— . wvovv.vv¢-"Womogo~om« .m-ovflm EFFECTS or easomm BY FATHEAD mums, mamas PROMELAS, on PUNKTONIC commutes m SMALL. EUTR-OPHIC Ponce Dissertation for the Degree of Ph. D. MiCHlGAN STATE UNiVERSlTY LOWS A. HELFRICH 1976 .4“ A m m”Inuujwulwuflw”smut!W . I Militia] Univexsity ABSTRACT EFFECTS OF PREDATION BY FATHEAD MINNOWS, Pimephales promelas, ON PLANKTONIC COMMUNITIES IN SMALL, EUTROPHIC PONDS By Louis A. Helfrich This experiment was designed to examine the effects of predation by fathead minnows, Pimephales promelas Rafinesque, on planktonic communities in a series of eight artificially enriched ponds. Replicate fish populations were established in half of the ponds and the remaining ponds were kept unstocked for reference. Physical, chemical and biological parameters, including phytoplankton and zooplankton densities, were measured at weekly intervals. The fish populations and their food habits are described. Fish predation significantly reduced the densities of both small and large zooplankton species. The gut contents of fatheads suggest a size selection of prey that was related to the size of the minnow. The decrease in the total abundance of herbivorous zooplankters permitted an increase in the total standing crop of algae, primary productivity and turbidity and ultimately effected a shift in algal composition from one dominated by edible green algae, diatoms and cryptomonads to one dominated by inedible blue-green algae. There was a strong inverse relationship between the average density of blue—green algae and the average concentration of free carbon dioxide. EFFECTS OF PREDATION BY FATHEAD MINNOWS, Pimephales promelas, ON PLANKTONIC COMMUNITIES IN SMALL, EUTROPHIC PONDS By \\ c‘ of} g Louis A? Helfrich A DISSERTATION submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Fisheries and Wildlife 1976 ACKNOWLEDGMENTS I wish to express my gratitude to my maJor professor, Dr. C. D. McNabb, who advised and encouraged me throughout this study, and to my graduate committee members, Dr. T. G. Bahr, Dr. W. T. Porter, Dr. E. W. Roelofs, and Dr. P. I. Tack for their thoughtful review and suggestions concerning this thesis. I would particularly like to thank Dr. R. A. Cole and Diana L. Weigmann for their special attention to the problems involved in this study and for their continuing advice, criticisms and encourage- ment throughout this study. Special thanks is also extended to Joan Duffy, who helped in the field sampling and zooplankton enumeration with an appreciation for detail and accuracy that was outstanding. Finally, I wish to acknowledge the understanding, interest and inspiration of my family. Financial support was provided by the Michigan Agricultural Experiment Station and the Rockefeller Foundation under funds administered by the Institute of Water Research at Michigan State University. ii TABLE OF CONTENTS INTRODUCTION . . . . . . . . . . . . MATERIALS AND METHODS . . . . . . . . . The Experimental Ponds . . . . . . Fish Populations . . . . . . . . Water Samples . . . . . . . . . . Chemical Analysis . . Primary Productivity . . . . . . . Phytoplankton . . . . . . . . . . Zooplankton . . . . . . . . . . Data Analysis . . . . . . . . . RESULTS . . . . . . . . . . . . . . . . Fish . . . . . . . . . . . . . . . Population Changes . . . . . Stomach Contents . . . . . . Zooplankton . . . . . . . . . . Total Zooplankton Density . Zooplankton Composition . . Phytoplankton . . . . . . . . . . Gross Primary Productivity . Phytoplankton Density . . . . Phytoplankton Composition . . Physical Conditions . . . . . . . Chemical Conditions . . . . . . . DISCUSSION . . . . . . . . . . . . . . LITERATURE CITED . . . . . . . . . . APPENDICES . . . . . . . . . . . . . . iii \O\OCD-]O\O\|—' 10 10 ll 13 13 13 1h 19 19 22 2T 27 3o 30 37 37 1&6 53 58 Number A-ll LIST OF TABLES Page Relative importance and seasonal variation in 17 utilization of food items for large (>h5 mm) and small (<20 m) fathead minnows collected from four experimental ponds during the second weeks of July, August and September. Species list of the phytoplankton encountered 58 during quantitative counts. Species list of zooplankton encountered during 59 quantitative counts . iv Number 10 LIST OF FIGURES Proposed interrelationships between planktivorous fish, herbivorous zooplankters, planktonic algae and chemical parameters in eutrophic ponds. Size-frequency distribution of the minnow popula- tions harvested in September. Mean zooplankton densities (i one standard error) in the stocked and the unstocked ponds. Mean numerical densities of the major zooplankton species in the stocked and the unstocked ponds. Average gross primary productivity (i one standard error) in the stocked and the unstocked ponds. Mean total phytoplankton densities (i one standard error) in the stocked and unstocked ponds. Mean numerical densities of the major phytoplankton taxa in the stocked and the unstocked ponds. Mean percent transmission values (i one standard error) in the stocked and the unstocked ponds. Mean total phosphorus (as PO ) and free carbon dioxide values (i one standard error) in the stocked and the unstocked ponds. Relationship between the log of the average free carbon dioxide concentration and the leg of the average density of blue-green algae. The straight line was fitted using the Least Squares Method. 16 21 25 29 32 3h 39 A2 hh INTRODUCTION The potential role of fish as regulators of plankton communities has been recognized at least since Clements and Shelford (1939) preposed that carnivorous fish may ultimately control plant and animal planktonic densities. Although physical and chemical attributes more often are credited as important regulators of algal abundance and species composition; biological mechanisms, including fish predation, zooplank- ton grazing, competition and parasitism, also have been recognized as potentially important (Edmondson, 1972; Hutchinson, 1973; Lund, 1965; Hrbacek et al., 1961; Porter, 1973). Hurlbert et al. (1972) believed that excessive phytoplankton growth, a principal symptom of eutrophica- tion, may be more directly attributable to man-caused alterations in fish populations than to nutrient influx. Although previous workers recognized the potential role of planktivorous fish as regulators of phytoplankton populations, little published work has directly addressed the problem. This experiment was designed to clarify some of these trophic relationships. The general purpose was to test the impact of zooplankti- vorous fish on the plankton community, with particular emphasis on algal dynamics and associated changes in nutrient chemistry. This study addresses a sequence of hypotheses that are designed to test the premise that zooplanktivorous fish can influence phyto- plankton productivity which, in turn, may modify water quality and cause successional changes in algal composition. The model schematized in Figure 1 was structured from observations and speculations of previous investigators. Among them, Hrbacek (1962) considered fish as the decisive factor influencing the composition and quantity of plankton and possibly several physical and chemical properties of water. Many authors, including