A comparison between predation effects on zooplankton communities by Neomysis and Chaoborus

Community level effects of predation by two invertebrate predators, the opossum shrimp (Neomysis intermedia), and the larva of the phantom midge (Chaoborus flavicans) were studied and compared. N. intermedia appeared abundantly in the shallow eutrophic Lake Kasumigaura and had a significant impact on the zooplankton community. The predation pressure by Neomysis was highest on cladocerans, followed by rotifers, and finally copepods. At high densities (maximum nearly 20 000 individuals m^–2), Neomysis excluded almost all cladocerans, rotifers and copepods from the lake.Zooplankton communities were established in experimental ponds, into which C. flavicans was introduced. The predator's density was around 1 individual l^–1, and was probably controled by cannibalism. Although Chaoborus larvae feed on various zooplankton species, their predation impact on zooplankton populations was markedly selective. They eliminated medium- and small-sized cladocerans and calanoid copepods from the ponds, but rotifers increased.Although the feeding selectivities of Neomysis and Chaoborus individuals were similar, the predation effects on zooplankton communities by the two predators were different.     

Effects of a storm event on the structure of the pelagic food web with special emphasis on planktonic ciliates

Storm-induced changes in the water column structure and thepelagic food web were investigated by daily sampling duringa 3 week period (October 24 to November 10, 1988) at a permanentstation in the southern Kattegat (Denmark) Subsequent to a stormthe primary production increased and the size-distribution ofthe phytoplankton changed towards larger cells. Some componentsof the zooplankton community responded to the increase in potentialfood, either in terms of a functional response (copepods) orin terms of a numerical response (cladocera), whereas no changesin the populations of mixo- and heterotrophic aliates were evident.The dilates were not food limited since growth rates determinedin situ were comparable to maximum growth rates measured inthe laboratory. Calculations suggest that the total clearancecapacity of planktonic copepods (as fraction of water columncleared per unit time) was of the same magnitude as the measuredinstantaneous growth rates of the ciliates (0.55–0 85day^–1). Ciliate populations were therefore probably limitedby copepod predation. Ciliates, however, contributed only insignificantlyto the diets of copepods. Although the total zooplankton productionincreased by at least 30% subsequent to the storm (and thatof the copepods by 50%), the zooplankton community was unableto assimilate the entire increase in primary production. Therelative zooplankton (copepods, cladoceran and ciliates) grazingrate on the primary production decreased 30%, and the flow ofcarbon to biomass accumulation, other grazers and in particularsedimentation is predicted to have increased by 100% followingthe storm.     

Zooplankton community structure driven by vertebrate and invertebrate predators

Summary A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.     

Impacts of nutrients and zooplankton on the microbial food web of an ultra-oligotrophic lake

In ultra-oligotrophic lakes and the sea, calanoid copepods arethe dominant mesozoo-plankton and cladocerans are generallysparse or absent. To determine the effects of predation andnutrient enrichment on the pelagic microbial food web of anultra-oligotrophic lake, we added copepods and cladocerans atlow biomasses (<60 µg l^–1 to in situ enclosuresin Lake Wakatipu, New Zealand, in the presence and absence ofadded nutrients (nitrogen and phosphorus). In response to nutrientfertilization, the concentrations of phototrophs >3 µmand heterotrophic bacteria increased by 50 and 15%, respectively,over 4 days, but those of cyanobacterial picoplankton decreasedby 68%. The presence of calanoid copepods (Boeckella dilatata)at ambient densities (1 and 4 l^–1) rapidly and severelysuppressed ciliate population growth over 4 days and also loweredthat of flagellates >3 µm, even when microbial growthwas enhanced by added nutrients. The presence of a small cladoceran,Ceriodaphnia dubia, at double the densities, but similar biomasses,to those of copepods, depressed the net growth rates of ciliatesand flagellates to a lesser degree. The net growth rate of heterotrophicbacteria after 4 days declined with flagellate abundance, consistentwith the possibility of regulation by flagellates. Althoughbacteria and algae increased in response to nutrient fertilization(bottom-up control), predation (top-down control) appeared toplay an important role in structuring the microbial food webof this ultra-oligotrophic lake in summer.     

