the offshore area of the lake. M. granulata is of widespread occurrence in the summer plankton of eutropiiic temperate lakes. It characteristically occurs in lakes with blue-green algae blooms, particularly Anacystis cyanea, but their maxima do not necp'^'sa'^i ]y coincide (Hutchinson ]9<^7). Green algae St aura strum spp. were found only in low concentrations at the be- ginning of the sampling period (Figure 26 and 27) . Thereafter a bloom of Staurastrum pingue occurred at Station A on April 30 when 2,548 organisms/ liter were present. This was followed on the next sampling date by a less abrupt increase at Station B. Part of this abundance at Station B could have been a result of horizontal redistribution from Station A rather than in situ growth. If that were so, then the horizontal currents did not carry plankton to Station C as there was no evidence for an increase there. S^. pingue generally occurs in hard, productive waters and is often associated with blue-green algae and diatoms (Hutchinson 1967). Figure 26.- Seasonal changes in abundance of Staurastrum leptocladum and S^. pingue. 110 0 1 0 1 0 _C Staurastrurn ieptocladum _B 0) CM. b 0 X C 3 0 4 0 A *-r-» — *~1 — > -M 2 Z) QU JFMAMJJASO MONTH 113 S^. 1 eptoc 1 ad'jgp. var . denticulatum was the least abundant species of desmid and no large increases occurred until September when a maximum of 124.3 organisms/liter was reached at Station A (Figure 26). S. toho- pekaligense pulsed twice during the study period, simultaneously reach- ing high densities at all three stations (Figure 27). The April 30 high (466 organisms/liter at Station B) occurred when S^. pingue bloomed at Station A and the increase before September 19 coincided with the increases of S^. pingue (particularly Stations A and B) . Pediastrum s imp 1 ex var. duodenarium showed an overall increase from the beginning of the sampling period until May (Figure 27). At Station A the density began to decrease prior to the decrease at Stations B and C, a sim.ilar pattern to that of S_. pingue. The subsequent decline was followed by a higher peak in late July and August, reaching a maximum of 262 organisms/liter on July 29 at Station C. The most striking feature of the seasonal changes in the green algae was the prevalence of two periods of increase for the majority of species and stations, the first during April-May and the second during August-September. Blue-green algae Anacystis cyanea was the only species of blue-green algae present at all times of the year and in quantities worthy of reporting. Modest densities were present in January, but subsequent lower densities per- sisted until July (Figure 28). The extremely high densities encountered on July 29 at all tliree stations (543-899 organisms/liter) was such an abrupt increase that its occurrence could not have been predicted on the basis of increases prior to that date (except porliaps Station B). At tlie Figure 28.- Seasonal changes in abundance of phycomycetes and Anacystis cyanea. 115 (D O X in +-» E 3 J F M A M J MONTH A 5 O 116 time of this pulse, green algae and diatcns were present in low densi- ties relative to other sampling periods. Other algae Ceratium hirundinel la was present in low numbers (up to 19 organisms/ liter) during some periods and completely absent from the plankton at other times. Some forms of C_. hirundinella are eurytopic, but the species is most characteristic of eutrophic warm waters with a slightly alkaline pH. It is present in the epilimnion of many temperate lakes in the summer, but also was found in the small lakes of Java (Ruttner 1952, in Hutchinson 1967). Eudorina sp. was never more abundant than 1 colony/liter and was often absent, while Coelastrum sp. was nearly always present and reached a maximum density of 17 colonies/liter. Phycomycetes One of the most interesting observations was the occurrence of an organism believed to be an aquatic phycomycete. The free-living cottony masses did not appear to be attaclied to particles or otlier organisms. It was abundant in nearly all of the plankton samples until its dis- appearance at the end of the sampling period in September and October (Figure 28). The pulses of abundance did not coincide witli pulses of the dominant phytoplankton. The first pulse, evident at Stations B and C, occurred in February and March and preceded tlie dry- season blooms of phytoplankton. Likewise, the maximum abundance for the year was observed at Stations A and B whicli preceded wet-season pliytoplankton blooms. Failure to find cases wlicre tills or similar phycomycetes liave been re- ported previously as a component of lake plankton makes this observa- tion an unusual curiosity. 117 Copepods Cyclopoid copepods were represented by Therniocyclops inversus and Mcsocyclops edax, the latter being more abundant. Adults and copepo- dids were grouped together and were more abundant during the first half of th.e sampling period thnn the second (Figure 29). No abrupt changes v.'ore noted, and the lake appeared to be relatively homogeneous at all sampling stations for any particular sampling date. Of the calanoid copepods, Diaptomus dorsalis was far more abundant than Pseudodioptomus culebrensi s . Calanoids were generally less abun- dant than cyciopoids during tlie beginning of the sampling period, while tlie opposite vius true for July-September (Figure 29). Tliere seemed to be more similarities in cl\0-nges between Stations A and B than for Station C and the ctlier stations. However, the nauplii (of both calanoids and cyciopoids) had a very homogeneous distribution th.roughout the lake for any particular collection. Relative to collections preceding and after July 9, densities on that date were markedly low. There was some evi- dence of decreased egg numbers between May and June before the decrease in nauplii in July, but no comparable explanation exists for the sharp increase in nauplii at the end of July. Cladocera Diaphanosoma brachyurum increased throughout the dry-season months (March-May) at all stations (Figure 30). The decrease that followed was most abrupt at Station A, less at Station B, and least at Station C. Moina micrura and Bosmina longirostris showed no evidence of seasonal synclirony in abundance at the sampling stations. Little can be said about Figure 29.- Seasonal changes in abundance of copepods, 119 J F M A M J J A S O MONTH Figure 30.- Seasonal changes in abundance of cladocera. 121 Ceriodaphnia J FMAMJ JASO MONTH 122 Ceriodaphiiia lacustris except that it was present in moderate numbers throughout most of the sojupling period. Rotifers No distinct seasonal trends in abundance could be detected for rotifers. IVhere rapid increases did occur, they were more conmion in colonial rotifers at Station A (Figure 31). For example Conochilus unicornis increased to a maximum of 79 organisms/ liter at Station A during August, Conochiloides dossuarius increased rapidly to 24 organ- isms/liter at the same station in April, while the greatest increase in Sinantherina was between September and October at Station A (to 59 organisms/liter). The non-colonial rotifers showed no distinct seasonal trends (Figure 32) . This was likely due to the long interval between collection dates relative to the generation times of most rotifer popu- lations. Possible Controlling Factors The most apparent feature of seasonal changes in phytoplanlr.ton populations was the late dry-season pulse in April and May and a later wet-season pulse in August and September. Only the dry-season pulse in Melosira granulata at Station A can be attributed to horizontal movement rather than in situ growth. The synchrony between stations and between species of algae for the two pulses indicate that the factors controlling growth were present at the same time throughout tlie lake and had similar effects on most of the species. Environmental factors commonly accepted as regulators of phytoplankton growth are solar radiation, temperature, and nutrients. Of these, only solar radiation and temperature were Figure 31.- Seasonal changes in abundance of colonial rotifers, 124 20 10 0 Q Sinantherina 0 10 Q Conochiloldes J P'MAMJ JASO MONTH Figure 32.- Seasonal changes in abundance of solitary rotifers. 126 Brachionus 5 0 5 0 5 0 10 0 5 0 10 ■i-> ■^ 10 •t^ 0 ^ 10 0 10 0 10 0 10 0 10 0 - *^ -!-^ r^ t ■ -B I P ' ■ » I [I) ll ..I » I t I ft .. 1.1 ( 1 9- Filinia Keratella JFMAMJJA50 MONTH 127 monitored at sufficient intervals to enable detecting seasonal trends; saJTiples for nutrient analysis were collected only twice ("April 14 and October 23). Grazing by zooplankton may be an additional consideration, but it is unlikely that the zooplankton would have effectively grazed some of the larger phytoplankton such as Pcdia strum and Anacystis. Diatoms, however, may have been subject to some control by grazing. 7\nderson (195S) noted a phytoplankton minimum when nutrient deficiency was unlikely in Lake Lenore, Washington, but Moina hutchinsoni were moderately abundant. Thus, it is intere?ting to note that M. mi crura experienced an annual maxim.um at Station A when diatoms had decreased (Figure 30) . For the lake stations, and particularly Stations A and B, the sur- face and bottom temperatures were highest during the dry season and decreased in July at the beginning of the wet season (Figure 13) . This was a result of an increase in discharge to the lake of river water of lower temperature, noticeable in the deep samples from Stations A and B. The lower insolation in July also could have contributed to the decrease in water temperature. It is possible that the decrease in solar radia- tion could liave been partially responsible for the lower numbers of phytoplankton between the April-May and August-September pulses. Secchi disk transparencies in July for the three stations were among the lowest observed during the year (Figure 44). In the absence of large densities of plankton, the decreased transparency might be attributed to higher turbidity induced by wet- season inputs of silt and detritus- laden waters from the watershed. Floating debris, ranging from leaves to large logs, were especially abundant in the area of Station A, and as a result, navi- gation was often hazardous. 128 In the search for an explanation of the decrease during July for many of the planktonic species, the possibility of inhibitory substances cannot be overruled. This v/as the month of initial flushing of the colored waters of the lagoons (Amatillo, Lagartos) and rivers (El Padre Creekj Rio Oscuro) of the Polochic Delta. The water exported from this area to the lake differed sharply from lake water in its organic matter concentration (Figure 34) , dissolved oxygen, pH, total alkalinity, and conductivity (Figure 14). Although a complete list of potentially in- hibitory substances from these anaerobic or near-anaerobic waters is not in order here, a few would include hydrogen sulfide, tannins, and heavy metals (Cu, Zn, Al) either in ionic form or present as complexes with organic matter. In spite of the possibility of inhibitory srbstanres, some of the largest pulses in phytoplankton occurred soon after the July 9 low. It is interesting that some substances, inhibitory to certain species, are apparently stimulatory to others. For example, Lefevre et al. (1952, in Hutchinson 1967) collected pond water at different times of the year to test its effect on plankton cultures. Water collected in October, when there was a great deal of decomposition of higher plants, was stimulatory to two species of Pediastrum (£. boryanum and P^. clathraturm var. punctu- latum) but was inhibitory to two species of Cosmarium. Similarly, sub- stances which may be inhibitory at high concentrations may be stimula- tory at low concentrations. If the low numbers of phytoplankton sam.plcd at a time coinciding witli the initial flushing of the watershed were evi- dence of inhibition, then subsequent dilution of the lake by wet-season rains could have rendered such substances innocuous or even stimulatory. 