most outstandingly, Hrbacek and Novatna-Dvorakova (1965), Brooks and Dodson (1965) and Galbraith (1967), have established that certain species of fish are capable of altering zooplankton composition. Others, Fryer (1957), Burns (1968), Jorgensen (1966), Porter (1973), have found that crustacean zooplankters exhibit selective feeding behavior. In at least some instances zooplankton grazing has had a measurable depressive impact on algal abundances and productivity (Cushing, 1958; Menzel and Ryther, 1961; Hargrave and Geen, 1970; Haney, 1971). Another group of investigators have noted that primary productivity in eutrOphic systems may become limited by carbon dioxide availability (King, 1970, 1972; Shapiro, 1973), and that blue—green algae tend to be favored in nutrient enriched environments where carbon dioxide is scarce. Development of blue-green algal blooms is usually one of the least desirable consequences of eutrophication. Presumably, then, blue-green algal dominances in enriched environ- ments may be favored by restricting zooplankton grazing which, in turn, increases primary productivity and promotes the rapid develop- ment of large algal standing crops that are often referred to as "blooms." Algal composition could change as a consequence of carbon dioxide scarcity generated by the increased rate of carbon incorpora- tion in an otherwise unlimited aquatic environment (King, 1972). Figure 1. Proposed interrelationships between planktivorous fish, herbivorous zooplankters, plankton algae and chemical parameters in eutrophic ponds. FISH STOCK + NATURAL RECRUITMENT [INTENSITY 0F PREDATION PRESSURE ZOOPLANKTON GRAZING POTENTIAL PRIMARY 1 , PRODUCTIVITY CARBON DIOXIDE AVAILABILITY 2 PH . IALGAL ,J COMPETITION , ALGAL E _ COMPOSITION _ Hypothetically, uninhibited zooplanktivorous fish populations have the capacity to decimate zooplanktonic herbivores, at least during the short term, as long as all sizes and species of grazers are readily consumed. Planktivorous fish populations of diverse age and size structure probably would be required to obtain this result. With these hypotheses in mind, I stocked four of eight physically and biologically similar ponds with adult fathead minnows (Pimephales promelas) Rafinesque, which soon developed a diversified age and size structure through natural recruitment. Subsequent to fish stocking, data were gathered on fish populations, fish stomach contents, zoo- plankton and algal density, primary productivity, carbon dioxide.~ and other limnological parameters. The results generally confirm the predictions of the model. MATERIALS AND METHODS The Experimental Ponds Eight identical ponds, 7.2 x 7.2 x 1.8 m deep, previously used as wastewater treatment ponds served as the experimental units. The ponds were constructed of concrete and had a bottom of peastone gravel. Three months before the experiments, each unit was filled to a depth of 1.5 m with a mixture of untreated well water and domestic sewage. Constant, identical water levels and similar fertilities were main- tained in all experimental ponds during the study period by the weekly addition of a homogeneous mixture of domestic wastewater. Wastewater applications were carefully conducted to insure that each pond received identical volumes and equivalent nutrient enrich- ment. Wastewater was pumped from a municipal sewage line into a head tank, thoroughly mixed and then uniformly distributed over the surface of each pond through a network of suspended PVC tubing. The total phosphorus and ammonia concentrations of the influent wastewater were measured weekly; ancillary data on pH and temperature were taken and spot checks for heavy metals were conducted. Throughout the study, the pH of the influent wastewater remained relatively constant at 7.8, the average temperature was 22 C, copper and chromium concen- trations remained below detectable levels, total phosphorus values (as P) ranged from 5.8 to 7.2 mg/liter, and ammonia nitrogen levels ranged from 16.1 to l7.h mg/liter. All ponds were similar in their chemistry and biology at the initiation of the treatment. Fish Populations On July 1, l97h, 200 adult fathead minnows (Pimephales promelas), h.5 to 6 cm total length, were introduced into each of four randomly selected ponds. The remaining four ponds were kept unstocked for reference. The initial stocking density, 38,500 fish/ha, was chosen to provide a relatively high level of predation pressure on the z00plankton community. .The adult stock was in spawning condition; I assumed that natural reproduction and the consequent recruitment of larval fish in early summer would provide a size structure capable of utilizing the full-range of food particle sizes available and increase the intensity of predation through the summer. The fathead minnow has received considerable attention as a forage fish well suited for pond culture. This minnow is widely distributed and extensively propagated in the United States; it is a popular bait fish which is commonly used as food for hatchery-reared game fish. Fathead minnows were selected for use in this study because of their potential influence on plankton communities. This species has generalized food habits encompassing the majority of herbivorous zooplankton. These minnows are characteristic of small warmwater ponds, often are found in wastewater lagoons and are quite tolerant of the chemical conditions that typically occur in hypereutrophic waters. The initial stock used in this study was obtained from a waste stabili- zation pond at Belding, Michigan. ll llllll'lll A 1.]... Food habits were determined for 100 fathead minnows seined during the second weeks of July, August and September. Large minnows (>h5 mm) were injected with a 50% alcohol solution to retard digestive processes and all fish were preserved in a 10% formalin solution. Total lengths of individual fish were recorded and gut contents were rinsed into a circular counting chamber where food items were identified and enumerated. During the second week of September, the total fish populations were harvested from two ponds chosen randomly from the four stocked ponds. These ponds were cleared of macrophytes, treated with rotenone solution and repeatedly seined. The fish were immediately preserved in formaldehyde solution. Later they were counted, measured to the nearest 1 mm, and separated into two size categories; small, larval minnows (