Feeding behaviour of major 0 + fish species in a shallow, eutrophic lake (Tjeukemeer, The Netherlands)

The forage base and the food selectivity of 0+ representatives of six abundant freshwater fish species were studied in a shallow, eutrophic Dutch lake. Most species relied on the zooplankton; the size-selective predation in early summer was directed to the smaller copepods and in late summer to larger cladocerans and copepods than concurrently present in the lake. Daphnia spp. and cyclopoid copepods were the main zooplankton taxa for smelt, perch and pikeperch. Energetically, the large cladoceran, Leptodora kindtii, was especially important for pikeperch. Bream and roach preyed upon smaller zooplankton than the other fish species. The influence of the zooplankton predation by abundant 0+ fish was clear from a small mean Daphnia size in September; this size is to be used as an indicator in fishery management. Neomysis integer, the most important macrofauna species, was consumed by perch, pikeperch and ruffe; pikeperch was most size-selective in this respect. The 0+ ruffe was à typically benthivorous fish. Only the 0+ pikeperch became piscivorous, especially in years when smelt was abundant.     

Planktivory in the changing Lake Huron zooplankton community: Bythotrephes consumption exceeds that of Mysis and fish

1. Oligotrophic lakes are generally dominated by calanoid copepods because of their competitive advantage over cladocerans at low prey densities. Planktivory also can alter zooplankton community structure. We sought to understand the role of planktivory in driving recent changes to the zooplankton community of Lake Huron, a large oligotrophic lake on the border of Canada and the United States. We tested the hypothesis that excessive predation by fish (rainbow smelt Osmerus mordax, bloater Coregonus hoyi) and invertebrates (Mysis relicta, Bythotrephes longimanus) had driven observed declines in cladoceran and cyclopoid copepod biomass between 2002 and 2007. 2. We used a field sampling and bioenergetics modelling approach to generate estimates of daily consumption by planktivores at two 91‐m depth sites in northern Lake Huron, U.S.A., for each month, May–October 2007. Daily consumption was compared to daily zooplankton production. 3. Bythotrephes was the dominant planktivore and estimated to have eaten 78% of all zooplankton consumed. Bythotrephes consumption exceeded total zooplankton production between July and October. Mysis consumed 19% of all the zooplankton consumed and exceeded zooplankton production in October. Consumption by fish was relatively unimportant – eating only 3% of all zooplankton consumed. 4. Because Bythotrephes was so important, we explored other consumption estimation methods that predict lower Bythotrephes consumption. Under this scenario, Mysis was the most important planktivore, and Bythotrephes consumption exceeded zooplankton production only in August. 5. Our results provide no support for the hypothesis that excessive fish consumption directly contributed to the decline of cladocerans and cyclopoid copepods in Lake Huron. Rather, they highlight the importance of invertebrate planktivores in structuring zooplankton communities, especially for those foods webs that have both Bythotrephes and Mysis. Together, these species occupy the epi‐, meta‐ and hypolimnion, leaving limited refuge for zooplankton prey.     

Creating conditions for changes in prey community structure by Chaoborus spp. in a lake in Sweden

In Lake G»rdsjön (Southwest Sweden), liming as an experimental improvement of living conditions for pelagic algae, resulted in a significant increase of algal biomass and a reduction of mean cell size. The algal development was beneficial for small sized filter feeding zooplankton, particularly rotifers, which showed a significant increase. The increase in abundance of small sized zooplankton created better food conditions for the smaller instars, and thus a much better overall survival of Chaoborus larvae. The resulting, 6–7 times larger population of Chaoborus larvae significantly changed the structure of the crustacean zooplankton community. Bosmina coregoni, the fastest swimmer of the crustacean species suffered most and was strongly reduced by the increased predation from Chaoborus. The share of cladocerans decreased, while copepods increased in importance.     