129 Data on critical nutrients arc both scanty and unreliable, but it is unlikely that these would have been limiting. For example, silica ranged from 10-14 mg/liter for both sampling periods (Appendix, Table F.) well above 0-1 mg/liter, a range for which there is evidence suggesting a limitation of diatom growth in nature (Lund 19S0) . Because no samples were analyzed for nutrients during the July 9 low in phytoplankton den- sity, no conclusions can be drawn from control by nutrients. The dramatic increase of Anacystis cyanea on July 29 is noteworthy because it marked the increase of the other phytoplankton populations. Blue-green algae are typically found in waters with high dissolved organic matter and some of the highest concentrations of organic matter in the lake were detected in July (Figure 34). However, no specific organic substance has been found to facilitate their growth (Hutchinson 1967). Nevertheless, the sudden appearance of high densities of A. cyanea on July 29 marked the beginning of conditions apparently favorable to the growth of other phytoplankton. A. cyanea persisted until the end of the sampling period. A. cyanea is one of the greatest problem algae in eutrophic lakes of northern latitudes, particularly when it forms massive summer water blooms. In India, it develops permanent blooms in artifi- cial temple tanks. Maximum numbers occur in July and decrease to a mini- mum a month later in August. No changes in temperature or phosphate concentration is observed during the bloom reduction (Hutchinson 1967). Grazing by zooplankton can be discounted as a factor contributing to the July 9 low in phytoplankton not only because it is likely that most of the algae were too large to serve as a direct food source, but also because many species of the zooplankton were present in low numbers 130 on that date. Nauplii underwent a sharp decrease as well. An exception was the pulse in Moina micrvira at Station A which reached a maximum of 21 organisms/liter. In spite of the homogeneous distribution and the distinct bimodal pulse demonstrated by the net plankton during 1972, it would be tenuous, at best, to expect this to be a regular annual occurrence. However, Nordlie (1970) also has evidence of seasonality in the plankton abundance of Lake Izabal. In August 1969 the phytoplankton diversity was approxi- mately the same along the east-west axis of the lake. Presumably it was also relatively abundant. Heaviest zooplankton populations were at the west end and their abundance decreased in the eastern region. This may have been a situation similar to my August 1972 observations. During March 1970, Nordlie's samples showed the greatest phyto- plankton diversity at the east end of the lake, while almost none were present from the middle and west end. Zooplankters were similarly dis- tributed as in 1969 but more dense. Although 1 found phytoplankton to be much less abundant in March 1972 than August 1972, it was much more abundant than implied by Nordlie for his March 1970 samples. It is possible that Nordlie's sampling in March preceded a dry-season pulse in abundance, sucli as that observed in April and May 1972, Benthic Community The most interesting curiosity of the bottom fauna was the occur- rence of Tanaidacea, first reported from the lake by Nordlie (1970). Several species of this family are known from fresh waters, but it is likely that all are dependent on slightly more saline water than is usual far from the coast (Hutchinson 1967). The highest density recorded 131 was 296 organisms/m^ at San Felipe Bay (Table 9) where the penetration of brackish water from the coast is at least a seasonal event. It is doubtful, however, that the specimens collected from the outfalls of the Rio Polochic distributaries (Comercio and Coban) ever come in con- tact with water of conductivity greater than ca. 200 ymlio/cm. More sampling, both on a spatial and seasonal basis, would be necessary to adequately establish distributional and temporal patterns. Nordlie 2 (1970) calculated abundances of Tanaidacea as high as 1,378/m in pre- vious collections from the lake. Members of the family Chaoborinae, presumably Chaoborus , were present in the bottom deposits both in larval and pupal stages. Den- 2 sities were as high as 77 organisms/m and they were present at all localities sampled except Station 15 (Figure 39) where none were found. They also were collected occasionally in net plankton tows. The Tendi- pedinae were found at all stations and ranged between 10-77 organisms/m Adults were seen frequently and in massive numbers above the lake. Oligochaeta were present at five of the twelve stations with densities 2 ranging from 19-153 organisms/m . The gastropods, most of which were not alive when collected, were present at six of the twelve stations. The average density of 29 organisms/m" for the Chaoborinae was 2 below that found by Deevey (1957) in Lake Amatitlan (40/m ), and much 2 lower than his mean for Lake Giiija (l,27S/m ). Deevey found no Tendi- 2 pidac at Lake Giiija but reported a mean of 673 organisms/m for Lake Amatitlan, an order of magnitude greater than my samples at Lake Izabal (Table 9). The density of bottom fauna was strikingly low compared to lakes in temperate latitudes (Deevey 1941), but it must be remembered that 132 \D I O ^1 (U o +J o o •xi (D ■P O o o (U CTi fH to in -H • w w V) a u rt +J txi rt pL c 3 •H CL, fH o 0 43 03 o > a ^l x: oj u ►J rt 0 o oJ 13 ■H oi C 03 E- ■p ^— 0 > ' CO i^ rsi vC' vi; O 00 o o CC '^ LO CJ -^ O) CO o CTl CO ^ CO f-H r^ >* ^ I^ r\l r— 1 K5 (Nl (N .— 1 K! 1— I 00 o CTl LO LO CTi CM O h- r-- CO CO 1"^ r-- 00 r^ t^ cn r-- LO 1-^ to ^g- LO vO ■^ LO LO rsi r-- o CO to o CTl cr> 03 to CTl CD OI CTl CTl rH to r-H rsi LO CM o 1 — 1 LO CM o LO e e ct3 03 o3 03 03 rt c P m C O o c P P P P P p 03 to O w u u u to to t/j 00 to CO CO o CM r-- ^o CTl o ■^ to O r^ CO CT> o z f-H 1—4 f— 4 1 — 1 r— * . — 1 1—4 1-4 (N 00 CTl 00 1— I LO 00 1—4 LO CM LO C •H 03 P U 0 o c 3 0 03 O o bO •H o o c 0 t/) 03 •H p o p 0 13 13 0 to o fii E o o 0 13 P 0 p t=J3 03 03 &, U 0 0 > 1—4 < Ph E o3 CO IJJ these numbers represent standing crop and not rates of biomass increment. Although the higher year-round temperatures of tropical lakes may result in growth rates of benthic organisms as much as tX'io or three times those of temperate lakes, it is doubtful if this factor would make their pro- duction comparable. A plausible explanation for the difference is that the more intense metabolism of the free water would tend to be more com- plete in tropical lakes, so that less surplus energy reaches the sedi- ments and their fauna (Deevey 1957) . This may be true for monomictic lakes (studied by Deevey) which rem.ain stratified throughout most of the year, but for lakes such as Lake Izabal that mix frequently to the bottom, it is unlikely that the benthic organisms would be restricted by energy availability. A more reasonable explanation may be predation by bottom- feeding fishes. Such fishes would have access to benthic organisms in all areas of the lake since they are not restricted by an anaerobic zone in Lake Izabal as they are seasonally, at least, in monomictic lakes. Bottom-feeding fishes are well represented in the ]ake and include marine catfish and several species of cichlids. The presence of substantial amounts of organic matter in the surface muds will be discussed as a potential source of food for detritus consumers in the section, "Metabo- lism and Organic Matter." Svurmiary Statement The net phytoplankton assemblage of Lake Izabal was characteristic of that found in many other shallow productive lakes with moderately hard waters. Most of these species have a wide latitudinal distribution. The predominant taxonomic groups included diatoms, myxopliycetes, desmids, and 134 members of the chlorococcales. The zooplankton composition had a slightly more tropical flavor, especially with regard to the copepods. For exam- ple, species of Mesocyclops were present, a genus of predominantly tropi- cal distribution, and Diaptomus dorsalis, also present, is common in countries bordering the Caribbean. Cladocera were less abundant than copepods, and while the rotifers were more diverse than other zooplankton groups, their abundance was much lower. The only surprising feature of the benthic community was the relatively abundant occurrence of a member of the Tanaidacea which is a group characteristic of marine and brackish waters. The most apparent feature of seasonal change was the late dry-season pulse of phytoplankton in April and May followed by a wet- season pulse in August and September (Figure 53). These pulses did not seem to coincide with rates of gross primary productivity except in late September and October when both phytoplankton abundance and primary productivity rates decreased (see following section. Metabolism and Organic Matter) . Apparently differences in the standing crop of net phytoplankton over periods of 3 or 4 weeks were an insensitive indicator of daily rates. This could be attributed to the high variation in rates of turnover possible in a system with low storage capacity. In most cases the pulses could be attributed to in situ growth rather than to distribution by hori- zontal currents. The interluding low abundance between the two pulses coincided with the inception of the wet season when runoff increased dramatically, discharging large quantities of silt and organic debris into the lake. The resulting decrease in transparency of the lake water may have been partly responsible for the low phytoplankton abundance during >% ^ -a o +-> c a> tn (U Vh fi| 0 Vi / N u 01 •p •H r-l u 0 fll w 6 w •H • c U) rt 0 bO 4-> 5-1 O O u X i-i p: o O o U) >> -t-J ^ •H Ph c 3 -tJ ^^ — ^ c cd O •\ c c CS o t) +-> c ,^ 3 c rO rt rt .— 1 PL, C o O o +-> t^ ^ C ^ rt c rH o Ph*-> 1 A^ 4-> c Q) rt C T-H CL, rH o 03 •p 4-> >^ O r^ E- Ps • ^0 to o u 3 w 1 •H m 136 o o o o o o lO o in o o o o o o m o in o C O o o o o o o o ID o 10 -^ ^ sz c O O o CVJ J9|!l /suus!ue6jo 137 this period. Other controlling factors thnt cannot be overruled are the influx of inhibitory or toxic substances, moderate decreases in tempera- ture, or changes in nutrient availability. The low abundance In June and July was interrupted by a bloom of Anacystis cyanea in late July whicli shortly preceded an increase in most other species of phytoplankton. Zooplankton populations on the whole were comparatively more stable than phytoplankton populations. METABOLISM AND ORGMIC MATTER This chapter, which represents the core of the present study, attempts to estimate the magnitude of the principal energy flows respon- sible for the metabolism of the lake ecosystem. One of these energy flows, that from allochthonous detritus input, received special atten- tion. In lakes that approximate the surface area of Lake Izabal, alloch- thonous energy sources are generally regarded as insignificant contribu- tions to total metabolism. This is undoubtedly true for lakes whose re- placement time for the water mass is in the tens or hundreds of years. However, by virtue of the relatively small volume of Lake Izabal in rela- tionship to its surface area, as well as it location within a watershed receiving high rainfall, the annual replacement of a large portion of the water mass by runoff could conceivably produce an energy surplus or deficit. This would ultimately depend on the organic matter concentra- tions of the inflowing and outflowing waters. Other important metabolic compartments in aquatic ecosystems are the plankton and the bottom muds. In the gradient from deep to shallow lakes, the proportion of total metabolism attributed to the bottom muds becomes greater. Thus some attention was given to the energy flow of the benthos, although the metabolism of the planktonic compartment was ex- pected to be of greater magnitude. 138 139 Methods Chemical Oxygen Demand The concentration of organic compounds in the lake and river waters was determined by oxidative digestion with potassium dichromate and sul- furic acid, commonly known as "wet oxidation." Several procedures were reviewed and a number of modifications were made before routine analysis was established. Silver sulfate (Ag SO ) used as a catalyst for the oxidation is particularly effective for straight-chain alcohols and acids (Golterman 1969} but certain phenolic compounds are generally resistant to oxidation, even in the presence of a catalyst. In spite of this and other disadvantages, the method was generally useful in the absence of more sophisticated instrumentation. Fraction definitions and conversion criteria The limnological application of organic analysis by quantitative di- chromate oxidation has been reviewed and tested by Maciolek (1962). To maximize the information that could be extracted from a single test, the samples were separated to distinguish three size fractions. Total_ COD rep- resents all the organic matter sizes in the sample, dissolved and particu- late. Dissolved COD is the fraction remaining after vacuum filtration through a O.SO-y pore membrane filter and theoretically should contain par- ticles no greater in diameter than the filter's pore size as well as solu- ble organic matter. Particulate COD refers to organic matter greater than O.SO \} in diameter, calculated by subtracting the total COU from the dis- solved COD concentration. Net Particulate COD is the fraction collected with a No. 20 plankton net wliose mesh aperture is 76 y (Welch 1948). 140 Theoretically tliis sliuulu represent particles whose diameter is greater than the aperture size (approximately 100 times the size of the particu- late COD) and corresponds to the organic matter of the net plankton. Maciolek (1962) reported COD as Oxygen Consumed (O.C.)- He pointed out that a theoretical Oxygen Equivalent (O.E.) must be assumed in order to convert COD (or O.C.) to organic matter. The O.E. is constant for an individual pure compound (e.g., the O.E. of hexose is 1.06). The mg Oxygen Consumed divided by the assumed O.E. for representative organic compounds yields the weight of the organic compound, mg O.C. (or COD) „ • m ^^ —2 ;^— = — — ^^ = mg Organic Matter. mg O.E. " Several approaches can be used to determine the O.E., including calculation from elemental composition and from proximate composition determined from proteins, lipids, and carbohydrates. Table 10 lir,ts characteristic O.E. 