Diel vertical migration in zooplankton: rapid individual response to predators

While diel vertical migration in zooplankton has been shownrecently to be a predator avoidance behavior, the mechanismby which predators induce and maintain such behavior has beendebated. We report results of an in situ predator manipulationexperiment during which enclosed populations of the marine planktomccopepod Acaraa hudsonica rapidly changed their vertical distributionand diel migration behavior depending on presence or absenceof the planktivorous fish Casterosteus aculeatus These resultspoint unambiguously to phenotypic behavioral plasticity of individualplanktonic prey, not, as previously hypothesized, population-geneticlevel behavioral changes caused by selective fish predation,as the mechanism underlying changes in diel vertical migrationin this copepod.     

Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied,focusing on fish larvae and their copepod prey. The major objectiveswere to describe distributional overlap and evaluate the predatoryimpact by larval fish. The study was carried out across DoggerBank in the North Sea, August-September 1991. Sampling transectscrossed tidal fronts off the Bank and plankton at all trophiclevels showed peak abundance within frontal zones. Also Verticallythere was a significant overlap in distributional patterns ofthe plankton. Seven species of fish larvae were abundant, ofthese sprat (Sprattus sprattus) dominated. The abundance ofone group of fish larvae peaked in the shallow water close tothe Bank, whereas other species, including sprat, were foundin deeper water. Prey preference and predation pressure of fishlarvae were assessed using information on prey sizes and growthrates of larvae and the copepod prey. We estimated larval removalof preferred prey sizes to 3–4% day^–1, counterbalancedby a 3–7% day^–1' replenishment from copepod productionand growth. Additional predation pressure on copepods by aninvertebrate predator was estimated to 1–3%day^–1.In conclusion, the dynamics of fish larvae and other zooplankterswere closely linked. At peak abundances of fish larvae (>35mg dry weight m^–2), the accumulated predation on specificsize ranges of copepods, exerted by larvae and other predators,could exceed the ability of copepod replenishment and intra-/interspecificcompetition among predators might take place.     

Coexistence of two similar copepod species, <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and <Emphasis Type="Italic">E. graciloides</Emphasis>: the role of differential predator avoidance

Habitat choice in relation to environmental factors of two coexisting calanoid copepod species, Eudiaptomus gracilis and E. graciloides, was studied in a mesotrophic lake and in large indoor mesocosms. Both species and sexes showed pronounced diel vertical migration (DVM) in the field. In 12 m deep mesocosms with free ranging fish DVM was observed and species increased day depth over time. No changes were observed in copepod day depth over time in experiments with fish kairomone. It is hypothesized that fish kairomone acts as an early warning system to copepods which respond by moving deeper, but only as far as the thermocline. For full DVM, a nearby mechanical stimulus is necessary. Thus, as fish go deeper to feed, copepods retreat. The response of copepods to fish predation, in the presence of low and high numbers of Daphnia,shows that copepods effectively use Daphniaas living shields to avoid predation. The two species adopt different vertical migration strategies depending on whether there are high or low numbers of Daphniapresent. A dominant feature of mesocosm experiments was the night time aggregating (lekking) of E. gracilis males at the surface. When the spring and autumn percentages of risk takers in the epilimnion were compared, E. gracilis, particularly males, suffered the greatest cost.     

Copepod reaction to odor stimuli influenced by cestode infection

The cestode Schistocephalus solidus uses copepods as first andsticklebacks as second intermediate hosts. For transmission,an infected copepod has to be preyed upon by a stickleback.We used copepods of the species Macrocy albidus to test whetherinfected and uninfected copepods differ in their reaction totwo kind of simultaneously presented odors: odors of sticklebacksand odors of sticklebacks and conspecificz. By giving this choice,we attempted to force the copepods to make a trade-off betweenthe benefit of risk dilution and possible predator confusionand the costs of food competition and other disadvantages inducedby conspecifics. Within 1–8 h after last feeding, uninfectedcopepods clearly preferred the odors of conspeciflcs under thechemically simulated threat of predation. This was in contrastto the infected copepods, who tended to avoid the odor of conspecifics.When the time between experiment and last feeding varied, infectedcopepods showed an increas preference for fish water only (oravoided conspecthcs) with increasing hunger level This suggeststhat S. solidus benefits from hunger-induced behavioral changesof its copepod host by influencing its microhabitat selection.The same effect could be found in both sexes; however, it wassignificantly more pronounced in male than in female copepods.We propose several hypotheses that could explain the differencebetween the sexes in their infection-dependent microhabitatselection.     