Food-web manipulations influence grazer control of phytoplankton growth rates in Lake Michigan

Stocking piscivorous salmonids in Lake Michigan produced dramaticalterations in food-web structure, including higher numbersof large-bodied zooplankton (especially Daphnia pulicaria),lower summer chlorophyll concentrations and increased watertransparency. Experimental determinations of epilimnetic phytoplanktongrowth rates and of zooplankton grazing rates indicate thatherbivorous zooplankton controlled algal dynamics during thesummer of 1983 because grazers occupied the surface waters throughoutthe day. In 1985, however, both large- and small-bodied Daphniamade approximately equal contributions to total grazer biomass,and all grazers displayed pronounced diel vertical migrations,visiting epilimnetic waters only at night. This prohibited zooplanktonfrom controlling algal dynamics because grazing losses did notexceed phytoplankton growth rates. The changes in zooplanktoncommunity composition and behavior observed in summer 1985 probablyresulted from increased predation by visually orienting planktivorousfish, especially bloater chub (Coregonus hoyi). Effects of food-webmanipulations on phytoplankton dynamics were evident only duringJuly and August. During spring and early summer copepods dominateLake Michigan's zooplankton community. Owing to their smallbody size, copepods are less susceptible to fish predation andexhibit much lower filtering rates than Daphnia. Variabilityin zooplanktivorous fish abundance probably has little effecton phytoplankton dynamics during spring and early summer.     

Toxic and inhibitory effects of the blue-green alga Microcystis aeruginosa on herbivorous zooplankton

Blooms of blue-green algae are often associated with declinesin populations of large-bodied cladocerans and increased importanceof small cladocerans, copepods, and rotifers. We conducted toxicityand herbivory experiments, using a wide range of herbivore taxa,to test the hypothesis that the blue-green alga Microcystisaeruginosa most strongly inhibits large cladocerans. For a varietyof herbivore taxa, M. aeruginosa was toxic or non-nutritious,and inhibited feeding on co-occurring nutritious food. The rotiferBrachionus calyciflorus was unique in several respects: it wasunaffected by M. aeruginosa toxins, it showed some ability togrow and reproduce on a diet of M. aeruginosa, and it maintainedhigh feeding rates on co-occurring nutritious food in the presenceof bloom densities of M. aeruginosa. There was a strong relationbetween the toxicity of M. aeruginosa and its inhibitory effecton herbivore feeding rates. Copepods strongly avoided consumingM. aeruginosa, but all cladocerans and rotifers tested filteredunicellular M. aeruginosa at rates similar to or higher thannutritious Chlamydomonas reinhardi. Our results indicate thatthere are a variety of mechanisms whereby herbivorous zooplanktoncan coexist with blooms of M. aeruginosa, including resistanceto toxic chemicals (B. calyciflorus), and avoidance of consumptionof M. aeruginosa by chemosensory means (copepods), or by theinability to consume large colonies (some small cladocerans). ¹Present Address: Department of Biology, George Mason University,4400 University Drive, Fairfax, VA 22030, USA     

Copepod and cladoceran success in an oligotrophic lake

Cladocerans have proved to be much less successful members ofthe Lake Tahoe limnetic community than the calanoid copepods.An analysis of temporal and spatial heterogeneity of the cladoceranBosmina longirostris and the herbivorous calanoid copepod Diaptomustyrrelli revealed important differences between the two typesof zooplankton. Although the productivity of both species islimited to varying degree by the abundance of certain algalspecies and the availability of particulate nitrogen, they differstrongly in the mechanisms responsible for mortality. Correlationanalysis suggests that B. longirosris is severely limited temporallyand spatially by predation from the omnivorous calanoid copepodEpischura nevadensis. In contrast, D. tyrrelli is not noticeablylimited by predators. Instead, abiotic seasonal events and starvationcontrol its population dynamics. Mortality and natality differencesbetween Bosmina and Diaptomus in Lake Tahoe help to explainthe general lack of success of cladocerans in oligotrophic lakes.     