h5 mm). Subsamples from each of these groups were dried in a forced-air oven for 2h-hours and weighed to estimate production. Water Samples The sampling regime provided for replicate physical, chemical and biological samples for each pond during every week of the study. To determine sampling points, a grid was established for each pond and random numbers were selected to determine the sampling area. All samples for water chemistry and phytoplankton analysis were collected with a one liter PVC Kemmer bottle at mid-depth between 10:00 a.m. and 1:00 p.m. the day before wastewater application. Chemical Analysis Total phosphorus (as phosphate) was measured by the stannous chloride method (American Public Health Association, 1965). Nitrate nitrogen measurements were made by the brucine method of Jenkins and Medesker (196A). Ammonia nitrogen was determined by the direct Nesslerization test (American Public Health Association, 1965). The hydrogen ion concentration was measured using a Corning Glass electrode pH meter; total alkalinity was determined by acid titration (American Public Health Association, 1965). Free carbon dioxide concentrations were estimated by nomograph using alkalinity, pH and temperature determinations (American Public Health Association, 1965). Temperature and oxygen profiles were measured each week over a 2h4hour period in all ponds with a YSI oxygen meter which was periodically standardized against Winkler determinations of oxygen. In addition, continuous 2h-hour temperature and oxygen measurements using a Rustrak Temperature- Oxygen Recorder and probe (Model 192), were made each week in one randomly selected pond. Primary Productivity_ Gross primary productivity was calculated from the diurnal oxygen curves by the single station method of Odum (1956). The values for the gas transfer coefficient (K) across the airawater interface were calculated from the rate of change of oxygen concentrations for each diurnal curve and corrected for temperature (Odum, 1956). Water clarity was estimated by measuring the percent transmission at mid- depth using a Schueler Submarine Photometer. 10 Phytgplankton Duplicate whole water samples were collected from each pond during every week of the study at the same time samples for chemical analysis were taken. One sample was preserved with a 5% formalin solution and the other with a 1% Lugol's killing and fixing solution. Two preserva- tives were used because of their differential disruptive properties; the formalin fixative may destroy small flagellates while Lugol's may disrupt the integrity of colonial algae (O'Brien, 1970). Individual phytoplankters were counted on membrane filters following the technique of McNabb (1960); the filters were examined at 200x and hsox using a dark phase microscope. Diatoms were identified on separate, permanent mounts which were prepared according to Weber (1971). Phyto- plankton taxonomy was based on the keys of Smith (1950) and Prescott (1962). A list of the planktonic algae encountered in the experimental ponds is given in Appendix I. Zooplankton Triplicate zooplankton samples were collected each week in all ponds with a Wisconsin-style plankton net (7hu, No. 20 mesh size). Vertical tows were made to integrate samples over depth. To reduce distributional errors, all samples were taken at night, beginning one hour after sunset, when the vertical distribution of the animal plankters was assumed to be more uniform. All zooplankton densities undoubtedly were underestimated since the efficiency of the No. 20 plankton net has been estimated at h0-60% (Hall et aZ., 1970). The values presented represent the actual estimates obtained, since any 11 correction factor for net efficiency would not change trends in the data. All samples were immediately preserved in a 70% ethanol and 3% formalin solution. Subsamples, 1—10 ml, were enumerated in a zooplankton counting wheel under a dissecting microscope. Crustaceans were identified according to Brooks (1959), Wilson (1959) and Yeatman (1959) and rotifers according to Edmondson (1959). A list of the zooplankton species encountered in the experimental ponds is given in Appendix II. Data Analysis The results were arranged to show differences between the stocked and unstocked ponds. The sampling regime provided for duplicate phyto- plankton and physicochemical samples and triplicate zooplankton collections from each of the 8 ponds every week. Phytoplankton and physicochemical means for a given date represent 8 samples for each of the two sets of ponds. Similarly, zooplankton means for a given date represent 12 samples for each of the two groups of ponds. A one-way analysis of variance was applied to most of the data. Log- arithmic transformations of the plankton values were used to reduce heterogeneity among variances and percentage values were normally distributed by an arc sine transformation. The plankton data were analyzed with standard parametric statistical techniques because the short generation times of planktonic organisms made autocorrelation between the sampling dates unlikely. Standard errors presented in association with treatment means contain several sources of variation. No attempt was made to separate within-pond variation, which was high since the area sampled is a small fraction of the total pond area, 12 from within—treatment group variation. A large component of the variation in the plankton data was the result of asynchronous popula- tion trends. The following results clearly demonstrate that among the factors measured, there were major, consistant trends and significant differences between the stocked and unstocked ponds. RESULTS Fish Population Changes Changes in the populations of introduced fathead minnows were investigated in two ponds which were chosen randomly from the four stocked ponds. The data on total numbers, biomass, production of fry and adult survivorship were similar for both ponds. I assumed that changes exhibited by these two ponds adequately represent population changes for all of the four ponds stocked with minnows at the beginning of the experiment. In establishing the minnow populations each pond received 200 fathead minnows (38,500 fish/ha) weighing h00 grams in total (77 Kg/ha). The total numbers and biomass in both ponds increased as a result of high survivorship of the initial stock and natural reproduction through the summer. At the termination of the study in September, 8888 minnows weighing 226A grams (A35 Kg/ha) were recovered from Pond 2 and 6188 minnows weighing 1913 grams (3A9 Kg/ha) were recovered from Pond 5. Total biomass of fish had increased at least 5 fold in each pond over the study period. Large numbers of 5-12 mm larval fish first appeared in the zoo- plankton samples on 22 July (fourth week) in all of the stocked ponds. The larvae were apparently only susceptible to the plankton net in this size range, as fish larger than 12 mm were never caught. Since 13 1h newly hatched fathead minnows are about 5 mm long, the first major reproductive pulse probably occurred during the third week of July. The size-frequency distribution of fish populations in September indicated that reproduction continued throughout the study and that there was no recruitment of larval fish into the size-classes of the initial stock (Figure 2). The mode of the fry size distribution was about 15 mm; the maximum length attained by young-of-the-year fish was 35 mm. The total number of larval fish recovered in September from the two ponds, 8758 and 6125, undoubtedly were underestimates of the total larval fish populations, particularly of larvae less than 20 mm. These smaller larvae were especially susceptible to entrainment by the pumps used to regulate the water levels, and despite precautions to screen them from the intake, many small fish were not recovered. Therefore, the estimate of fish biomass in the ponds was conservative. Survivorship of adult minnows was determined for all four stocked ponds. Of the total 200 fish stocked in each pond, 59-82% were recovered in September. The survival rates obtained are somewhat higher than those determined for natural populations (Isaak, 1961). Evidently, the high post—spawning mortality commonly reported for this species (Markus, 193A; Hasler et al., l9h6) did not occur in this experiment. Stomach Contents The relative importance and monthly variation in utilization of food items for large (>h5 mm) and small (<20 mm) fathead minnows collected from all four stocked ponds during the second weeks of July, August and September are listed in Table l. Crustacean species were dominant in Figure 2. 