's that were reported by Maciolek (1962) in approxi- mate order of their limnological importance. In the section-- Balance of the Organic Matter Budget -- where the energy budget of the lake is calculated, a value of 1.44 O.E. was selected to convert COD values to organic matter. The relationship between combustion calorimetry and oxygen consumed is close enough to permit an accurate caloric estimate by quantitative oxidation (Maciolek 1962). A value of 3.4 gcal per mg of O.C. is suggested, which would be equivalent to approximately 4. 86 gcal per mg organic matter. Collection and treatment of samples Water samples were collected in the field in clean 500-ml narrow- neck amber glass bottles. When deep samples were collected a 2- or 3- liter Van Dorn bottle was used. Surface waters were generally taken by 141 Table 10.- Characteristic oxygeji equivalents [O.E.j in approximate order of their limnological iipportance^ Repr esentative Sample Type O.E. Range O.E. Dissolved matter 1.42-1.47 1.44 Seston 1.39-1.53 1.40 Organic sediment 1.46 Phytoplankton 1.40-1.47 1.44 Aquatic invertebrates 1.48-1.69 1.54 Net plankton 1.40-1.55 1.52 From Maciolek (1962) . 142 immersing the collection bottle beneath the surface" (10-25 cm). The sam- ples were stored on ice during transport to the laboratory. Generally there was a maximum lapse of 7-8 hours between the first field collection and the laboratory treatment of the samples. The procedure for chemical oxygen demand (American Public Health Association 1965) was modified slightly. Mercuric sulfate (Hg SO ) was not added to eliminate interference by chloride because of the low concentration of this ion (<12 ppm) . One hundred-ml samples were oven- dried at 95 C in 200 or 250-ml Erlenmeyer flasks requiring approximately 18 hours for complete evaporation of the water. Ten ml of 0.05N^K^Cr^O were added to the flask followed by 25 ml of concentrated H SO in which 5 g/ liter of Ag SO was dissolved as a catalyst. The sainples were digested for three hours in boiling rain water in unsealed pressure cookers to facilitate even heat distribution of the samples. The mouths of the flasks were covered with aluminum foil and the necks of the flasks pro- vided a refluxing surface which may have minimized the loss of volatile organics. After digestion, 100 ml of distilled water were added and the flasks were cooled to room temperature in a water bath. Titrations of blanks and samples were made with a 0.025N (approx.) Fe(NH^)2(S0^) 2- 611 0 solution and three drops of ferroin solution (G. Frederick Smith Co.) were used as an indicator. To determine the dissolved fraction of the total organic matter, 100 ml of sample were vacuum filtered through a O.SO-ypore Metricel GA-4 membrane filter (Gelman Instrument Co.) The fiJters were first rinsed with distilled water to rid them of possible organic contamination and were then purged before filtration of the sample with approximately 200 ml of 143 of the sample to clear the distilled v.'ater. The dissolved organic matter samples were treated as described above and the particulate organic matter was determined by subtraction. A No. 20 plankton net (25-cm diameter) was used for filtering water and concentrating samples for the determination of large particulate organic matter. River samples were collected by holding the net below the surface in the current (of knoun velocity) for a known time period (usually less than one minute). When velocities were too great for hold- ing the net in position, a bucket was used to remove samples just below the surface and 200 liters were poured through the plankton net. Samples were stored on ice until they were transported to the laboratory. They were always analyzed the same day as collected. The procedure for digestion and analysis was modified from the method of Golterman (1969) . Samples were concentrated to 50 ml or, if the total sample was too high in organic matter for this procedure, an aliquot (1/2 or 1/4 of the total sample) was used instead. Since the sam.ples were high in organic matter concentration, they were not dried and reagents that were added for digestion were the same as described above except that l.OOON^K Cr 0 was the oxidant. The aluminum foil-capped flasks were digested for three hours. After dilution vv'ith SO ml of dis- tilled water, the samples were coiled and titrated with 0.25N (approx.) Fe(NH ) (SO ) '6 HO in the presence of 3 drops of ferroin indicator 4 4i 4 2 2 soluti on. Dissolved Oxygen Concentration Determinations The precision of methods for the determination of dissolved oxygen in natural waters makes possible the measurement of the metabolic activities 144 of cojisumers and producers over relatively short periods of time. The method used for dissolved oxygen in this study is a modification of the Winkler method (Golterman 1969] . The modification employs a much higher concentration of KI than normally used, thus reducing errors due to vola- tilization of I_ and interference by organic matter. Hydroxides dissolve more readily and the starch endpoint is sharper. Nitrite (NO ) interfer- ence was eliminated by use of sodium azide. Approximately 0.010N_ sodium thiosulfate was used to titrate duplicate 100-ml aliquots from each bottle resulting in a precision of about 0.03 mg 0^/liter. Ground glass-stoppered BOD bottles of 300-ml capacity were used for nearly all water collections for oxygen determination. The MnSO reagent and alkaline iodine-azide solution were added immediately after sampling and acidification was delayed until immediately before titration in the laboratory. Biological oxygen demand Large plastic containers (9.5 or 18.9 liters) were used to collect water sajnples in the field for transport to the laboratory. For the river and lake surface samples, the mouth of the jug was held approximately 20 cm below the surface and allou-ed to fill. Deeper samples were collected with a 3-liter Van Dorn sampler and the water was drained into the plastic jugs. A subsample was usually collected and fixed at the field site for measuring the in situ dissolved oxygen concentration. In the afternoon or evening of the day of collection, the oxygen concentrations of the water in the plastic jugs were measured with a YSI oxygen meter (Model 51A) . If the concentration was below 5 mg O^/liter, the samples were aerated until they contained 6-8 mg/liter oxygen. 145 After thorough inixing of the v\'ater to ensure hoinogeneity of organic matter and oxygen, samples were carefully siphoned into eight darkened BOD bottles. The first and the eighth bottles filled were immediately analyzed for oxygen. The other six BOD bottles were submerged in a darkened water bath to prevent air leaks and to minimize ambient fluctua- tions in temperature. The bottles were agitated daily to resuspend par- ticulate matter that may have settled to the bottom. Duplicate bottles were analyzed for dissolved oxygen after one, three and seven days of incubation. Due to the limitation in number of BOD bottles, several trials were incubated for five days only, i.e., duplicates were analyzed on day zero and after five days and respiration rates were averaged on a daily basis. A directly comparable 5-day daily rate can be determined from the other procedure by averaging the 3-day and 7-day oxygen concentrations, subtracting this from the zero-day con- centration, and dividing the difference by the number of days (5) . Respiration of bottom muds and their organic content Samples of bottom mud were collected during October 1972 with an 2 Ekman sampler (522.6 cm ). When each haul (2 per station) was lifted into tlie boat, efforts were made to disturb the structure as little as possible. The top leaves of the sampler were folded back to gain access to the mud and a hypodermic syringe was used to remove 5-ml subsamples from the upper 2 cm of the mud in the Ekman sampler. The orifice of the s>'^ringe was enlarged to 7-nmi diameter to allow the collection of parti- culate matter. Two 5-ml subsamples were placed in a 300-ml BOD bottle which was stoppered and returned to the laboratory. Three BOD bottles were filled from eacli liaul, for a total of six per station. 146 In the laboratory, the BOD bottles were carefully filled by slowly siphoning lake water down the side of t!ic bottles to minimize disturbance of the mud. The water was initially turbid, but became clear after one hour with the settling of suspended mud particles. An initial bottle was filled for determination of the 0 concentration in the lake water as well as three control bottles for determining the respiration of lake water only (without mud) . The bottles were incubated in darkness at ambient room temperature (ca. 25 C) . Subsamples from these BOD bottles were drawn from ca. 4 cm above the mud layer into 75-ml bottles with an aspirator. The volume of re- agents used for oxygen concentration determination were adjusted to the size of the sample. The initial bottle (lake water) was analyzed immedi- ately after all samples were prepared. Duplicate bottles with mud and one buttle witli lake water weie analyzed for OAygen after Lwo, f oui , and eight hours of incubation time. Respiration rates were calculated from the differences between these determinations. To convert the rate of change of oxygen concentration to respiration rate per unit area of mud, the following calculations were made: Volume of water mass = 0.300 liter (total) - 0.010 liter (mud) = 0.290 Inside diameter of BOD bottle = 6.35 cm 2 -4 2 Cross sectional area - 31.67 cm or 31.67 x 10 m mg 0^/liter 0.29 liter ^ X 37 — 7^ = 91.57 mg 0 /m- hr hr 31.67 X 10 m" The respiration rate (mg 0„/]iter hr) was multiplied by the constant 91.57 2 9 to yield mg 0 /m" hr. 147 The Ekman samples, from which the small subsamples were removed for the mud respiration experiment, were passed through a No. 40 mesh screen (U.4i7-mm aperture) to retain particulate matter and mud-dwelling organisms. After removal of the organisms for counting (see Section-- Planktonic and Benthic Communities), the rem.aining samples were analyzed for organic matter content. Samples were oven dried (70 C) and weighed, then ignited at 550 C for 1.5 hours and reweighed. The weight loss by 2 ignition was calculated per unit area (m ) of mud surface and as percent of the total particulate matter retained by the screen (excluding the subsamples for respiration rates and the organisms). Light and dark bottle method Several attempts were made at the beginning of the study to esti- mate primary production by the use of diurnal changes in oxygen concen- tration of the water mass. These attempts were abandoned due to the erratic results from the error involved in the horizonal displacement of water masses of differing metabolic history. The oxygen light and dark bottle method (Vollcnweider 1969) was then used and stations for measurement were selected in the western, middle, and eastern areas of the lake (Figure 4). Water samples were collected with a three-liter Van Dorn bottle from the surface, and 1, 3, 6, and 11 m. After distributing each sample among an initial, light, and dark bottle (covered with black electrical tape and aluminum foil), the two latter bottles were returned to the depth from which they were collected for incubation. The initial bottles were fixed and placed in an insulated box to exclude sunlight and prevent excessive heating. The trials were usually duplicated at each station and the incubation time, in most cases, was for tliree hours, from about 0900 to 1200. 148 V/hen posrible, relatively cloudless mornings were chosen for the measurements arid the three stations were measured on consecutive days. Total incoming radiation was measured in El Estor with a pyrheliometer (Solar Radiation Recorder 9-401, R.E. White Instruments, Inc.). Secchi disk values were recorded using a standard 20-cm diameter disk. Results and Discussion The bulk of the data for organic matter, measured as chemical oxygen demand (COD), is available from March through October 1972. Before March the procedure had not been modified sufficiently to yield reliable measurements of COD in moderate to less dilute concentrations; only mea- surements of net particulate organic matter are reported for December 1971, January and February 1972 in the Appendix (Table F-6) . 9 The results will be reported as mg COD/liter or g COD/m" of lake surface, or as rates, i.e., g COD/m day or g COD x 10 /month. The re- sults vs'ill not be converted into organic matter equivalents until the section , Balance of the Organic Matter Budget, because (1) the tech- nique measured COD directly and organic matter only indirectly, (2) com- parisons among stations, collection dates, and with data in the litera- ture (often reported as mg COD/liter) can be made directly without conver- sion, and (3) the conversion requires some rounding off of data. In a few cases the reported concentration of dissolved COD is higher than the total COD concentration. This is apparently due to variation in sampling or to error in the analysis of the samples. When further calcu- lations were made, the higher of the two values was used as the total COD concentration. 