Mesozooplankton Grazing on Picocyanobacteria in the Baltic Sea as Inferred from Molecular Diet Analysis

Our current knowledge on the microbial component of zooplankton diet is limited, and it is generally assumed that bacteria-sized prey is not directly consumed by most mesozooplankton grazers in the marine food webs. We questioned this assumption and conducted field and laboratory studies to examine picocyanobacteria contribution to the diets of Baltic Sea zooplankton, including copepods. First, qPCR targeting ITS-1 rDNA sequence of the picocyanobacteria Synechococcus spp. was used to examine picocyanobacterial DNA occurrence in the guts of Baltic zooplankton (copepods, cladocerans and rotifers). All field-collected zooplankton were found to consume picocyanobacteria in substantial quantities. In terms of Synechococcus quantity, the individual gut content was highest in cladocerans, whereas biomass-specific gut content was highest in rotifers and copepod nauplii. Moreover, the gut content in copepods was positively related to the picocyanobacteria abundance and negatively to the total phytoplankton abundance in the water column at the time of sampling. This indicates that increased availability of picocyanobacteria resulted in the increased intake of this prey and that copepods may rely more on picoplankton when food in the preferred size range declines. Second, a feeding experiments with a laboratory reared copepod Acartia tonsa fed a mixture of the picocyanobacterium Synechococcus bacillaris and microalga Rhodomonas salina confirmed that copepods ingested Synechococcus, even when the alternative food was plentiful. Finally, palatability of the picocyanobacteria for A. tonsa was demonstrated using uptake of ¹³C by the copepods as a proxy for carbon uptake in feeding experiment with ¹³C-labeled S. bacillaris. These findings suggest that, if abundant, picoplankton may become an important component of mesozooplankton diet, which needs to be accounted for in food web models and productivity assessments.     

Drivers assessment of zooplankton grazing on phytoplankton under different scenarios of fish predation and turbidity in an in situ mesocosm experiment

Zooplankton play a key role in energy transfer within lake food webs, but we have a poor knowledge concerning their role as phytoplankton grazers in shallow subtropical lakes. In this study, we aimed to determine how zooplankton grazing upon phytoplankton is altered in different scenarios of fish predation and turbidity, and we explored the relevance of grazing compared to other environmental variables, to explain phytoplankton biomass changes. A mesocosm experiment was conducted by including the following treatments: fish, turbidity, fish + turbidity, and a control (without fish or varying turbidity). The experiment lasted 21 days, and samples were taken four times. Zooplankton grazing was only effective for the microphagous group upon Cryptophyceae, while large Chlorophyceae and small pennate Bacillariophyceae biomass were benefited in the presence of copepods and cladocerans, being negatively affected by depletions in nitrogen availability. In the turbidity treatment, a reduction in phytoplankton biomass was obtained, artificially increasing zooplankton grazing on phytoplankton, while fish presence inhibited grazing of adult copepods and cladocerans. The other groups of phytoplankton were only influenced by the environment. This experiment suggests that phytoplankton biomass variations would be more affected by the environment than by zooplankton grazing in shallow lakes from the Paraná River.