15 Size-frequency distribution of the minnow populations harvested in September. Number of Fish x I0 ‘ _0 NUbUIlem l6 Pond 2 initial stock Nahumslm 1“. Pond 5 Initial stock I . n“..— 5 IO IS 2025 30 35 4045 505560 6570 mm, Total Length 11‘ I! I'll .l‘cllll 17 .mponap o>Hpmomwo hpflfio spa: mo .mpownp o>fipmomfio apnea spas me .mpoanp o>wpmowfie Sense new: no .mpodhp o>fipmowwc apnea sea: :9 .mpoaAp o>wpmowwc apnea spas to ooa New 00H mma ooa mmm ooa mew ooa mow Hopoe m a o o a m OH >0 H m apogee mm Had mm mm o o s em m m mosses omen: Ha mm m ma m mm em Hwa N Pm noooonpmo as we m m Hm ems o o o o oNNnnosoe o o o 0 mm New 0 o o o shamans essence o o S OH H OH NH on a ma neaoaoaoao o o o o o o o 0 ma or assonaoem o o s e as one mm mas 0 mm trespass o o o o m m: o o H m aewsmom o o o o o o m om as me eessaeeoesoo m NH ma as o 0 ma an o o ssoaosoeaoee o o w ma 0 o a ow mm can seeeasa R .02 m .02 a .02 R .02 TR .02 sapH coca oHHmEm vownmq oaaaam Downed downed nopawpmom pmswd¢ Saab .Amomoo Ham aw om u av .nonEopmom can pmsmdd .haah mo mamas ocooom one wafinse meson Hapaoafinomxa snow Scam oopooaaoo msosaws consume Ase omvv Hanan can Ase many owned pom asap“ coon mo cofipoufiafips a“ aoHpast> Hoaomoom was ooaapaomafl o>prHom .H manna 18 the stomachs of 100 fathead minnows collected during the study; they comprised about 65% of the total number of all food items. 0f the crustacea, cladocerans were the most important prey item, accounting for 36% of the total number. Cladocerans were followed in numerical importance by the copepods, including nauplii (18%), and the rotifer, Keratella quadrata (18%). Ostracods and chironomid larvae each accounted for 11% of the total number of prey items. Minor food organisms included other rotifers, amphipods, mayfly and dragonfly larvae, snails and algae. Few phytoplankton cells occurred in the digestive tracts examined during the summer. However, filamentous algae, primarily species of’CZadbphora and Spirogyra, and detritus were common in the intestinal contents toward the end of the study, in September. Comparisons of the stomach contents of large fathead minnows (>h5 mm) and small minnows (<20 mm) suggests that fish size was an important determinant in the selection of prey items. Cladocerans predominated in the stomachs of the large fathead minnows, accounting for 5h% of the total number of all prey items. In contrast, the rotifers, essentially Karatella quadrata, and copepod nauplii were the most frequently utilized food organisms for the small minnows; they comprised AS and 23% respectively of the total stomach contents. Distinct changes in the utilization of food organisms by the two size categories of minnows were Observed during the study. In early July, the large cladoceran, Daphnia puZex, was the most important prey species, accounting for 53% of the total food items. At this time no larval fish were available for food habit analysis. During the second week of August, Chydbrus sphaericus, and the ostracods were the 19 the major food organisms of the large minnows, representing 22 and 27% of the total diet. At the same time larval fish were feeding primarily on the rotifer, KarateZZa quadnata (51%), and secondarily on copepod nauplii (28%). In September, all minnows switched from a planktophagous to a benthophagous habit and chironomid larvae became the most important food organism for all sizes of fathead minnows, comprising 59% of the total number of food items taken by the large minnows and 63% of the total stomach contents for the small minnows. In summary, the initial stock of fathead minnows introduced to four treatment ponds reproduced and their biomass increased by at least 5-fold. Analysis of the stomach contents of fathead minnows revealed that the majority of zooplankton species present in the ponds were consumed. In general, the mean size of the zooplankton Species found in the stomachs was directly related to the size of the fish. Fathead minnows represented the dominant planktivore in the experi- mental ponds and, unlike natural systems with an array of piscivorous fishes and other natural predators to limit their densities, the minnow populations were virtually unrestrained. Potentially, the fathead minnow population had the capacity to severely limit zooplankton population abundances. Zooplankton Total Zooplankton Density, The results of the zooplankton studies indicated that fathead minnows significantly reduced the total populations of crustacean zooplankters (Figure 3). Initially, the mean total numbers of 20 Figure 3. Mean zooplankton densities (i one standard error) in the stocked ponds and the unstocked ponds. Number/ Liter l500 IZOO 900 600 300 21 .——. Stocked Ponds o---o Unstocked Ponds ¢First Larval FiSh Date 22 zooplankters in all eight experimental ponds were relatively similar with average maximum densities for both stocked and unstocked ponds in excess of 1000 individuals/liter. During the first four weeks of July, the mean total number of zooplankton declined rapidly in all eight ponds; throughout this period the zooplankton densities in the stocked ponds were statistically indistinguishable from those in the unstocked ponds. Thereafter, total numbers of zooplankton in the stocked ponds were significantly (p < 0.05) lower than those in the unstocked ponds. The total number of zooplankton during the last six weeks of the study averaged 665ih5 (mean i one standard error) individuals/liter in the unstocked ponds and 196:28 individuals/liter in the stocked ponds. Zooplankton Composition The zooplankton community was numerically dominated by the large cladoceran, Daphnia puZex, the smaller rotifer, KerateZZa quadTata, and copepod nauplii. Combinations of these two species and the immature copepods accounted for about 78% of the total number of zooplankters in the experimental ponds during the study. Most major differences in zooplankton densities occurring between the stocked and the unstocked ponds can be attributed largely to shifts in the relative abundance of these three taxa. The total number of zooplankton species occurring in the experi- mental ponds was relatively low. Only nine species of cladocerans, seven Species of c0pepods, nine Species of rotifers and two Species of ostracods were encountered in the samples (Appendix II). Since the majority of these Species exhibited only sparse and isolated 23 occurrences, no clear differences in Species diversity could be detected between the two groups of ponds. Quantitative changes of the zoo- plankton Species and groups which