149 The concentrations of COD will be examined first, followed by the rates of input, that is, the concentrations multiplied by the discharge rates of the rivers. Concentrations of Chemical Oxygen Demand The concentrations of COD in all samples (as mg/liter) are reported in the Appendix, Tables F-lto F-6 . For simplification and clarity, the data are partitioned into water types recognized earlier in the discussion of water characteristics (p. 66). Swamp waters The waters of the Rio OscurO; Amatillo, and El Padre Creek are coffee-colored, visibly suggesting the presence of dissolved organic matter. The highest concentrations of total and dissolved COD were mea- sured from this water type (Figure 54a) . July represented the month of greatest total COD concentrations, with Rio Oscuro highest (63.13 mg/ liter), Amatillo next (54.05 mg/liter) and El Padre Creek third (46.30 mg/liter). These maxima coincide with the first rains and appear to be a result of initial flushing of the swamp forest. During the dry season (March-June), Rio Oscuro showed marked dif- ferences in COD concentration between the surface and 5 m (Appendix, Table F-1). This was due to the independent origin and stratification of the two water masses (as explained in the section-- Hydrology and Water Characteristics) with the more concentrated and warmer surface waters originating from the swamp. The water column was thermally stratified at the Amatillo station also, but the two water masses were of the same origin and had similar Figure 34.- Concentrations of particulate and dissolved COD (mg/liter) during the sampling period for (a) swamp waters, [h) Rio Polochic distributaries, (c) small rivers, and (d) lake stations (A, B, C, and San Felipe). March, April, and May are dry-season months; n.d. indicates that no data were collected. 151 60 50- 40 30 20- 10- A "^ _j D- Particulate COD D- Dissolved COD MAMJJAO MAMJJAO Oscuro Amatillo M A M J J A O El RadreCr 0 Q 20 o U 10 B R E - 0 — =aBs« iS(£. M A M J J A O Comercio MAMJJAO Cob&n I M A M J J A O Bujajal M A M J J A O San Marcos M A M J J A O Sajce M A M J J A O Manacas Cr 10 D MAMJJAO M A M J J Station A Station B A O 10 M A M J J A O Station C i«!3^ nd .^- M A M J J A O San Felipe 152 COD values. The slightly higner COD concentrations at 3 m (Appendix, Table F-1) in March and April may represent real differences (relative to surface) since the 3-m water was anaerobic. Inorganic reducing substances could have contributed to the COD at 3-m depth since the test does not distinguish between organic and inorganic reducing compounds. The generally higher dry- season measurements of COD at Amatillo as compared with Rio Oscuro and El Padre Creek were likely the result of intense localized phytoplankton production. In these stagnant waters current v\'as absent, whereas Rio Oscuro always had a slight dry-season flow. The high net particulate COD's from April through June support the visual observations that phytoplankton biomass was high. Rio Polochic distributaries The COD conceiitrations of all three distributaiies show an almost continuous monthly increase until June, and a subsequent decline through October (Figure 34b) . The differences between the distributaries for any one month were probably variations due to sampling. The COD concentra- tions of the July samples were predominately due to particulate matter, unlike the swamp waters of Figure 34a. The Rio Polochic samples were also quite high in the proportion of particulate COD in October. Small rivers Three small rivers were sampled for COD from March through October (Figure 34c). The range in total concentration of C0!1 (3.69-24.06 mg/ liter) was similar to the Rio Polochic distributaries (4.97-27.76 mg/ liter). The Manacas Creek samples showed the least variation; particulate COD never increased above one-fifth of the total COD (August 21). Since 153 the only noticeable discharge at Manacas Creek was during July and August (the months of the two highest COD concentrations), samples from other months contained dissolved and suspended COD probably unrelated to runoff. The Rio Sauce had no detectable flow until June, accounting for the sharp increase in both particulate and dissolved COD at the beginning of the wet season. The July sample had an extremely high net particulate concentration (Appendix, Table F-3) which exceeded the sensitivity of the test used. The Rio San Marcos was in constant flow throughout the year and differed from the Rio Sauce in that the watershed was mostly deforested, possibly contributing to the visibly higher silt load. Total COD concen- trations never reached the high values of the Rio Sauce. Lake stations and outlet Stations A, B, and C, and the San Felipe outlet demonstrated fewer differences in seasonal COD concentrations than all river stations (Fig- ure 34d) . As a result, seasonal trends are harder to discern. However, by comparing dry- season with wet-season COD concentrations some differ- ences can be noted. At Station A, March through June samples were below 9.00 mg total COD/ liter, while from July through October total COD values were above 10 mg/liter. At Station B the differences between the two seasons were not as great. Station C had no samples above 10 mg total COD/litcr. Only in June, July, and August did they exceed 9 mg/liter. At San Fciipe there was no evidence of seasonal trends in COD concentra- tion and values fluctuated around a mean of 8.96 mg/liter. 154 Monthly Flows of COD The discharge rates of the rivers and watershed areas that are listed in Table 2 were used to calculate the rate of organic matter flow into Lake Izabal from March through October, the months for which COD measure- 3 ments were available. The concentration of COD (mg/liter or g/m ) mul- tiplied by the monthly discharge (m x 10 ) yields the monthly flow of total COD (g X 10 ) . These calculations were performed on all COD frac- tions for the three Rio Polochic distributaries (Comercio, Coban, and Bujajal), Rio Oscuro, Rio Amatillo, Rio San Marcos, Rio Sauce and the remaining watershed areas to the north and south of the lake (Appendix, Table G) . The COD export at the San Felipe outlet was calculated from the outflow rates estimated by balancing the water budget (Table 5) and the COD concentrations measured from that station. These data are summarized in Table 11 on the basis of calculations in the Appendix, Table G. Dry- and wet-season total inflows increased by approximately one order of magnitude between May and June, the transi- tion to the wet season. Total COD inflow nearly doubled between June and July, and declined thereafter (Figure 35). The output at San Felipe was relatively constant and close to the total inflows for March, April, and May (dry season) . June, July, and August outflows from San Felipe lagged behind the total inflows, but by October, the flow values were approximately the same. The average monthly inflows for the wet season (June, July, August, and October) were approximately 11 times tlie monthly inflows for the three dry-season months. For the purpose of later calculations, the September flows, for whicli no data were obtained, will be defined as the average of the August and October values. Accordingly, total September 155 w s c O -H i-H ^H bO C R •H tH F? X c 1—1 -H X W) +j o C J3 o 6 w p: i-H .p-l rt rt +-> ^1 O 4-> C o •> tn W ctf -a o (D tn X I t/) ■!-> U Q> (D & ■»-> 03 S • (L) T) P-i C -H rt r-l rt •H CA) U *-> r-l 04 IP; 03 b 3 S f-H T3 O Um •H .—1 C > ^^ •.H •H +J -O 3 Q C O O •H U 1—1 U rt 4^ O +J O tw o ■M o ^^ w NO -a OJ o c (D I— 1 nS ^H O X >^ c O 1-1 •H o ^ U •!-> P^ c fn M O rt ^ s ^ M • W TJ S C rt O C3 .— I C U-i -^ •H C —1 •H c3 t3 43 O Q ca +-> O M 1— ( U )-• 3 10 X •-3 0) rH o ■p o o to 3 3 X 3 0) c 3 H rt ^ LO o o Ol •^ o o U-; t--. o) •^ o \C rj ro NO LT) CM to O —I 00 00 o o ■H u 4-> 3 03 O +-> to c/^ o LO •^ r^ NO O to vO t^ NO LO \o NO 1 — 1 CM a, rsi to nd rf to to vO to CO 1—1 r en LO t^ to 00 t^ CN r-- \D rg o '^ t-- ■y- to to cr> vO CT> CM LO LO to to CM '^ \0 3 to to O "3 o m (u o o X •H 1—1 ^H us to u rt rt u ;-. c •r-> (1) *i x^ o d) Vrt m U ■M 4-1 6 rCl •r-> 3 c u ui o o 3 rt C3 O 3= u u CQ c/: 00 .z. o o LO 00 LO o nO LO '* r-- LO t-^ CO to 1 — 1 LO ^ 'd- CM to NO '^t r-- to o r^ r^ 00 Oi NO o CM CT. en vO nO LO c^ CM 00 CM O LO en 1—1 r-- CTi I~^ NO LO ^ -* CM '^ LO CO 1 — ( CM LO NO r\ •\ •4 1 •^ •V •» *\ o CO f^J NO to CM CM NO Csl vf o o »4 NO CM CTl to CTi \0 C^J 1— 1 00 NO to r^ 00 to CM CM \0 vO to tfl CM to lO CTl CO o CM to CTl CO CM NO CTl LO NO CTl 1— 1 to 1 — ( 00 CM O CT3 u (1) ,Q 6 +-) PL, o CO Oj tiO to t/J 03 r—^ lO o O 4-> tJ o +-> O iH u a> 03 +-1 O 'm •H O 'h U-i o; &NJD O fcZ) 1-1 C •H X i-H CL,Q e o 03 U to • to (30 r-l q LO •H -V f-l NO 3 LO 0 to ^ 03 03 3 (DNi) -c; o ■M 1-1 o ■p c •H o c 3 Si a o u c>o NO CM O tJ CM • H •> C LO CS CM ^1 L|_| O O 1—1 (D 03 3 ■M 1-1 O 03 H > ct3 Figure 35.- Rates of organic matter inflows and outflows of Lake Izabal for the lake as a whole (g COD x 10^/month) and for an average m^ of surface area (g COD/m" day) , 157 40 ^30 c o E % O ^ ?0 10 Inflows Outflow 2.0 1.5 ns T3 CvJ E 1.0 o D) 05 0 i 1 J 1 1 1 i 1 1 1 1 1 KD 0 J FMAMJ JASON D^ MONTH 158 inflows would be 2b, 202. 6 x 10 g COu, and the San Felipe outflow would be 22,729.8 x 10^ g COD. To evaluate the relative contribution of watershed areas to the total organic matter inflow, the Izabal Watershed can be divided into two runoff components. The Polochic Valley component is defined as those rivers which empty into the lake through the Polochic delta and include the three Rio Polochic distributaries, Rio Oscuro, and Rio Amatillo. These two latter "swamp rivers" are included because the Rio Polochic, during wet season flooding, spread over the delta and mixed with the waters of the Oscuro and Amatillo. The minor runoff component of the basin includes the Rio Sauce, Rio San Marcos, and the watersheds to the north and south of the lake. Table 12 compares the monthly percentage contribution of organic matter runoff from the Polochic Valley and the minor watershed components. The Polocliic Valley watershed contributed between 87.8 and 96.2 percent of the total organic matter for the months sampled. In summary, the flow of organic matter into Lake Izabal was strikingly seasonal, and not due only to increased wet-season runoff rates, but also to increased organic matter concentration of the runoff. The monthly percentage of total organic runoff during the sampling period averaged 1.7% for each of the three dry-season months (March-May) and 19.01. for each of the five wet-season montlis (June-October) . At the beginning of the wet season, organic matter outflow at San Felipe lagged behind up- stream inflows for approximately three montlis; thereafter outflows and inflows did not differ widely. The Polochic Valley alone accounts for 80 percent of the watershed area but more than 90'' cti • S TJ •H C fH rt p-l X r-< rt rt TJ 6 1 w 1—1 ^-_-' ^^ — s o w V > (L> ■P •v rt 10 !-i (D •H n M o 03 •H p •P 3 ci3 ^ ^H •H •H ?H Ph-P m W p^ 162 ■TF i- >^ fd T> +-> I o <^ (D > O 0) asm I o l: in c in (13 03 O to 0) •H 03 2 ^-1 4-> I fH ■P o H Figure 40.- Respiration rates of mud samples from Lake Izabal. Arrows on the horizontal axes indicate midpoints between samples and the lengths of vertical bars represent differences between duplicate rate determinations. 172 1 2 3 4 5 6 Hours 173 Table 13.- Hourly and daily respiration rates of Lake Izabal bottom muds No.^ mg 0^/ '.' hr b g ^ / 2 , c O^/m day :,d Station 12 Oct 15 Oct 16 Oct 12 Oct 15 Oct 16 Oct 2 18. ,82 .452 8 15. ,32 .368 9 13. 90 .334 10 11. ,91 .286 14 12.96 .311 15 23.15 .556 16 17.39 .417 17 16.21 .389 18 13.24 .318 19 10.83 .260 20 10.76 .258 Station niambers are those of Figure 39. Average of replicate rate determinations between 2 and 4 hours and betvs^een 4 and 8 hours of incubation. Hourly rates multiplied by 24. Average daily respiration rate for all stations listed in table was 0.359 g 02/111- day. 174 temperature) . Results are about an order of magnitude higher than this for river muds where stirring is used in the incubation chambers. 