     

Zooplankton behavioral responses to solar UV radiation vary within and among lakes

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation(UVR), with rotifers and copepods tending to be more UV tolerantthan cladocerans, regardless of lake UV transparency. On thebasis of these differences, we hypothesized that UV avoidancebehavior would be greatest in zooplankton with low UV tolerance,particularly in high-UV systems. Both downward and upward movementsof zooplankton were examined in cylindrical acrylic columnsthat either transmitted full sunlight or blocked UV-B and shortwavelength UV-A radiation. Columns were suspended verticallyin the surface waters of a high- and low-UV lake at varyingtimes of day and cloud cover. In the high-UV lake, the cladoceranDaphnia catawba displayed a distinct avoidance of the surfacewaters in the UV+ treatment but often showed a preference forthe surface in the UV– treatment. Copepods were less responsiveto UVR with calanoids displaying a somewhat greater UV avoidancethan cyclopoids. In the low-UV lake, no behavioral differenceswere observed among UV treatments. These results suggest thatUVR may influence the vertical distribution and habitat partitioningof certain zooplankton taxa in high-UV lakes while other factors,such as predation and food limitation, may be more importantin low-UV lakes.     

An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)

1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire. 2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods. 3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments. 4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed. 5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.     

Zooplankton composition and distribution off the coast of Galicia, Spain

During June and September 1984, zooplankton samples were collectedwith other hydrographic and biological data along the Galiciancoast (NW of Spain). In June copepods contributed {small tilde}60%to the total zooplankton community, with larvaceans, siphonophoresand cladocerans also abundant. In September >90% of the zooplanktonsampled were copepods. The dominant species of copepods in bothJune and September were Acartia clausi, Paracalanus parvus andTemora longicornis. The meroplankton was dominated by echinoderms,bryozoans, barnacle larvae and bivalve larvae. In June the averagezooplankton biomass was 31.08 mg C m^–3; the Septemberaverage was 41.69 mg C m^–3. The relationship between theslopes of the regression equations (biomass versus abundance)suggests that the zooplankton assemblage in June was composedby larger animals than in September. The major concentrationof zooplankton was between 0 and 50 m, with both June and Septemberdaytime surface samples having 6–7 times the amount oforganisms than the lower water column (50–100 m). Therewere no distinct differences in total zooplankton abundancesat the inshore and offshore stations; however, the inshore stationsoften had a higher percentage of meroplankton than the offshorestations. In June zooplankton abundance at the northern transectsand the western transects was similar. In September there weregreater concentrations of zooplankton in the western Galicianshelf as compared with the northern shelf. These differencesin the horizontal distribution of the zooplankton were relatedto upwelling events.     

Trophic interactions of the freshwater jellyfish Craspedacusta sowerbii

The predatory impact and the trophic role of the freshwaterjellyfish, Craspedacusta sowerbii, was studied using microcosmand enclosure experiments as well as a 3-year pond survey. Theresults showed a significant decrease of small herbivorous crustaceans,i.e. Bosmina longirostris and juvenile cyclopoid copepods, inthe medusa treatments of the microcosms and the enclosure experiments.Chlorophyll concentrations in the enclosure experiment weresignificantly increased in the medusa treatment, suggestingthat C. sowerbii may cause cascading effects in the food chain.A comparison of daily zooplankton losses during the pond surveycaused by medusae and fish (roach, Rutilus rutilus), and theirfood selectivities suggest food separation of these two predatorsand reveal a strong negative impact of medusae on the copepodpond community. In the case of a jellyfish bloom, our resultsshow that both food chains can co-occur in lakes because ofa weak interaction between these top predators, fish and jellyfish,with simultaneous impacts on the zooplankton structure.     

Long-term Annual and Seasonal Changes of Meta- and Protozooplankton in Lake Müggelsee (Berlin): Effects of Eutrophication,Grazing Activities,and the Impact of Predation

Annual changes of rotifers, copepods, cladocerans, the ciliate Epistylis rotans, and larvae of Dreissena polymorpha were analysed for the period 1908–1990. Though food resources increased 6–10 fold in the course of eutrophication, only rotifers and Epistylis increased accordingly. Probably as a result of increased predation pressure crustaceans increased only twice. The seasonal pattern of metazoans and protozoans (flagellates, sarcodines, ciliates) were analysed for 12 and 3 years, resp. During winter and spring, large heterotrophic flagellates and ciliates dominated the zooplankton and were responsible for a pronounced - formerly underestimated - grazing pressure on phytoplankton. In early summer, metazoan filter-feeders were often able to cause a significant reduction of phyto- and protozooplankton. However, during some years, phytoplankton declined in the absence of a pronounced grazing pressure. Field data and experiments revealed that predators were able to regulate the density of cladocerans in early summer (mainly cyclopoids) and summer (mainly Leptodora, smelt and fish juveniles).     