occurred most frequently in the samples are illustrated in Figure A. The rotifer, Karatella quadrata, was the most abundant animal plankter in the experimental ponds; this species constituted 3h% (8-65%) of the total number of all zooplankters and about 98% of the total number of rotifers in the experimental ponds. The remaining species of rotifers contributed few individuals, seldom exceeding lO/liter. During the first five weeks of the study, K. quadrata populations fluctuated asynchronously; their densities in the stocked ponds were indistinguishable from those in the unstocked ponds. After July, the populations of K. quadrata increased in the unstocked ponds, to densities h to 25 times greater and significantly (p < 0.05) different from those in the stocked ponds. The number of K. quadrata during the last five weeks of the study averaged 36h/liter in the unstocked ponds and h0/1iter in stocked ponds. In the stocked ponds, K. quadrata populations reached maximum abundances (ADO/liter) during the fourth week then suddenly declined; this period of rapid depression coincides with the appearance of large numbers of minnow larvae. Analysis of the stomach contents revealed that K. quadrata accounted for 51% of the total diet of larval fish at this time. The cladoceran, Daphnia pulex, represented the largest zooplankton Species and the most numerous cladoceran in the ponds; this species comprised an average of 17% (11-25%) of the total number of zooplankters in all ponds during the study. D. pulex was most abundant during the first week of the study when the mean densities in all ponds exceeded Figure A. 2h Mean numerical densities of the major zooplankton species in the stocked and unstocked ponds. Number] Liter 500 250 300 ISO 40- 20' ‘OOT 200' O IOOT 0'4 Unlocked Ponds 50* . I III Ill] ‘1 A II: I I‘ll! 26 ZOO/liter. Shortly after the introduction of fathead minnows the population of D. pulex declined. By the end of July (fifth week) D. pulex populations in the stocked ponds were significantly (p < 0.05) lower than those in the stocked ponds. During the last six weeks, the number of D. pulex averaged 102/1iter in the unstocked ponds and 7/liter in the stocked ponds. Stomach analysis of the large fathead minnows at this time Showed that D. pulex was the dominant prey item, accounting for 53% of the total number of all stomach contents. Similarly, Hrbacek (1962), Brooks and Dodson (1965) and Galbraith (1967), all found that large Daphnia were the preferred prey of planktivorous fish and the first zooplankton species to decline at high levels of fish predation. The copepod nauplii were considered separately from the adult copepods Since they represented a numerically important group. Copepod nauplii comprised 27% (9-52%) of the total number of zooplankters in the ponds during the study. The only statistically significant differences in nauplii densities between the stocked and the unstocked ponds occurred during mid-summer (7/22, 7/29). Immature copepods became particularly abundant during late summer when the populations of K. quadrata and D. pulex had been substantially reduced. The adult copepods and ostracods contributed little to the total number of zooplankters in any of the experimental ponds. Comparisons of these two taxa between the stocked and unstocked ponds were characterized by asynchronous fluctuations of little magnitude with small non-significant differences. Subtle, statistically non-significant, but consistent differences in the densities of the small cladocerans appeared between the stocked 27 and the unstocked ponds. The average numbers of Bosmina Zongirostris, Ceriodhphnia reticulata, and Diaphanosoma Zeuchtenbergiana were generally lower in the stocked ponds than in the unstocked ponds; while the mean densities of Chydbrus sphaericus were consistantly greater in the stocked ponds than in the unstocked ponds. In summary, the populations of certain zooplankton Species were strongly reduced following the introduction of fathead minnows and the subsequent recruitment of larval fish. Other zooplankton Species decreased less obviously, but somewhat constantly over the study period. Chydbrus sphaericus seemed to Show a positive response to increasing levels of fish predation. The overall effect of stocking fathead minnows was a significant reduction of the total abundance of herbivorous zooplankters and presumably grazing pressure. Most of the zooplankton Species present in the experimental ponds are considered relatively efficient filter feeders (Jorgensen, 1966). Potentially, the zooplankton populations have the capacity to limit algal growth and influence primary productivity. Phytgplankton Gross Prima§y_Productivity Gross primary productivity, calculated from diurnal oxygen curves, was used to estimate the total phytoplankton response to the introduction of planktivorous fish (Figure 5). Primary productivity corresponded closely with the total phytoplankton development in both groups of experimental ponds. Gross primary productivity was consistently higher in the stocked ponds. Primary productivity increased throughout the 28 Figure 5. Average gross primary productivity (i one standard error) in the stocked and unstocked ponds. Oxygen produced 9/ m2 / day 29 H Stocked Ponds 25 ' on". Unstocked Ponds 20- IS" IO“ 30 study in the stocked ponds; whereas productivity in the unstocked ponds declined during the last half of the experimental period. After July, the average weekly primary productivity was Significantly (p < 0.05) higher in the stocked ponds. During the last five weeks of the study, gross primary productivity averaged 17 i 1.3 g 02/m2/day in the stocked ponds and 8.3 i 1.0 g 02/m2/day in the unstocked ponds. Phytoplankton Densities The total standing crop and community composition of the phyto- plankton demonstrated marked differences between the stocked and unstocked ponds by the end of the study period. Initially, all eight ponds had relatively similar cell densities and similar species representation. The total number of phytoplankton cells remained consistently lower in the unstocked ponds while the number of algal cells increased dramatically in the stocked ponds (Figure 6). The mean total weekly cell counts were statistically (p < 0.05) higher in the stocked ponds after July. Phytoplankton densities during this period averaged 30,h86 cells/ml in the stocked ponds and 2,818 cells/ml in the unstocked ponds. Phytoplankton Composition Weekly differences in the numerical densities of the major phyto- plankton taxa between the stocked and the unstocked ponds are illustrated in Figure 7. In general the successional composition of the phyto- plankton in the stocked ponds shifted from an early summer dominance of mixed greens, to a mid-summer association of diatoms and cryptomonads 31 Figure 6. Mean total phytoplankton densities (i one standard error) in the stocked and unstocked ponds. IOO Cells/ml X IO 3 6 32 a—a Stocked Ponds o---o Unstocked Ponds “first Larval Fish 33 Figure 7. Mean numerical densities of the major phytoplankton taxa in the stocked and the unstocked ponds. Calls/ml 1: l0 2 I000 l00 l0 I00 5 I00 I Bluoaruns o-o Unlocked Ponds H Stocked Ponds 