2 McDonnell and Hall (1969) reported hourly rates of ca. 0.22 g 0 /m for a "mildly polluted eutrophic stream," and similar values (0.10-0.20 g 2 0 /m hr) were reported for river muds in England by Edwards and Rolley (1965). The central, nearly fiat basin of Lake Izabal is a very uniform clay-gyttja of moderately high organic content, while inshore regions contain considerable sand and gravel of alluvial origin (Tsukada and Deevey 1967) . The moderately high organic content of the clay-gyttja (8.7-12.9% loss on ignition) is mostly carbonized plant fragments. Tsukada 's cores, for which these analyses were reported, are well be- low the surface of the mud (to 3.2-m depth) and have long been unavail- able to potential detritus feeders and decomposers. To estimate the quantity and organic composition of the mud sur- face, especially the particulate detritus, Ekman samples were collected and treated as reported in the "Methods." Figure 59 illustrates the results obtained from samples collected throughout the lake basin. Sam- ples from inshore stations near river outfalls and in bays and coves generally contained larger quantities and percentages of combustible particulate organic matter than did offshore samples. Offshore stations below 10.5-m depth (Nos. 6, 9, 10, 14, 17, 19), excluding stations where large pieces of plant material were collected (Nos. 15 and IS), yielded between 14.6 and 41.0 percent loss on ignition (x = 25.8%) for particu- late organic matter. These values, all being proportionately higlier in organic matter than the deeper muds, possibly reflect a bias toward higher 175 organic content of the particulate fraction, but also imply that the mud surface contained substantial amounts of organic matter potentially avail- able for detritus consumers. Measurements of Primary Productivity One of the chief difficulties of the light and dark bottle method for primary productivity measurements is expressing the results obtained during a 3-hour measurement on a daily basis. To do this the following calculations were made as follows: (1) The dissolved oxygen concentrations at the beginning of the experiment and those after incubation in the light and dark bottles were plotted on graph paper with oxygen (mg/litcr) as the abscissa and depth (m) of incubation as the ordinate. The area between the light and dark bottle curves was integrated by planimetry, the averages were cal- culated for the duplicate experiments, and the value was defined as gross primary production (Pg) for the incubation period. The area between the dark bottle and initial readings was the respiration (R) (Figure 41) . (2) The Pg and R during the incubation period were divided by the number of hours of incubation to yield hourly gross primary pro- ductivity (Pg/hr) and hourly nighttime respiration (R/hrJ . (3) The difference between the Pg/hr and R/hr was the net primary productivity (Pn/hr) . All these values are listed in the first three columns of Table 14. (4) Next, the hourly radiation received during incubation, cal- culated from the solar radiation recorder readings, was divided into the monthly average of daily radiation received. The result is the number of hours of effective radiation per day. Tlie assumption is that daytime Figure 41.- Example of curves generated from light and dark bottle experiments from which metabolism is determined plani- metrically. The area between light and dark bottle curves is gross primary productivity (Pg) , and the area between initial and dark bottle curves is respiration (R) for the incubation period. Net primary productivity is the arithmetic difference between Pg and R. 177 0 Oxygen (mg/ liter) 8 18June72 Station C 0912-1212 Q) 6 8 "--x Initial Concn. <^-oLight Bottle •— *Dark Bottle 10 Comoensation Depth 178 primary production maintains the same proporLiojial relationsliip to total daily light as hourly primary production during incubation does to hourly light received during incubation. It was believed to be more realistic to use a monthly average of daily radiation rather than radiation deter- mined on a single day for extrapolating daily metabolism over longer periods. A hypothetical example of this calculation is given in Figure 42. (5) Both Pn/hr and Pg/hr are multiplied by the effective hours of light per day to yield Pn/day and Pg/day respectively. (6) The ratio of gross community metabolism (Pg) to 24-hour res- piration (R-j.) was calculated from the following formula: Pg/day _ (Pg/hr) (hrs of effective light) R^^ ' (R/hr) (24 hrs) This is the P/R ratio used by Odum (1956) and Margalef (1963) as an indicator of biomass accumulation (P/R>1) or consumption (P/Ps O W) '^ M -O (H -H Q) O 3 -P •H -P +J W Oj rt ■r-i !~i O Pi 3 -H t/5 O C (D 03 ^ (1> M ^1 -P O O t+H r-l ^ c o CnI O -P •H - !/l 0) ^ C ^ M (1) P Cu -P (U 4-) ■p o e •H .C -H > H P •H W +-> 0) o • 3 U O -a -p O T3 w X - 3 • rt —) 1 < ij. J AepgUj/^O & similar seasonal trends were lacking; the P/R ratio was more commonly less than unity, and during no period did the P/R deviate far above one. Thus, the western and central parts of the lake (Stations A and B) seem to be areas where production seasonally exceeded respiration, whereas the eastern part of the lake (Station C) demonstrated less of a seasonal pulse in metabolism. Efficiency of gross primary productivity The efficiency of primary productivity will be defined as the fraction or percentage of incoming radiant energy (available to photosynthesis) that is converted into chemical energy by gross primary productivity. The energy of the spectral region available for photosvnthesis (400-770 my) was assuii'.ed to be 50% of the energy recorded by the pyrheliometer. Total solar radiation, recorded almost continually from October 1971 through November 1972, is reported in Table 15. Daily readings were aver- aged on a weekly and monthly basis. The monthly averages were used to calculate efficiencies of Pg/day, and the hourly radiation, determined during the incubation period, was used to calculate efficiency of Pg/hr (Table 16). Since solar radiation was expressed as kcal/time, the productivity had to be converted from grams oxygen to equivalent energy and time units. The choice of 4 kcal/g 0 lies between the value for one of the products of photosynthesis (glucose) and the final products of biomass (plankton) . If glucose were the only product of photosynthesis, approximately US kcal of solar radiation would be required per mole of oxygen, or 3.7 kcal/g 0^. If biomass were the product of photosynthesis, the 4.8-5.0 kcal /g bio- mass (or 0 equivalent) represents the energy content of plankton (Maciolek 186 Table 15.- Total daily incoming radiation averaged on weekly and monthly basis from October 1971 through October 1972 Dates of Measurement No . Days Measured kcal/m day (weekly) kcal/m day (monthly) Oct. 8-15, '71 16-23 24-31 7 7 8 4,142 4,558 3,635 4,112 Nov. 1-7, '71 8-14 15-21 22-30 7 7 7 9 3,325 3,113 3,479 3,258 3,294 Dec. 1-7, '71 8-14 15-21 22-31 7 7 7 10 3,906 3,515 3,240 3,335 3,499 Jan. 1-7, '72 8-14 15-21 22-31 7 7 7 10 3,617 3,698 2,707 3,872 3,474 Feb. 1-7, '72 8-14 15-21 22-29 7 7 7 8 2,959 4,115 3,372 3,643 3,522 Mar. 1-7, '72 8-14 15-22 23-31 7 7 7 9 4,522 3,628 4,953 5,098 4,550 Apr. 1-7, '72 8-14 16-22 23-30 7 7 7 8 4,111 4,879 4,499 4,776 4,566 May 1-7, '72 8-14 15-21 22-31 7 7 7 9 4,561 4,477 4,487 4,718 4,561 Jun. 1-7, '72 8-14 15-21 22-27 7 7 7 6 3,555 4,372 4,906 4,762 4,399 187 Table 15.- continued 2 2 Dates of No. Days kcal/m day kcal/m day Measurement Measured (weekly) (monthly) Jul. 2-7, '72 8-14 19-25 26-31 6 7 7 6 4,041 4,070 4,327 3,459 3,974 Aug. 1-7, '72 8-14 15-21 22-31 7 7 7 10 4,441 4,137 4,544 4,816 4,485 Sep. 1-7, '72 8-14 17-23 25-28 7 7 7 4 4,163 4,271 4,097 4,155 4,172 Oct. 1-7, '72 9-15 16-22 23-28 7 7 7 6 4,016 3,498 3,386 3,972 3,718 Daily average for October 1971-1972.... 4,025 188 Table 16.- Daily efficiencies of energy conversion froiri visible solar energy to energy fixed by gross primary productivity (Pg) . The energetic equivalent of one gram of oxygen was assumed to be 4 kcal and visible solar radiation was assumed to be 50% of total incoming radiant energy , 50% of Total ^^'^^y Radiations Percent g Oo/m^ kcal/m2 (kcal/m^ day) Efficiency 1,737 0.95 1,737 0.81 1,761 0.26 2,275 1.07 2,283 1.28 2,281 0.85 2,281 1,03 2,200 0.78 2,243 0.80 2,086 0.33 1,859 0.57 0.79 Ave. Station - Date Station A 11 Jan 29 Jan 2 Feb 4 Mar 24 Apr 6 May 28 May 20 Jun 9 Aug 27 Sep 27 Oct Stat ion C 5 May 29 May 18 Jun 7 Aug 25 Sep 4.11 16.44 3.51 14.04 1.15 4.60 6.09 24.36 7.31 29.24 4.84 19.36 5.85 23.40 4.30 17.20 4.50 18.00 1.72 6.88 2.67 10.68 Station B 3 May 3.30 13.20 2,281 0.58 5 Jun 6.41 25.64 2,200 1.17 19 Jun 4.80 19.20 2,200 0.87 8 Aug 5.90 23.60 2,243 1.05 26 Sep 2.42 9.68 2,086 0.46 0.83 Ave 3.05 12.20 2.19 8.76 2.75 11.00 3.44 13.76 1.22 4.88 2,281 0.53 2,281 0.38 2,200 0.50 2,243 0.61 2,086 0.23 0.45 Ave. ^ Average daily radiation for tlie month in which station was sampled. 189 1962) . Since neither glucose nor highly structured plankton biomass are the sole products represented by 0^ evolution in the experiments, the intermediate value of 4 kcal/g 0 seems justified. The daily efficiencies of gross primary productivity reported in Table 16 ranged between 0.23-1.28°6. Solar radiation was more constant that primary productivity which resulted in efficiencies being more pro- portional to productivity than to radiation. Efficiencies greater than \% were exceeded only three times at Station A and twice at Station B 2 when the primary productivity was greater than 20 kcal/m day. Light penetration and compensation depth At least twice monthly, Secchi disk transparency measurements (except July) were recorded at Stations A, B, and C (Figure 44) . There was a general trend of decreased transparency as the dry season progressed; the least transparent readings were during July (1.5-1.7 m.) . Thereafter, a trend toward greater transparency increased values to 3.1-4.2 m in October. A submarine pliotometer was not available at Lake Izabal to measure the percent extinction at the depth of Secchi disk transparency. The extinction coefficients were calculated from Secchi disk transparencies from the relationship 1.4 where ri is the extinction coefficient and z is the depth in meters of the Secchi disk transparency. The choice of the constant 1.4 is explained in the following paragraphs. The light intensity at the limit of Secchi disk transparency is about 15°o of the subsurface intensity according to several authors (Poole and rt o •H +-> U (D • > w 13 C C cti cd ^ *i 1— ( < X P m c c 0 0 e •H +J (/) nJ p ■P 0 W 0) c ■p c nJ 0 0 TJ (U (D tJ > fH u 0 w~i 0 0 0 ^^ 'D r* w P ■P C na 0) C e 03 (D h (/) 3 W) tn PI rt •H (U TJ e rt (D X u 0 C Mh (D 0 M 03 (D ft W) 10 C C 03 Clj U ^ p <0 -C Ai +-> 1/) •H +J T) c (a • H i/> x; ^^ 3 bO 191 u u LlI en u c o ■*-> CO X u u LlI to c o 03 -t-' LO I u u UJ LO < c o +-' 05 to c ro O JL U I c o to o £ a E C o in 03 E 03 E c o u c o to 03 E 03 E c\j n '^ Lf) ( uu ) A V 1 1 q ! s I A (D :z O 192 Atkins 1929; Clarke 1941; Ichimura 1956; Beeton 1957). According to Beer's Law of light extinction Iz = I e"'^ ^m o where I represents the light intensity at depth z, I the incident light z o impinging upon the water surface, and n the extinction coefficient. If I is 15% of I , then, z o 15 = 100 e"^ ^m n 100 - - In 15 = n Zni 4.605 - ■ 2.70S = n Zm 1.897^^ Assuming that the factor 1.9 remains constant, then the extinction co- efficient can be calculated from the following formula: 1.9 n ^ ^m Using the factor 1.9 would result in an extinction coefficient of greater than one for low Secchi disk values such as the one recorded on April 24 at Station A (1.45 m] . An extinction coefficient greater than one would be unlikely, especially since the Secchi disk was visible down to the 1.45-m depth. Vollenweider (1969) reported that this value varies from 1.4-3 and more depending on local conditions and the observer. Thus a factor of 1.4 was chosen for calculating extinction coefficients since it is less than the shallowest Secchi transparency (1.45 m) which avoids a n > Ij and it is within the range of values reported by Vollenweider (1969). Back calculation, using the formula from Beer's Law, resulted The es / — \ 'p P X •H rt ^-^ W (-■ XX S P P +-1 (D 13 tp •H P, c X a> 0 l/l (D •H 4J 4-> c -o rt U (D a. 03 C w 0

/-> H ■ H U •H d) 3 tA) Q C •P fn t/5 0 0 ni rt P •H M w Cl, 0 "P ^H 0 0 H ri>< f-l c •P t/l 0) •rH Oi -H Pi -o -a X -o 0) +J -H C p 0 •H X CC rt ^ — y bO w 0 p B^ 03 C U /-^ lA f-( 0) CO ^— ' 1/1 ^ r^ > c C P < •H T) +-> 0 P^ n c •H CD bO ■M Oi 0) P Q -a c X -H Pi C -H box 0 G C 0 -T3 •H +-I -H 3 0 0 03 ■-t Ph<+H tn •H 0 0 0 t4H w P CO Ci ^4-1 -a a> cj rt 0 0 (/) bO c 0 0 C P C c 0 P (D 10 -H 0 -H C P ^ -0 •H +J 0 bO E •H 03 •)-> OJ -H C 0 tt. 0 ni (/) -p •H CJ tx •— 1 c 0 T3 3 0) C f-l 0 Ph-H 0 i-i 6 P 0 rt 0 K 0 0 1 rt P 03 Q r-- ' 0) I-l Xi a o • H p 03 P CO bOt^vooot~ocri'rs-cof^ t-^t^r-jtNcnrsico,— iCTi (NirMrMt-orsit-orsioNi"— ' cooocr)\orotooot-~o OOr- ivDt-OfOt^vOO vOvOLOCnCTiCTiLOsOUO tot^t^^LOt-oo-— 100 !NrMCNi-H'-HrHrMCN)CNJ ooLor^ocot-oooo v0'd-rM\0M000-lK)O r^vor^^LOLOLOcoLO^ LO LO LO O LO o r^ K) t^ LO o o LO LO LO cri LO vo O LO O LO o o LO (^ o t^ r^ o LO LO O CO LO \D o3 o3 o3 CCf^^nXXCbOP, o3o!rtP,oJrt330 <^-Di-3S zi ■p 13 t/) 0 ursi 13 p e C o ■ rt -o CO •H ■p i/i w 3 5 o Cm o f^ r^ 1—1 hH l4-l 0) bO s f-i rt o (D fH ^ — ' +J O +-> > •1- rt ■^d e /•-^ OtNl o <— s • H H c s rt O bo ■ U X ■— ( O -o cfl 3 +-> f-H +j O rt t/i r--( ft, V ^ •H ^-^ t— I o O c to •H -T3 X ^ O rt CX, •H ^1 -u (U o K1 £ Ph • +-> \o o I— t Uh X o •p r-H M •:» 0 (U -a > o o o OO E rt T3 !-i -o O O g; « i +J ■H ri 1/1 U-l ^-1 ■H o J-i .— < > m O. ri +J e +j •H tn rt CO rt e 1 • ■* 0) J^ 3 CO •H 1 p:. 