The life cycle dynamics and production of zooplankton in Øvre Heimdalsvatn

The zooplankton in Øvre Heimdalsvatn was studied from May 1969 to Aug 1973 in order to obtain knowledge about their functioning and role in the lake ecosystem. The paper concentrates on the main species: Holopedium gibberum, Bosmina longispina, Cyclops scutifer. Heterocope saliens, Conochilus unicornis, Polyarthra vulgaris and Kellicottia longispina but Megacyclops gigus, Daphnia longispina and ciliated cells are also considered. The development of the populations was fairly synchronized and made it possible to determine life cycles, development times, growth, reproduction and production from field data. The total production of the multicellular zooplankton was 2–3 mg dry wt m⁻² yr⁻¹. The cladocerans dominated and their share of the total was 84–92% compared to the copepods 5–8% and the rotifers 3–8%. The ciliate production was not estimated, but their biomass exceeded that of the copepods and rotifers. The spring spate, June temperatures, food availability and invertebrate predation were found to be the most important regulatory factors for life cycle dynamics and production. Allochthonous organic matter was found to be a necessary food resource in addition to algae.     

Composition and seasonality of nocturnal peracarid zooplankton from coastal New Hampshire (USA) waters, 1978-1980

Seasonal changes in the nocturnal peracarid zooplankton froma nearshore area of the Gulf of Maine were studied from January1978 through December 1980 as a requirement of an ecologicalmonitoring program for the Public Service Company of New Hampshire'sSeabrook Generating Stanon. Numerically dominant species includedthe mysids Michteimysis mixta and Neomysis americana. the amphipodsPontogeneia inermis, Themisto gaudichaudi and Monoculodes edwardsi.and the cumaceans Diastylis polita and Lamprops quadriplicata.A general seasonal cycle was documented with a more abundantand speciose assemblage during fall and winter than summer.Peak abundance occurred during spring months prior to emigrationof juvenile M.mixta from the study area. Abiotic and bioticfactors which may affect the structure and composition of thisassemblage are discussed.     

The predator-prey relationship of perch,Perca fluviatilis,larvae and zooplankton in two Scottish lochs

Synopsis Food consumption of perch larvae and the impact of this on zooplankton were examined in two adjacent shallow Scottish lochs. Maximum annual abundance of zooplankton occurred in mid-May at L. Kinord with minimum values in mid-June. Copepods were prominent in spring but were followed by a multi-species community of cladocerans and rotifers in summer. At L. Davan zooplankton biomass remained high through summer with cladocerans dominating andDaphnia longispina the most frequent species. Availability of food items was a principal factor governing feeding behaviour of larvae. Copepodite stages were initially the most common item in the diet in L. Kinord in 1976 and 1977 and rotifers the principal food in June 1977, reflecting the dominance of these items in the zooplankton. Cladocerans were dominant in the plankton community in L. Davan and constituted the greater part of food intake. Overlying this general pattern there was an increase in the size of food items taken by larvae with time and also a definite pattern of food selection for copepods, with initially selection for smaller copepodite stages and later for larger stages and adults. On most occasions larvae selected forCyclops strenuus abyssorum andPolphemus pediculus and selected againstDaphnia longispina. The reduction in the total zooplankton biomass attributed to perch larvae was minimal, with the exception of mid-June at L. Kinord in 1976. However, predation on particular species and copepodite stages was occasionally intense and may have impacted the zooplankton populations.