3h Fl l00 Euglanoida Cryptornonads 35 and finally to a dominance of blue-green algae in late summer. The unstocked ponds were also initially dominated by mixed green algae which progressed to a mid-summer association of diatoms and cryptomonads that continued into late summer. In contrast to the stocked ponds, no substantial development of blue—greens occurred in any of the unstocked ponds. The green algae (Chlorophyta) of the orders Volvocales and Chlorococcales dominated the phytoplankton community during the first three weeks of July; they comprised h6—86% of the total number of algae in the unstocked ponds and 60—90% in the stocked ponds. The mean total number of green algae fluctuated in a bimodal pattern with peak abundances in early July and late August in both groups of ponds. The early summer maximum was comprised mostly of Schroederia Sp. and Chlamydbmonas spp., while the main participants in the late summer pulse were Scenedesmus quadricauda, Gonium‘pectorale and Chlamydbmonas app. Although the mean total number of green algae was consistantly higher in the stocked ponds, differences between the stocked and the unstocked ponds were statistically identifiable (p < 0.05) only during the final two weeks of the study. At that time densities averaged h,092 cells/ml in stocked ponds and 366 cells/ml in the unstocked ponds. The diatoms (Bacillariophyceae) reached dominant proportions in ' mid-summer when they comprised h3-66% of the total number of all phytoplankton cells. In the unstocked ponds, the diatoms did not clearly dominate the phytoplanktonic community until the last two weeks of the study when they accounted for 50-51% of the total number of phytoplankton. Individuals of the genera Cyclotella, Stephanodiscus and Nitzschia were the major representatives of the mid-summer planktonic 36 algae in all ponds. The mean total number of diatoms in the stocked ponds (8,992 cells/ml) was significantly (p < 0.05) higher than in the unstocked ponds (867 cells/ml) after July. The cryptomonads (Cryptophyceae) exhibited similar patterns of development in both sets of experimental ponds throughout most of the study. They consistently represented about 20% of the total phyto- plankton community during mid-summer in all ponds. The cryptophycean flagellates, Rhodbmonas spp. and.Crypt0m0naS Spp. were the sole representatives of this taxa found in the ponds. The mean total number of cryptomonads in the stocked ponds (3,9h3 cells/ml) was significantly (p < 0.05) higher during the last two weeks of the study than (567 cells/ ml) in the unstocked ponds. Blue-green algae (Cyanophyta) were not found in the ponds during the first weeks of the study, but by late summer they comprised hl-80% of the total phytoplanktonic community in the stocked ponds. In the unstocked ponds, the blue—greens never exceeded 9% of the total number of the phytoplankton. The large filamentous species Anabaena fZOS-aquae and Anabaena affinis and the colonial blue-green Microcystis aeruginosa were the major representatives of this taxa in both sets of experi- mental ponds. During the last four weeks of the study, the mean total number of blue-greens (17,850 cells/ml) in the stocked ponds was significantly (p < 0.05) higher than that (130 cells/ml) in the un- stocked ponds. The dinoflagellates (Dinophyceae) and the euglenoids (Eugleno- phyceae) never exceeded 8% of the total phytoplanktonic community during the study. The dinoflagellates were represented by Ceratium hirundinella, Peridinium Sp. and Gymnodinium sp. and the euglenoids by i it I all 1 I‘ll ’ II 'l 'I [I l 1' ‘ ll. Ill III I I Ill-ll ‘ I' 1‘ 't I all I III.|I '- ‘II All? NA [Ill 37 EugZena Spp., Phacus Sp. and Trachelomonas sp. The average summer concentrations were about 171 dinoflagellates/ml and 229 euglenoids/ml. The mean weekly densities for both taxa in the stocked ponds and the reference ponds were statistically indistinct through the summer, except for a sudden pulse of euglenoids in the stocked ponds during the final two weeks of the study. Physical Conditions There was no meaningful difference in the water temperatures between the stocked and the unstocked ponds during the study. The average daily summer temperature was about 22 C in the unstocked ponds and 23 C in the stocked ponds. The maximum temperature recorded was 25 C; the average daily range was about 2 C. The percent transmission of light (water clarity) was consistently lower in the stocked ponds than in the unstocked ponds (Figure 8). During the last half of the study the stocked pond waters were signifi- cantly (p < 0.05) more turbid than the unstocked pond waters. The mean percent transmission at mid-depth (i one standard error) for the last five weeks of the study was 70.57 i 3.13 in the unstocked ponds and 32.22 i h.65 in the stocked ponds. Chemical Conditions All ponds were heavily enriched with domestic wastewater to produce relatively high nutrient levels. Based on phosphorus and nitrogen concentrations all ponds were eutrophic according to the classification of Vollenweider (1968). Of the phosphorus and nitrogen compounds 38 Figure 8. Mean percent transmission values (i one standard error) in the stocked and the unstocked ponds. Percent Transmission I00 50 39 H Stocked Ponds .--.. Unstocked Ponds hO measured, no statistically significant differences were determined between the stocked and the unstocked ponds. However, mean total phosphorus values were consistently higher in the stocked ponds with only one exception, during the fourth week of the study (Figure 9). During the study, total phosphorus (as phosphate) averaged 0.h6 i 0.0h mg/liter in the stocked ponds and 0.38 i 0.03 mg/liter in the unstocked ponds. Nitrogen values sampled at irregular intervals through the study averaged about O.h2 mg/liter nitrate nitrogen and 0.31 mg/liter ammonia nitrogen over all ponds during the study. Free carbon dioxide concentrations decreased substantially in all experimental ponds throughout the study period (Figure 9). During the last half of the study period, when blue-green algae were abundant in the stocked ponds, the free carbon dioxide values were consistently lower in the stocked ponds than in the unstocked ponds. Statistically significant (p < 0.05) differences between the two sets of experimental ponds occurred only during the final two weeks of the study, when free carbon dioxide values averaged 2h.77 t 3.57 H moles/liter in the unstocked ponds and 7.37 i 1.79 u moles/liter in the stocked ponds. The average density of blue-green algae for each experimental pond during every week of the study showed a strong negative correlation (r = -0.83) with the average concentration of free carbon dioxide in each experimental pond (Figure 10). Alkalinity and the hydrogen ion concentrations in the two sets of ponds were statistically indistinguishable. The total alkalinity (as CaC03) ranged from 232 to 1A6 mg/liter and mid-day pH values varied from 7.6 to 9.5 units. The highest pH values and the lowest alkalinities were recorded in the stocked ponds during late Summer when algal hl Figure 9. Mean total phosphorus (as POh) and free carbon dioxide values (i one standard error) in the stocked and unstocked ponds. l00 t- 2 '3 \ m 2 O E =1» IO N O o d) O h- u. a; 0.6 3 \ at 0.4 E 5’ a. 02 2.5 ,2 0.0 142 H Stocked Ponds ..