196 f- H o w o H CO 1 +-> O .ii +-> c X 03 X r-H Cu P< X X ^1 X rt rt -M T3 e -H w CM •H > s 6 ex, +-) a. W) u 2 o w n n to W 13 r~ o o • M M K5 u c X X ■U CM 6 bO rsi to vD ^1 (U •p ■p 2 3 O c c c3 I— I Wl U o 197 of the inflows and outflows from the pool. Inflows are represented by importation of organic matter from the watershed and by the contribution of gross primary production. Losses from the pool include exports of organic matter, loss by planktonic respiration, and by benthic respiration. The components of the model parallel measurements that were made in the field over an eight-month period, except for benthic respiration (one month of data) . It must be remembered, however, that these were not mea- surements for every day of the month, but rather one to several measurements during a single month at a limited number of sampling sites. The eight- month sampling period included three months of dry season (March-May) and four months of wet season (June-October). Since the Izabal Basin was characterized by a longer wet than dry season, the balance of the sampling period was toward the wet season, thus closely representing the hydrolog- ical regime over a full year. Also the sampling period included what could be considered the maxima of the two seasons, i.e., the dryest part of the dry season, and the wettest part of the rainy season. Table 18 summarizes the organic matter flow data according to the flows defined in the model. The values are daily rates and are assumed to be the average for each day in the month. The calculations on which Table 18 is based are given in the Appendix, Tables I and J. Oxygen pro- duction and consiunption values were assumed to be equivalent to organic matter (OM) values. Some interesting ecosystem cliaracteristics become apparent upon examination of Table IS. For example, Pg remained relatively constant from Marcli-August (above 4 g OM/m" day) and then declined to below 2 g 2 OM/m day be October. However, when Pn was exajnined, there was a net 198 u o ■M o o X o ^H crt S 6 o ^H <4H r-H 03 ^ nj t-J 1— 1 : o o ^ — > bO M C (D •H +-> +J -o rt ID e W (/I o CJ •H !h c fl( rt X W) 0) fn O CO < I CO f-< o o ■M C +-> O 03 -H V) ?^ -1-^ t/5 u o U 3 >-l •H -n tj c o 03 ^ bOCu U O O f-i n (.3 w ^H 2: O O Ph X !/) S O O Ci, 6 to 00 •^ K1 o o 1 o CM O o I O .-I o o o + O CTl o o CO LO CO 00 o o o en + o to CM rj- r-i r^ VO r-i r-J r-( O rsi •* (N CT) CTi CT> CT> I^ (M i-< to ■rf LO LO LO t~0 t-0 K) to to \0 to o I o I o o I o I o I o o c LO LO ■=3- O 03 a-, o \D 00 i-H o o o O O r-( 1 + + o + o + o to 03 vO «* C^I to to to o r^ ^ o CO O LO o3 LO •^ O .-H Csl CO r-1 to to o to to r^ CO o en LO to CO o + to CO o I o to en to LO o o 1 en CO o en to r— ( 00 to to o o 00 CO o to o en c (-1 u X o 03 a, 03 D 3 3 Qj o > •-J i-j < 00 O < 10 (L> 3 I-H o3 > U o +-> o o X) c 03 4-> 3 W) -1 < (1) ■M Uh O 0) 00 03 > o3 O t/1 03 -a +-» o3 f-H 3 CJ i-H 03 u O U I V) U o CL, ■P •H > •H •P O 3 O X •H rt ^1 —I Oh to 03 O J-i ^( <+H Oi U W 0) &0 C P. C3 0) T3 -H C -P •H S ct5 O Q O o> 1—1 O 3 O $-1 c a. 2 c o < I— I CM 05 a, "-. Q U o o 2 Q 0 03 Oj(N D < ^ 3 ^ •P E U O § ^ •H H 4-> CO (U O AS O 03 C-, E VO O Oi u f-i • u^ • (/> vO r-- CM r^ --H tl CTl LO r- LO to CT) O^ CTl nJ (D vD I— ( O^ a^ en f~- \0 \0 ^D OhvO f-H t— * r-H Oi CTl CTi CT> ■P en W) r—i r— 1 ,— t 1— 1 O N 1 — I C X t/i X -H • H (1) •H 0 to O O (U C 3 O T-H > u > •H 4::x -C C -H 1— 1 f— 1 o u o s 'O'O ^3 3 rH o 03 o o o (D o o o Q !J t_) H Q Q Q J k; ci Dj o o^ \o to o 03 o •H !X O u o 'a- to —I CM CTl ■^ o3 •H e 03 C 03 a. < o •> tlO c f-1 o o +-> c o Nl I (U ■M 03 U 0) E o E- o o o r— < • • • 1— I to I I I to to LO o • • to o < CT) CO LO \£3 (Nl LO r\) o LO M- r-- 03 o3\n3 fi -P O -H ■P -P 2 J »j 03 E O C +-> \o3 03 <-< C3 O ■P •> 03 03 O 03 > 1 — I 03 3 •H UJ :3 o OS c OS ►J 10 a. u O -r-l •H ^ ^ CL, fx, o O !-i u u •H P CL, o o f-l <-< ■p cd O fH • H p rH a o c p 3 O OJ +-> 3 O Cu, o LO CTl C •H c OS P c o E o !h 3 to 03 O 202 Summary Statement In the Izabal Watershed, the transfer or flow of organic matter from the terrestrial to the aquatic ecosystem was tightly linked to the hydrologic regime. However, the flow did not perfectly parallel the move- ment of water because of the higher organic matter concentration that occurred at the beginning of the wet season. This initial "flushing out" of organic matter from the watershed occurred during June for the Rio Polochic, but a month later for the colored swamp waters of the delta. During these months, the waters contained far higher proportions of par- ticulate organic matter than during the remainder of the sampling period. During this initial flushing pulse, the inflow of organic matter exceeded the outflow at the San Felipe outlet. By August, the rates of inflow and outflow began to converge and both continued to decline at the end of the sampling period. Organic matter that discharged into the western end of the lake from the Polochic Delta accounted for approximately 90"o of the organic detritus contributed by runoff. No correlation was observed be- tween organic matter (COD) concentration and respiration rates (BOD) of any of the inflowing or lake waters. Gross primary productivity (Pg) by phytoplankton represented the major source of organic matter for the lake, and the efficiency of Pg to total solar radiant energy ranged between 0.23 and 1.28"o. IVliereas Pg remained relatively constant throughout most of the sampling period, respiration in the water column was relatively low during the dry-season months compared to respiration rates that followed the onset of the wet season. The resulting balance of Pg and respiration (e.g. net primaiy production) was positive during the dry season, and became increasingly negative during tlic wet season. 203 Respiration by the bottom muds was approximately one-tenth the rate of planktonic respiration per unit area of lake. Thus it represented an important sink for organic matter, as might be expected in a shallow lake. The particulate component of benthic organic matter m.ay have been an im- portant source of energy for activity in the muds. Over half of the organic matter contributed by the Rio Polochic in June was of the particulate frac- tion, yet it did not appear to measureably increase the particulate frac- tion in the lake samples during June or the months that followed Figure 34d). The disappearance of particulate matter would have been, by infer- ence, from the water column to the bottom muds through sedimentation. The profile-bound density current of the Rio Polochic outfall may have served to isolate the particulate matter from the overlying waters and thus facilitated its sedimentation. The primary productivity of Lake Izabal was within the range of polluted eutrophic temperate lakes, but below the productivity of many tropical lakes. The replacement of a large portion of the lake water by high-turbidity runoff water possibly reduced transparency sufficiently such that the primary productivity did not approach the maximum theore- tically possible for planktonic systems. Data are lacking to help determine the direct causal effects which resulted in a net positive balance of energy during the dry season and its apparent consumption during the following wet season. However, these seasonal pulses of production and consumption suggest a mechanism by which the Lake Izabal ecosystem, and perhaps other similarly structured eco- systems, can achieve a steady-state balance of organic matter storage. CONCLUSIONS In the preceding chapters it has become clearly apparent that Lake Izabal is not an isolated ecosystem, but a unit of a larger ecosystem by virtue of flows that couple it with the upstream watershed and the downstream marine environment. The overriding force that controls this coupling is the hydrologic regime, whose seasonality is responsible for the pulses and oscillations characteristic of the ecosystem. By determining the relationship between different components (sub- systems) of the Izabal Watershed, one can assign distinguishing features, or attributes, that make this ecosystem distinct from others. It follows lIiaL any implicaLluiis of the study for ecusystem managcjaeiit inay be appli- cable to other ecosystems with similar attributes and vice versa. The following are attributes that seem to be of importance for characterizing the Izabal Watershed: (1) The ecosystem possesses a seasonal pulse of organic detritus movement . (2) Tlie lacustrine component of tlie watershed experiences oscilla- tions in food concentration and consumer activity. (3) The connection of the lake to a marine environment provides easy access for euryhaline marine fishes that can adapt to fresh- water conditions . These attributes are illustrated in Figure 46 as storages and flows of energ)' and matter between the subsystems of the Izabal Watershed. The 204 NO u •H 206 207 energy diagram is representative of the regional situation before the additional influences of modern agricultural and industrial man. The four main subsystems are (1) the terrestrial ecosystem (watershed), (2) the lagoons and coves in the Polochic Delta, (3) the main basin of Lake Izabal, and (4) the coastal marine ecosystem. The forcing functions that provide the energy for ecosystem power, maintenance, and inter- subsystem coupling are (1) sun and wind which drive primary productivity and evapotranspiration, (2) rainfall, which in connection with seasonality, controls the hydrologic regime and water movement, and (3) tide, gravity, and wind whose seasonal regimes act with water level to control fish mi- gration and salt water movement from the coastal marine ecosystem. Terrestrial ecosystem material exports to the lagoons and coves and the lake basin are carried by the energy provided by the do^mhill flow of water. Exports from the lagoons and coves to the lake basin include organic matter originating in the swamp forest of the delta region as well as organic matter from algal production. These exports occur in proportion to the water level of the lagoons and coves. The lake basin not only receives inputs from upstream ecosystems, but also receives salts and fish from the coastal marine ecosystems. Many of the internal flows and storages of the lake have already been discussed. Fishermen take advantage of the seasonal migration of fish which is possibly stimulated by factors such as salt-water movement, water levels, and other seasonal phenomena, especially the food concentration resulting from dry-season net productivity of phytoplankton in the coves and lagoons. These interactions and ecosystem attributes will be dis- cussed in relationship to their ecological implications for the aquatic ecosystem. 208 Seasonal Pulse of Organic Detritus Movement It was shown that nearly half of the total organic detritus move- ment from the watershed to the lake occurred during 3 months, or just one-fourth of the year. The timing of this event coincided with the transi- tion from a positive balance of gross primary production and respiration, to a negative balance. Although there was no direct evidence to demon- strate that this transition was a result of increased organic matter input to the lake, the change from an autotrophic (P/R > 1) to a heterotrophic (P/R < 1) balance suggests that it was at least a seasonal phenomenon associated v;ith the hydrologic regime. Rates of gross primary productivity remained nearly constant during the dry season and continued at nearly a constant rate during the initial months of the wet season. Thus the change from a P/R ratio of greater than unity to a P/R ratio of less than one was due to an increase in respiration rate. The wet-season inflow of allochthonous organic matter represents an auxiliary energy source for the maintenance of high respira- tion rates of the lacustrine ecosystem. Similarly, increased respiration rates have been documented for estuarine ecosystems during periods of increased fresh-water inflows which contain organic detritus (Cooper and Copeland 1973; Odiim 1967). Lake Izabal thus has metabolic characteristics similar to shallow estuaries, although physical features such as depth and salinity may be different. Movement of Organic Matter to Site of Consumption The swamp forest in the Polochic Delta is the source of some of the organic detritus that is discharged into the lake. This represents a 209 displacement from anaerobic conditions (the swamp forest) where organic matter consumption occurs slowly, to an aerobic environment (the lake) where the consumption of organic matter is more rapid and complete. Thus the physical flushing of the swamp forest is a mechanism that prevents large accumulations and storages of energy. The movement of organic matter from other regions of the watershed occurs in two phases. First, there is the initial flushing at the begin- ning of the wet season, when a high percentage of the organic detritus arrives at the lake in particulate form. Following this, the composition of influent detritus is predominately in dissolved form, and represents a more steady leakage from the terrestrial ecosystem. It is not clear whether this movement is from an environment of relatively slow organic matter consumption to a more rapid one, or to what extent metabolism that occurs in the rivers is responsible for altering the quality and quantity of detritus before it reaches the lake. In the case of the particulate detritus, the bottom muds of the lake and its associated fauna may provide conditions for the effective consumption of particulate organic matter. By dividing the total annual OM runoff from the Izabal Watershed (163,051.25 X 10 g OM) by the area of the watershed (6,860 km ) a value 2 of 23.8 g OM/m is obtained. This is well above the annual runoff of 7 5.3 g OM/m for the Hubbard Brook Experimental Forest (Likens 1972). The higher value for the Izabal Watershed might be due to increased OM runoff from a partially deforested ecosystem. However, it is likely tliat terrestrial ecosystems in the humid tropics may experience more OM leak- age than their temperate counterparts. Until more data on OM runoff from a broader spectrum of latitudes and rainfall regimes are available, this 210 supposition is tentative. However, based on what is known about ecosystem strategy toward the conservation of mineral nutrients, it is unlikely that OM would be regarded a "scarce" material because of its abundance in relatively productive terrestrial ecosystems. There is evidence in this study and others (Nelson and Scott 1962; Bormann et al. 1969) that in a given watershed OM runoff increases in greater proportion than hydrologic runoff. Thus, in an assemblage of watersheds with a gradient from low to high hydrologic runoff on an annual basis, O.M runoff could be expected to increase in greater propor- tion than hydrologic runoff. Assuming that the OM produced in one part of a watershed is utilized or consumed in another region leads to the possibility that regional coupling mechanisms are more predominant in areas of higher hydrologic runoff, i.e. higher rainfall. The conceptual image that emerges is a region composed of a mosaic of subsystems where upstream subsystems export potential energy in the form of organic matter to the more predominately heterotrophic sybsystems do^^mstream. The export of this energy is subsidized upstream by the weathering of rocks which yields nutrients for primary production. Mechanisms for Steady-State Balance According to the measurements during the sampling period, the organic matter budget shows a deficit, but his should not be interpreted to mean that the lake is a strictly "heterotrophic" ecosystem. Interpretation of the budget needs some qualification with respect to tlie year of study and adequacy of sampling. The hydrologic regime during 1972 was unusual due to greater than average rainfall in the region of the lake. Not only was rainfall intensity greater during the wet season, but the dry season 211 (<]00 nim per month), which typically begins in December, did not start until March 1972. Thus, the 1972 dry season was only three months duration, while five months constitutes the average dry season (Snedaker 1970). The balance of organic production to consumption is positive during the dry season and negative during the wet season as the data clearly demonstrate (Table 18). A year of atypical wetness, such as that experi- enced during the year of study, may be responsible for the "heterotrophic" character of the lake. Measurements would be required on more t>i)ical years in order to establish if the lake is characteristically hetero- trophic. Regardless of the nature of the annual balance, positive gains of organic matter during the dry season are followed by net losses during the wet season. This may be a mechanism by which the lake achieves steady state with respect to organic matter loading. Flows or sinks which prevent organic matter overloading are respiration (planktonic and benthic) and export at the lake's outlet. Lesser flows which were not measured would include the fossil sink for organic matter in the sedi- ment of the lake and emigration of fish to the marine environment. Positive gains in net production were probably underestimated be- cause the intense dry-season primary productivity of the deltaic lagoons (which are some of the highest daily rates for the region) (Brinson 1973), were not calculated as part of the budget. Although these lagoons re- present only a small percentage of the total surface area of the lake, their regional importance is magnified by attributes found in no other part of the Izabal Watcrslicd. 212 Oscillations in Food Concentration for Consumers As mentioned in the description of the study area, the shallow coves and lagoons of the Polochic Delta are popular areas for the activities of fishermen and fish-eating birds during the dry season. The high rates of net primary productivity and high densities of planktonic standing crop provide conditions attractive to fishes. The shallow depth relative to that of the lake basin offers the distinct advantage of reducing energy expended in feeding by concentrating the food in a compressed column of water. Dry-season activities of the consumers coincide with cloudless days which provide optimal conditions for photosynthesis and low or negli- gible rates of flushing during the hydrological m.inimum. Cichlasoma gutulatum that spawn in these shallow waters have an adaptive life cycle that entails breeding during the dry season when high planktonic densities provide a concentrated food source for their offspring. The seasonal availability of a concentrated food source in fresh-water systems could also be of selective advantage to those marine euyhaline species that can reach this energy source before runoff and flushing of the lagoons and coves dilute the accumulated organic matter by the time it reaches the sea. A similar account of seasonal food exploitation is the tismiche described by Gilbert and Kelso (1971) in the estuary at Tortuguero, Costa Rica. Following these dry-season activities, wet- season rains flush the lagoons of their plankton with anaerobic black waters from the surround- ing swamp forest, and the majority of the fish leave to become dispersed throughout the lake. The pulse is somewliat analogous to management prac- tices used in fish pond culture, whereby ponds are periodically drained before a new crop of fish is cultivated. 213 Consequences of the Connection to a Marine Ecosystem The dominance of the lake's fishery by marine fishes provides clear evidence that the Rio Dulce-El Golfete waterway has profound effects on the faunal composition of the lake. The extent to which a brackish water gradient to the lake regulates migration or facilitates the adaptation of euryhaline marine species to fresh water is probably dependent on the in- herent physiological mechanisms for osmoregulation possessed by each species. However, the dry-season penetration of brackish water into the lake may serve to stimulate migration. The periodicity and intensity of migrations is an obvious research need before the fishery can be rationally managed. There is substantial evidence for control of the Na:Cl ratio in the lake by seasonal inflow of brackish water from the sea. Regardless, the lake water did not approach oligohaline concentrations during the study period, but there is evidence that suggests salinities are higher during some years. Because of the short residence time of the water in the lake [<1 year), it would be difficult for higher salinities to escape being diluted and flushed out during the ensuing wet season. Therefore, those euryhaline species that do populate the lake must either be extremely well adapted to fresh water, or have behavioral patterns that facilitate extensive and frequent migrations to conditions of higher salinity. Ecosystem Management In the "Introduction," I suggested that watersheds, because of their well-defined boundaries, make conceptually attractive ecological units for demonstrating regional relationships and coupling. The I-abal Watershed 214 has been described a<; one that fits this criterion and many of the mechan- isms by which this ecosystem sustains or manages itself have been discussed, Wherever man lives, he manages or manipulates ecosystems to some degree and the Izabal Watershed is no exception. There has been a long history of moderate human activity there, but the recent rate of increased development could conceivably put a strain on the free services the eco- system provides to man without man-made reciprocal investments of energy or materials (feedback rewards) . The apparent decline of the fishery exemplifies this notion. Fishery Management In the section--"Regional Setting"--the status of the lake's fisher- ies was; briefly described and the need for management was emphasized. In addition to the extensive management of the natural fish populations through fishing controls and regulations, intensive management methods, by means of aquaculture, deserve consideration. Where aquacultural techniques are employed successfully in other humid tropical regions (Hickling 1961), they are usually closely associated with cultural diet patterns and often accompany wetland rice cultivation. Raising fish in cages has been suggested for Lake Izabal (T.C. Dorris, personal communica- tion), cind rearing of Cichlasoma gutulatum fingerlings to marketable size has been tried in an enclosure near the lake's edge by one individual. The question is not whether aquacultural tecimiques will work in Lake Izabal, but whetlier the energetic subsidies required for fish production ^ See H.T. Odum (1971) for a full discussion of man's partnership with nature. 215 can be provided. The present fish production in Lake Izabal is provided as a free service by nature. Raising fish in cages necessitates additional energetic subsidies in equipment, feed, and human labor, not to mention possible disease control measures. Projects in which aquaculture is attempted should account for these extra costs, and balance them with the marketable value of the fish produced. At the same time these costs should be compared with the cost of extensive management techniques that would be necessary for maintaining a maximum sustained yield for the free- living fish populations. Modern Agricultural and Industrial Man The energy diagram in Figure 47 illustrates the possible influences and modifications that industrial and agricultural development could have in the Izabal Watershed. Additional forcing functions not included in the previous diagram (Figure 46) include fossil fuel and outside energy and economy. Supplies from outside the ecosystem, such as nylon gill nets, outboard motors, and gasoline have already increased the rate of fish removal from the aquatic ecosystem. Preliminary mining activities have stimulated immigration of people into the region and have increased the exchange of money, goods, and services. The eventual export of mining products would presiunably provide capital inputs for more land develop- ment. Since the hydrologic pattern of the watershed is the overriding feature that regulates the attributes associated with special character- istics of this ecosystem, schemes that involve alteration of this pattern should receive careful study and consideration. Two schemes for development that could potentially alter hydrologic patterns are (1) impoundment of water in the Polochic Valley for hydropowcr 13 0) X in H OJ •p rt & C fH rH iJ C (D tw e O Ph o W .—1 !U 0 bJ3 > rt ^ jj ^ rt -H • 6 c/i C tA) nj -d o 6 C P^ rt .— 1 CD rt X^ •H bO -P f-l fH +-> (L> 4-1 W C O 3 (D -O (U C ^ S •H o o w T3 3 If) 03 fH o bO-O rH rt 3 rt •H t-t f-l TJ o 3 c +J X-H I— ( M 3 rt X o e o •H E -H f-l 3 X W) CO s rt 1 -* 0) ^^ 3 bO •H tJL, 217 218 and C2) channeling the shallow El Golfete to facilitate access to the lake by ships with deep draft. The first alteration might have downstream effects on the Polochic Delta. Some regions of the delta are currently growing toward the lake as determined by comparing old maps with present conditions. Entrapment of the sediment load of rivers by upstream impound- ments would reduce the alluvium available for delta growth. The small erabayments protected by the levees of the Polochic distributaries are the areas where high rates of primary production during the dry season provide a concentrated food source for consumers. The continued maintenance of these coves depends upon the degree to which the protective levees are dependent on sediment loads received during the wet-season flood stage of the Polochic distributaries. Also the upstream impoundments would tend to dampen oscillations between wet and dry season discharge rates. These pulses currently function as the seasonal controls on consumer acti- vity in the lagoons and coves. Channeling the relatively shallov\? El Golfete (4.5-m depth) that provides a formidable barrier to brackish water penetration may aid in dry -season movement of saline water to the lake. Water impoundment in the Polochic Valley may deter the upstream penetration of brackish water by discharging wet-season storages during the dry season. However, the effectiveness of a fresh-water current from the lake in displacing brackish water must be weighed against the opposing forces of tides, winds, and gravity that facilitate the upstream penetration of brackish water. In the event that channeling El Golfete facilitates the upstream movement of coastal waters, one can only speculate on the consequences of massive in- puts of saline water into the lake. Some clianges surely could be expected. 219 and Lake Maracaibo, Venezuela may be a good model upon which to judge these effects. Some of the basic data obtained in this study may be of use in determining the consequences of large scale hydrologic changes. Ques- tions raised by this investigation, especially with respect to fisheries management, will require specific data for answers. However, the unknown periodicity of shallow tropical lakes referred to by Tailing (1969) has been established for at least one of these lakes by demonstrating that the watershed is closely coupled with activities in the lake. 