-—o Unstockad Ponds 1&3 Figure 10. Relationship between the log of the average free C02 concentration and the log of the average density of blue-green algae. The straight line was fitted using the least squares method. IIII I000 l00t 5 I Average Blue-Green Cells/ml x IO2 0 Log Y- 5.78-2.36 Log X . ’ l l0 I00 Average Free 002 ,1 moles/Liter AS productivity was maximum. In general, the pH values in the stocked ponds were slightly higher than in the unstocked ponds, but well below lethal thresholds for zooplankton (O'Brien and deNoyelles, 1972). DISCUSSION The results of this study point to the potential role of plankti- vorous fish as regulators of planktonic communities in eutrophic environments. In this experiment, stocking of fathead minnows in otherwise fish-free ponds led to a significant reduction in the total number of zooplankton and the associated grazing pressures. The depression of zooplankton, in turn, allowed an increase in the total density of phytoplankton and ultimately effected a shift in algal composition from one dominated by green algae, diatoms and cryptomonads to one dominated by blue-green algae. These changes were associated with increased primary productivity and reduced water clarity. This transition resembles the frequently described succession of lakes from mesotrophic to eutrophic stages. It is unlikely that the changes observed in the stocked ponds were due to water quality alterations unrelated to stocking since this experiment was designed to minimize extraneous effects. The nutrient concentrations, exposure to overhead light, temperature and sediments were indistinguishable among ponds at the beginning of the experiment. Temperature and overhead light remained the same in all of the ponds during the experiment. Slight changes in nutrient concentrations originated from the stocking of fathead minnows alone. Several Species of invertebrate predators were also present in the ponds including: .4spZanchna 8p., Cyclops vernalis, throcyclops albidus and.Cha0b0rus Sp. as well as a few hemipterans, odonates and A6 A7 beetles. Zooplankton predators were no more abundant in the stocked ponds than in the unstocked ponds, so their effect was considered constant in all ponds. Holling (1965), Hall et al. (1970) and Kaczynski (1970) all discounted the relative impact of invertebrate predation on zooplankton populations as compared to food availability and fish predation. Fathead minnows were the dominant planktivores in the experimental ponds. The significant reduction of zooplankton populations in the stocked ponds was considered a direct effect of predation rather than the result of other environmental changes. Independent evidence from fish stomach analysis supports this conclusion. Crustacean zooplankters and rotifers were the most important summer food items for fathead minnows in the stocked ponds. Similarly, Held and Peterka (197A), Dobie et al. (1966) and Pearse (1918) all found predominating quantities of zooplankton and aquatic insects in the guts of fathead minnows. The fish in this experiment exhibited the capacity to influence the zooplankton composition of the ponds. The impact of fish predation has been investigated primarily in regard to the effect on prey diversity and size distribution. Although no previous authors have remarked on size-selection by fathead minnows, the gut contents of stocked fatheads suggest a size-selection of prey that was related to the size of the minnow. Other Species of fish are strongly food-size selective according to Hrbacek (1962), Hrbacek and Novatna-Dvorakova (1965), Brooks and Dodson (1965), and Galbraith (1967). Large fatheads recovered from the stocked ponds in this study contained relatively large zooplankters, particularly Daphnia pulex, while small minnows appeared to select smaller food items like A8 Keratella quadrata and copepod nauplii. However, it is possible that fathead minnows were simply selecting the most abundant zooplankters within a size range they could easily consume, since D. pulex and K. quadrata were the most numerous species in the ponds. Most previous investigators have shown that predation by plankti- vorous fish results in decreased proportion of large zooplankton species, probably because of size selection and a complimentary increase in the abundance of smaller zooplankters (Hrbacek, 1958; 1962; Hrbacek and Novatna—Dvorakova, 1965; Grygierek at al, 1966; Brooks and Dodson, 1965; Hall et al., 1970; Losos and Hetesa, 1973). Hall et al. (1970) found that even though fish predation increased the diversity of zooplankton, the compensatory response of the small zooplankters maintained the biomass of the original population, so that, in terms of biomass alone, there was no persistent effect from fish predation. Unlike the results of most previous studies, the density of both large and small species of zooplankton in this study was depressed by stocked fish while diversity remained relatively unchanged. However, in the sequence of events that followed stocking, the density of smaller K. quadrata at first seemed to increase and did not begin to decline until several weeks after the study began. The larger D. pulex began to decline immediately after stocking. The decline in small zooplankton corresponded to the time when large numbers of larval fish appeared in the ponds and began consuming K. quadrata in particular. The combined impact of a high initial stocking with adult fish, high adult survivorship, and a high reproductive potential with continuous recruitment produced a diverse size structure in the predator population which enabled it to decimate all size categories A9 of zooplankton relatively uniformly. In a similar study, Hurlbert et al. (1973) found that mosquito fish, Gambusia affinis, eliminated D. pulex and greatly reduced other crustacean and rotifer populations. The intensity of predation and the structure of the predator population appears to determine the zooplankton response to fish predation. At low and intermediate levels of fish predation, the abundance of large zooplankters such as Daphnia puZex, has been known to increase (Archibald, 1975). Grygierek et al. (1966) examined the effect of different stock density of carp fry and noted that the total abundance of zooplankton increased with increasing fish stock up to a critical stock density (about 22,500 fry/ha) beyond which total abundance decreased. Hall et at. (1970) concluded that predation by bluegills at low to moderately high intensities strongly depressed mean prey size and increased diversity without markedly altering the total biomass of the zooplankton. However, Hall et al. (1970) were using a fish species with discontinuous, single-cohort recruitment in a "weedy" habitat which was more Spatially complex than the ponds used in this study. Larval bluegills which feed on small zooplankters exist in the population only for two or three weeks of the year. Bluegill population size structure is relatively Simple compared to that of fathead minnows. It may be generally assumed that when planktivorous