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Depth Month (m) Mar S 10 Apr s 10 May s 10 Jun Jul s 10 Aug s 10 Oct s 10 233 Table F-6.- Net particulate organic matter concentrations (mg COD/liter) for December 1971, January 1972, and February 1972, when other values were not obtained Net Particulate (mg COD/liter) Station Dec Jan Feb Oscuro .013 .018 .017 Amatillo .046 Padre Creek .004 Coraercio .024 Coban .103 Bujajal >.170 Station A .024 Station B Station C San Felipe .009 ,033 ,037 .121 .119 .186 048 .106 038 .029 050 .049 033 .070 234 in c o •> •H tfl +J rt U D O nJ ^H U ^^ rt cti •t S > ca o >-< X •N Mh +-) < 3 X o to I— ( lA c .c o +-> -a •H c c +-> o rt Cj 6 +-> x: W 0) 4-J ^ ^ ^1 +J o o c ^ X) C "^ to rt w c ^H o to (D •H c > +-> O -H oJctJ •H U ^ 0 +-> +-> O O f-l C rt rt o O TO % — / U — 1 M-i t3 O O t4-i q tiO O ra to rt E fn TO O C C e D •M C o ,c 3 3 X ■!-> O O O 6 6 0 ctS m o Cl, O fH • H 4-) ri TO C3 .-H C/D -O M 3 tu u u > c TJ C O to C O U cti TO O 1 — 1 LO lO (M 1— I (N /— N o •H 4= X O 4-1 O M) C o Jh O 1— 1 TO E o X ■^. C- U\0 ^^ — ' to o • H .— ( o Q X •H to c; e ^1 CM t^ to vD to vD C30 LT) "-H 00 O en t^ rH O (M [^ rt .— ( CO (M oi Lo r^ CO to t^ 00 o r^ \D to (^ — H r~- to o (N <3^ .— I LO \0 O O 1^ to r-H to I— I I— I LO to CN M- VD •^ (~sl vC •V •^ *v •S to tN to I— I 1-H rg^rj-iooooo <~M t— 1 to O to O o \o ^a- in t^ G) OT o 1 — ' I— t rs! 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C) O^D Pu t/1 O ■H •H i-H 1 — 1 Q X (D K) tu e C 03 in 3 C •H +J C o o I 0) a o u nS 2 Oi t^ \D K> O^ O CI (N ■^ \0 ^ \D r^ LO O O^ r~~ 00 v£5 (NI CTl •=a- ^ LO -^ oo -^ vO LO O t-O O Oi \D 00 v£) O LO t^ Ol VO t^ rO — I vO M- CTl r-^ to CM to o vo o r^ to \D vO O^ O 00 CM 00 CM cj> o r^ o 00 \D CM CM CM -si- CO LO LO o o o o o o o o o o o o o o LO CM O to LO CTi vO OO ■ct LO LO CO LO CM ■— I O r-H O O l-H O CO o CT) 00 oo .— I r^ to \D O to t^ CO CM r^ 00 CO CT) t^ CO t^ to o c^j to t^ r^ LO \D O O CT) \0 to LO CO CTl CTi CTl 00 o r^ -a CM E a o u u -a c CO ca v> C O •H (J 05 +-) CO o (D bO| r3 !-i < Q O U CC E f—i CO ^D vD O Cr> vD 00 CO to rH CM O vO •-H a> •<* t^ -^ o o t-H lO "g- CM ■— I vD (M LO CM r- I-l cri cTi 'S- 00 "^ vo ^ cy> 1— I o \D -JD CTl ■^ \D CM vO O 'S- LO -q- CM r-» r-l 00 r^ ^D 00 >— I o <7i CTl to O CO to r-H o I^ CM ^ VD 00 CO CO LO r^ to LO r-l \0 LO CTi CO 00 CTl CM CM O (3^ 'J- 00 t-^ r~- o LO r— I .— I 1— I CM LO 1— I '^ . — I < — I • — ^ 1 — I f^I . — I CM CT^ CM to -* O CO r^ r-~ o) o o C7^ LO O O I— I I— I 1— I o o LO to vo o en CN] ^ to ■— I LO O P^ vD CM r^ \£> LO r~- CTi CO CO O to vO CT) to CO -* LO CO «-J- "^ LO "^t CM 00 r-- r^ CO o o cji r-l r-( ■a QJ C ■ H -a OS 3 o •H ■P 03 CD c 03 P 03 1 — I O u p^ -a > r-H O fn H X C rH bij P 03 p, rj 3 3 D O s < s: ►O f-j < O fn ^H X C .-I Wi P CO (i, rt D D 3 O 2 < S f^ "^ < O Q O U LH o 10 c o •H P o 03 U o o 03 CO Pi p lh o o o x: p 03 U P c o o c o CJ Q o u u-l o 3 r-H CO > U 0) P Oh -a X P Q O U bO E t/i C o oj u p c 0) o c o CJ Q o u ^1 o LH TJ to 3 f-l 0) I/) u c o CO P c o u c o •H p 03 P (L) U c o o X -a p 03 1—1 3 O 1—1 CO U 239 Table i-1 - Results of respiration experiments (BOD) in which duplicate samples were sacrificed after one, three, and seven days of incubation. Values represent oxygen concentrations Cmg 02/liter) and values in parentheses are the rates (mg 02/liter day) between sampling dates^ Collection Date Initial Day 1 Day 3 Day 7 5-Day BOD Comercio (Polochic) 27 Dec 71 25 Jan 72 1 Apr 72 29 Sep 72 8.10 7.59 7.20 7.22 (0.14) (0.051 (0.30) (0.13) Coban 7.96 ( - ) 7.54 (0.08) 6.91 (0.20) 7.09 (0.04) (Polochic) 8.08 7.58 6.51 7.06 (0.04) (0.06) (0.07) (0.06) 7.94 7.16 6.49 6.81 (0.02) (0.07) (0.14) (0.06) 17 Jan 72 8.32 (0.07) 8.25 (0.10) 8.05 (0.13) 7.54 (0.11) 14 Mar 72 29 Sep 72 8.06 7.26 (0.15) Bu j a j a 7.11 (0.03) 1 (Polochic) 7.05 (0.05) 6.83 (0.16) (0.06) 17 Jan 72 8.30 (0.24) 8.07 (0.03) 8 . 00 (0.10) 7.61 (0.10) 14 Mar 72 1 Apr 72 10 May 72 7 Jun 72 2 Aug 72 7.85 7.68 7.35 6.64 6.42 (0.34) (0.27) (0.40) (0.33) Amatillo 7.34 (0.25) 7.08 (0.31) 6.25 (0.46) 6.09 (0.10) (Swamp Waters 6.85 6.46 5.34 5.89 1 (0.17) (0.05) (0.22) (0.09) 6.16 6.25 4.44 5.52 (0.12) (0.24) (0.20) (0.35) (0.14) 27 Dec 71 25 Jan 72 10 Apr 72 2 Aug 72 6.76 6.59 7.23 6.70 (0.60) (1.55) (1.63) (0.22) 6.16 (0.27) 5.04 (0.69) 5.60 (0.95) 6.48 (0.08) 5.63 3.65 3.70 6.32 (0.14) (0.58) (0.92) (0.09) 5.08 1 . 33 0.00 5.98 (0.28) (0.82) (1.08) (0.11) Padre Creek (Swamp Waters) 18 Jan 72 29 Sep 72 6.75 4.38 (0.43) (1.56) Lagartos 6.32 (0.29) 2.82 (0.18) (Swamp Waters^ 5,74 2.46 L (0.10) (0.16) 5.35 1.82 (0.24) (0.45) 10 May 72 29 Sep 72 7.23 6.81 (1.07) (0.56) 6.16 (1.52) 6.26 (0.19) 3. 13 5.87 (0.55) (0.14) 0.94 5.32 (1.04) (0.27) 240 Table H.- continued Collection Date Initial Dav 1 Day 3 Day 7 5-nay BOD Oscuro (Swamp Waters) 27 Dec 71 25 Jan 72 1 Apr 72 7 Jun 72 2 Aug 72 7.57 6.04 6.00 6.74 6.62 (0.41) (0.30) (0.48) (0.35) (0.32) Sauce ( 7.16 5.74 5.53 6.39 6.30 'Small (0.15) (0.14) (0.23) (0.31) (0.13) Rivers) 6.86 5.47 4.83 5.77 6.05 (0.12) (0.07) (0.12) (0.19) (0.10) 6.37 5.18 4.48 5.02 5.63 (0.19) (0.14) (0.26) (0.27) (0.24) 10 Apr 72 9 Jun 72 29 Jul 72 7.41 7.14 7.56 (0.25) (0.54) (0.18) 7.16 6.60 7.38 (0.15) (0.55) (0.17) 6.86 5.50 7.04 (0.22) (0.32) (0.17) 5.99 4.24 6.37 (0.20) (0.45) (0.17) San Marcos (Snia 11 Rivers) 19 May 72 9 Jun 72 29 Jul 72 6.92 6.99 7.45 (0.35) (0.23) 6.64 7.22 (0.25) (0.16) 6.14 6.90 (0.13) (0.08) 5.64 6.57 (0.16) (0.22) (0.14) Man acas Cr< 2ek (Small Rivers) 19 May 72 5 Oct 72 7.10 6.94 (0.22) 6.73 (0.11) 6.51 (0.09) 6.15 (0.14) (0.12) 3 Jan 72 7.96 10 Apr 72 7.56 10 May 72 7.71 19 May 72 7.54 8 Oct 72 6.99 (11m) Station A (Lake) (0.26) 7.70 (0.18) 7.34 (0.05) 7.14 (0.15) (0.27) 7.29 (0.20) 6.90 (0.32) 5.60 (0.26) (0.54) 7.17 (0.77) 5.64 (0.16) 5.00 (0.48) (0.18) (0.15) 6.86 (0.20) 6.47 (0.10) 6.08 (0.14) Station B (Lake) 3 Jan 72 6.73 (0.24) 6.49 (0.05) 6.40 (0.08) 6.06 (0.10) 19 May 72 (surf) 19 May 72 (15.5m) 7.30 CO. 17) 7.10 (0-1^) 241 Table H.- continued Collection Date Initial Day 1 Day 5 Day 7 5-Day BOD Station B (Lake) - continued 8 Oct 72 7.36 (0.08) 7.28 (0.11) 7.06 (0.05) 6.86 (0.08) (surf) 8 Oct 72 6.88 (0.17) 6.71 (0.10) 6.51 (0.07) 6.25 (0.10) (15 m) Station C (Lake) 19 May 72 7.56 (0.12) (surf) 5 Oct 72 6.53 (0.15) 6.59 (0.07) 6.26 (0.06) 6.00 (0.08) San Felipe (Lake) 3 Jan 72 8, ,21 (0. ,22) 7. ,99 (0. 23) 7. ,53 CO. ,10) 7. .13 (0. 16) 8 Feb 72 7, ,62 (0. ,25) 7, ,37 (0. 14) 7. ,11 (0. ,08) 6. .80 (0. 15) 29 Jul 72 7, ,36 (0. ,09) 7, .28 (0. 16) 6. ,97 (0. ,07) 6, .70 (0. 11) The 5-Day BOD rates (mg 02/li'cer day) were calculated by dividing the difference between the initial concentration and the Day 5 concentra- tion (average of Day 3 and Day 7 concentrations) by 5. 242 t/) T3 CO q I— ( rt 0) s rC O H i-H ^ C ■ -H (fl •H X W .—1 rt -H ^ rt -a >^ ■-I 0) -c x; •M ■!-> C o c 6 (U o T3 2 C -P 03 (U -Q X t— I / — s • H W (U 03 0>-t rH V— ' IW M-( -p H na ^1 C 0) ^1 03 m 0) m fl, W -H 3 T3 w o o rM O ■p M-l -H ■p C -P o3 •H (D fc: Q ,2 O O -P •H CJl -H c Jh 03 -d 03 M C U 03 P 01 C e ^( (D OJ 3 o W) X S 03 O -3 I-H -"^ 03 o o O

03 Q O tji O \0 tj^ o o IT) vO to to ^ o ^ ^ CTi en CO CO 00 o in •— I CO CTl o o 00 O LO t^) \0 CTl 00 (71 r-- (Nl O 00 o CM O to Oi u ^ X C r1 &, CT3 3 *i. < <^ "^ vO H O t— 1 vD to (NI vO -> CO CO (NJ (^ 00 CO o to CTl CM CNJ CnJ CM to to CM VO to Ol vD H .— ) \D (ji CTl vO OJ D to cn ■^T CO 00 r-- LO •1 o to to LO fN O CM CM •^ LO 00 (M vD LO LO i-H OJ 'S- LO OI CTl r^ >£> o r\! \0 O 00 (Nl r~- (71 CO •i CTl r— ( to CTl o a\ \D O LO oi '4- to o o to o I-H be P 3 3 U ■-5 < O 1) 3 r— ( 03 > Q O U CO > •H -3 X -3 (D P OS rH 3 (J t— < 03 o u o p p 03 C 03 00 1-1 O C O E (U to X o3 -a o to X J3 03 P O P X I-H X ■p c o (U bO C ■H -3 •H > •H X Xi -3 OO ■P X •H OJ X > x; o •H +-> ^— ^ •M O fn X 3 O rt ■13 ^4-1 T3 O ^ /— -rg PL, c: 6 Cl, --~^ X^ CN u o rt >N E 4-> M •H -H fH > O Ol-H fH +-> cfl W O W 3 V5 o -a o bC f-( --I &, rt Hh > O X U -1 10 rt r-l O E < ■M -H 03 fn f — s ^1 (^ • u o (D -P U p M O p rt c -o rt fH C c 0) 'O C3 > C U a ri - ■H ca c O '■ oi X /-^ '• txo +-<•*< fH CM^^ o M-l PS O >— ' •/) o (—■ p ceo O O -H +-> • H -H ■•-> c ■P •(-> 03 CD OS ri P i-H t-l fH W 03 3 -H > O Oh O •H i-t 10 ^ ri 03 0) n3 CJ" U ^1 r-i 0) •-» (D 1— 1 ,Q nj H o o t-- o to c o to Csl en to CM ^ r—i CTl •^ c T-H \0 to to t-~ CJ LO OO c- • • • • • • • • (NJ ,— ( o o o o 1 — 1 r-1 + + + 1 1 1 1 1 o to r^ o to t^ o to 0) ■<* CM •^ to C-, to o r-~ •— 1 Wl rg j—t r-- CO LO o 00 CTl 00 03 oi . . . • • • • • ^1 (N rsi to •<3- lO Tf to f1 0) O ro to O o r-- o o ■^ to t^ r- CD 00 CO r^ bO rg •^ , — 1 tNl rj o to O") a. • • • • • • • • lO LO r~- to o LO to LO c to LO f-H o to CN o vC Cl. . • • ■ • • • • o o o i-H o o c^g CN + + 1 1 1 1 1 1 u c "* o o 1-H vD o 00 CTl LO o CN LT, LO CO v£3 LO CN \C CO •r-l Ci . • • • • • • • P CN CM CM to to to to to OS •P r/1 LO LO •^ to 'a- to o o bO o O \D \D 1 — 1 o \D CN 0- • • • • • • • to to CN CN to lO r-H r-( LO LO rg o CTl LO cn o Cm vD '^ t-H LT. L-0 lO o C-i Ch • • • • • • • • O o CN CN o rsj CM .-H + + + + 1 1 1 1 oa C ■^ LO LO LO LO ^ v£) r—t LO o CN \D v£) to a^ CD LO CN to •H oi . . - • P C-M CN r-o CN LO r-- LO '^ 03 ■P in O O r- LO LO 1 — 1 CN Lf5 bO to to ■^ •^ to CN r-H ^ a. • • . • • • • • to to ■^ LO LO LO to CN \r> r^ ^ to 00 I^ LO 00 C f— 1 00 en ^ CN I-H vO en cu • . • • • • • LO to o CN r-H o o o < + + 1 1 1 + 1 ' r— ■rf I-H 00 LO vO vD LO 1 — 1 M- o CN CN o to ^~i O CO o CN •H c; • • • ■ • ■ • • P ' — 1 to vO r^ LO t') to to 03 ■P U) r^ LO I— H to 00 CN \D vO bO to en ^ r- to O to CN a. • . • . • • \0 U o ,Q u p 6 cu JZ JZ i-i w cu ^ 4-> u •H 4) X 3 p o c u fH X C f-H bO Cl, •p o ri c^ 03 D 3 3 o; CJ »±. -^ < *^ <-> '-5 < LO o LITERATURE CITED American Public Health Association. 1965. Standard Methods for the Examination of Water and Wastewater, 12th ed. APHA. 769 p. Anderson, G.C. 1958. Seasonal characteristics of two saline lakes in Washington. Limnol. Oceanogr. 3:51-68. Barlow, J. P., C.J. Lorenzen, and R.T. Myren. 1963. Eutrophication of a tidal estuary. Limnol. Oceanogr. 3:251-262. Beeton, A.M. 1957. Relationship between Secchi disk readings and light penetration in Lake Huron. Trans. Amer. Fish. Soc. 87:73-79. Bormann, F.H., G.E. Likens, and J.S. Eaton. 1969. Biotic regula- tion of particulate and solution losses from a forest ecosystem. Bio-Science 19:600-610. Bre7onik, P L. , W.M. Morgan, E.E. Shannon, and H.D. Putnajr;. 1969. Eutrophication factors in north central Florida lakes. Engineering and Industrial Experiment Station. Bull. Ser. No. 134. 101 p. Brinson, L.G. 1973. A comparison of seasonal plankton abundance and primary productivity in five tropical fresh-water deltaic embayments. M.S. thesis. University of Florida, Gainesville, Fla. Brooks, H.K. 1969. A preliminary report to the Organization for Tropical Studies, Inc. on Lake Izabal, Geology and Hydrology. 19 p. Carr, A.F., III. 1971. The commercial snook (Centropomus undecimalis) fishery of Lake Izabal, Guatemala. 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Glen and Geneva, until graduation from Shelby High School in 1961. He graduated with the degree Bachelor of Science in biology at Heidelberg College in June, 1965, where he was awarded membership to Who's Who in American Colleges and Universities and Tower Men, an academic honorary. In August, 1965, he entered graduate studies at The University of Michigan where he received the Newcombe Fellowship in botany. Upon receipt of a Master of Science in Botany in May, 1967, he v;ent to Central America as a Peace Corps Volunteer. There he worked as a fisheries bio- logist for two years in Turrialba, Costa Rica. He entered graduate school at the University of Florida in September, 1969, where he is presently a candidate for the degree Doctor of Philosophy While there he was awarded a Research Fellowship from the Program for Latin America and the Caribbean sector of the Foreign Area Fellowship Pro- gram. He is a member of the American Society of Limnology and Oceano- graphy, Phi Sigma Society, The Ecological Society of America, and the American Institute of Biological Sciences. 251 I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. J:^4^ Ariel E. Lugo, Chairman Assistant Professor of Botany I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. <=^. I J.^ .a-i- David S. Anthony Professor of Botany I certify that I have read tliis study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for "lie degree of Doctor of Phiiojoptiy ma G. Griffin, 1^1/ associate Professor of Botany I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Frank G. Nordlie Associate Professor of Zoology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Pliilosoph}-. Hugh L. Por Professor iioe Soils I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. ?r ^f.<^ Z:^ ■U\ Leland Shanor Professor of Botany This dissertation was submitted to the Dean of the College of Agriculture and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December, 1975 OaJIvs n. College of Agriculture Dean, Graduate School