fish effect changes in zooplankton populations there will be corresponding changes in the primary producers. Although data on fiSh-zooplankton-phytoplankton interrelationships are sparse and often inconclusive, some fish-induced changes in the composition and abundance of the phytoplankton community have been reported after the introduction of zooplanktivorous fish have lull! ‘Iv |. I ‘III I I. ‘l I III I II III I 50 effected a change in zooplankton Species composition (Hrbacek 8t aZ., 1961; Brooks, 1969; Straskraba, 1965; Grygierek et aZ., 1967; Hurlbert et al., 1972; LOSOS and Hetesa, 1973). In experiments where man served as the predator, manipulation of zooplankton densities signifi- cantly altered algal standing crops and determined the relative proportions of algal species (Pennington, 19h1; Ehrlich, 196A; Porter, 1973). Most significant differences in the plankton (quantity and composition) found in stocked and unstocked ponds occurred during the latter half of the study period when larval fish were abundant. At this time, the development of the phytoplankton was directly proportional to the increase in the number of minnows and indirectly proportional to the density of herbivorous zooplankters. Among the major phyto- plankton taxa, both the diatoms and blue-green algae were consistently more dense in the stocked ponds. Similar increases in the total standing crop of phytoplankton and blue-green algae were reported in ponds following the introduction of planktivorous fish (Losos and Hetesa, 1973; Hurlbert et al., 1972). The genera of algae which increased in the stocked ponds, with the exception of the blue—green algae, are commonly found in the guts of grazing zooplankton (Mullin, 1967; Fryer, 1957; Porter, 1973). Depressed grazing pressure appears to have permitted the increased abundance of these edible Species. However, it is unlikely that the strong development of blue-greens, essentially Anabaena and Microcystis, was a direct response to lower grazing pressure. The rejection of large filamentous and gelatinous blue-greens, notably Anabaena and Microcystis (Burns, 1968), has been attributed to their large size and ‘II I I'll. I’ll. I II 51 unmanageable dimensions. There also is evidence that certain blue- green species produce substances which inhibit feeding by zooplankton. Arnold (1971) found that Anabaena fZOS-aquae and several coccoid Species reduce ingestion, assimilation, survivorship and reproduction in D. pulex. Shangenberg (1960) found that.Microcystis aeruginosa was very toxic to D. pulex. Among the numerous adaptive characteristics of blue-green algae that have been suggested to explain why blue-greens reach maximum abundance in eutrophic waters, tolerance to low carbon dioxide tension (King, 1970) appears to be the most reasonable explanation for their success in the stocked ponds. Blue-green algae may also gain competi- tive advantages because at least some Species are more tolerant of high light intensities and temperatures (Jackson, 1965), are able to regulate their bouyancy to remain in the nutrient-rich photic zone (Reynolds, 1965), can chemically inhibit the growth of other algae (Boyd, 1973), and can fix atmospheric nitrogen (Dugdale and Neess, 1961). All of the experimental ponds in this study were enough alike in all respects other than carbon dioxide concentrations to disregard these other explanations for blue-green development. During the last four weeks of the study, free carbon dioxide concentrations in the stocked ponds averaged less than 10 u moles/liter, a value which is considered limiting to most green algae but adequate for blue-greens (King, 1972). Therefore, it is proposed that depressed grazing pressure, which allowed an increase biomass and productivity and an accelerated consumptive uptake of carbon dioxide by the edible plank- tonic algae, indirectly generated the conditions (limiting CO concen- 2 trations) which favored the development of blue-green algae. 52 Evidence gathered from eutrophic ponds in this study indicate that a planktivorous fish population can severely limit zooplankton and that the relative abundance of algal Species is, in part, dependent on the resultant depression of grazing by herbivorous zooplankters. Therefore, accelerated primary productivity accompanied by succession to blue-green algal dominances may result from changing trophic relationships between primary and secondary consumers. The success of future water quality management strategies oriented toward the regulation of objectionable growths of planktonic and filamentous algae requires not only the ability to accurately access the physicochemical processes involved, but a more complete under- standing of and appreciation for biological forces which drive aquatic ecosystems. The elimination of all nutrient inputs and complete control of other abiotic factors is not usually practicable. The potential importance of biological controls is evident from this study, although further investigations are required before phytoplankton densities can be routinely manipulated by management of consumer populations. LITERATURE CITED LITERATURE CITED American Public Health Association. 1965. 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Wiley, New York. 57 Yeatman, H. C. 1959. Cyclopoida, p. 795-815. In W. T. Edmondson (ed.), Fresh-water Biology, 2nd edition. Wiley, New York. APPENDICES APPENDIX I. Species list of the phytoplankton in the experimental ponds encountered during quantitative counts. Chlorophyceae: Actinastrum Sp. Ankistrodesmus thcatuS Carteria Sp. Chlorella vulgariS Chlorella Sp. Chlamydbmonas Spp. Closterium sp. Cosmarium Spp. Golenkinia Sp. Gonium pectorale Kirchneriella Sp. Micractinium 3p . Phndbrina morum Pedfiastrum boryanum Scenedesmus quadricaudh Scenedesmus Spp. r Schroederia Sp. Selenastrum Sp. Staurastrum Sp. Tetraedron Spp. VOZvox sp. Bacillariophyceae: Achnanthes lanceolata .Amphora ovaZiS Cyclotella meneghiniana CycloteZZa steZZigera CycloteZZa Spp. Cocconeis placentula Gomphoenema Sp. Melosira granulata Melosira Spp. Navicula Spp. Nitzschia acicularis Nitzschia paZea Nitzschia Spp. RhoiSOSphenia curvata Stephanodiscus Spp. synedra ulna synedra Spp. 58 Cyanophyceae: Agmenellum Sp. Anabaena affinis Anabaena fZOS-aquae Anabaena sp. 00mphosphaeria Zacustris Microcystis'aeruginosa Cryptophyceae: Cryptomonas sp. Rhodbmonas Sp. EuglenOphyceae: EugZena Spp. Phacus Sp. Trachelomonas Sp. Dinophyceae: Ceratium hirundtnella Peridinium Sp. Gymnodinium Sp. Chrysophyceae: MhZZomonaS Sp. APPENDIX 11. Species list of the zooplankton in the experimental ponds encountered during quantitative counts. Cladocera: Alana Sp. Bosmina longirostris Ceriodaphnia reticulata Chydbrus Sphaericus Daphnia pulex Diaphanosoma leuchtenbergiana Pleuroxus denticulatus Sbapholeberis kingi Simocephalus ventulus Copepoda: Copepod nauplii Cyclops vericans rubellus Cyclops vernalis Diaptomus pallidus Eucyclops agilis Mbcrocyclops albidus Mesocyclops edox Rotifera: ASplanohna sp. Brachionus sp. Kerotella cochlearis Keratella quadrata Leeane sp. Lepadella Sp. Mbnostyla Sp. Platyius sp. synchaeta pectinata Ostracoda: Cypridopsis Sp. Physocypria Sp. 59 "IIIIIIIIIIIIIIIIIIIII