Salinity tolerance of the gemmules of Eunapius fragilis (Leidy) and the inhibition of germination by various salts

We report results of a field test of the predator avoidance hypothesis as an explanation of the adaptive significance of diel vertical migration in zooplankton. We determined the vertical distribution and diel migration of the planktonic copepod Acartia hudsonica, concurrently with the abundance of pelagic fish, transparency and thermal stratification of the water column, on six cruises over a one year period in a temperate marine lagoon (Jakles Lagoon, San Juan Island, Washington, USA). Striking seasonal variability was observed in all biological and environmental variables. Linear regressions of the strength of diel vertical migration in A. hudsonica on these environmental variables resulted in only one statistically significant relationship, that between copepod diel vertical migration and predator abundance. These results, together with those of previous studies, point to diel vertical migration as a widespread behavioral response of planktonic prey to the presence of their predators.     

Effect of prey size on vulnerability of copepods to predation by the scyphomedusae Aurelia aurita and Cyanea sp.

A growing number of studies correlate changes in zooplanktonpopulations with abundance of medusae, but we cannot yet explainor predict the specific factors driving these interactions.This study demonstrates that the size of copepods has a significantinfluence on their vulnerability to predation by scyphomedusae.This finding is important because prey size, independent ofbehavior, has been neglected in theoretical models of predationby medusae. In experiments in a planktonkreisel, we used liveand heat-killed prey (Acartia hudsonica adults and copepodites)to separate the effects of copepod size and behavior on feedingrates by two medusae (Aurelia aurita and Cyanea sp.). Resultsrevealed that: differences in copepod size had a significantimpact on feeding rates, and thus small size can provide a refugefrom predation; behavior of adults diminished the liabilityassociated with larger size; and medusae with different morphologiesingested A.hudsonica at similar rates. Other experiments demonstratedthat medusae digested copepods at different rates based on preysize and predator species, findings that have implications forall future laboratory and field studies that assess feedingby scyphomedusae. Finally, this study illustrates how laboratorystudies serve as critical supplements to field observations.The effect of prey size on feeding rates can be confounded bydifferences in prey behavior, yet explains why small copepodswere typically ingested at relatively low rates by medusae.Size was clearly a dominant factor influencing copepod vulnerabilityacross scyphomedusan species, even those with very differentmorphologies. Future work should focus on the mechanisms ofsize selection, or the factors influencing contact and retentionrates.     

Is mating of copepods associated with increased risk of predation?

In laboratory experiments, we tested whether the cyclopoid copepodCyclops vicinus is more vulnerable to predation by juvenile(young-of-the-year) roach (Rutilus rutilus) and/or larvae ofthe phantom midge Chaoborus flavicans during mating than individualadult copepods of the same species. We also tested whether C.vicinusavoided mating, displayed shorter mating times and/or loweractivity when a predator (Chaoborus) was present, or when exposedto water in which predators (Chaoborus) were previously held.Fish ate the same quantity of pairs in copulation and individualegg-bearing females but significantly fewer females withouteggs. Chaoborus ate similar quantities of pairs and femalesbut fewer males. Pairs in copulation and egg-bearing femaleswere detected earlier by fish than individual non-ovigerousfemales or males, and pairs in copulation were captured mosteasily by fish. Cyclops vicinus mated less often when Chaoboruslarvae were present than in water in which Chaoborus was previouslyheld, or in pure tap water. However, predator presence or exposureto predator-treated water had no effect on (i.e. did not reduce)mating duration. Activity of C.vicinus pairs during copulationwas significantly lower when a predator was present, or in predator-treatedwater, than in pure tap water, and activity of pairs decreasedwith increasing activity of the predator. Although our experimentswere carried out under artificial conditions which cannot betransferred to the complex conditions in the field, they suggestthat mating in copepods is dangerous and that copepods haveevolved particular strategies to reduce the risk of predationduring mating.     

The Role of the Predaceous Copepod Parabroteas Sarsi in the Pelagic Food Web of a Large Deep Andean Lake

Parabroteas sarsi is a predaceous calanoid copepod that inhabits both shallow temporary fishless ponds and deep fish lakes of Patagonia and Antarctica. The aim of this study was to analyse the effect of P. sarsi on the plankton structure of a deep Andean lake (>100 m depth) and the zooplankton vertical distribution in order to asses a possible vertical refuge of the predatory copepod against visual fish predation. We tested the extent to which the trophic cascade effect of this predator propagates through the food web. We carried out a vertical sampling in Lake Rivadavia (Patagonia, Argentina) in order to assess zooplankton distribution. P. sarsi showed a vertical distribution towards deeper layers of the water column both at midday and at night, indicating that the copepod had an effective refuge against visual predation. Additionally, we carried out both field and laboratory experiments with the presence of P. sarsi. The predator was observed to affect significantly the survival of the copepod Boeckella michaelseni both in laboratory and field experiments. On the contrary, rotifers and adults of Daphnia cf. commutata were not substantially affected by the predator. B. michaelseni mouthparts revealed an omnivorous diet; therefore a broad phytoplanktonic size spectrum could be affected by this copepod. However, no cascade effect was observed due to the presence of P. sarsi despite the decrease of B. michaelseni abundance.     

Factors affecting feeding selectivity of visual predators on the copepod Acartia tonsa: locomotion,visibility and escape responses

Visual predation by fish on copepods involves prey encounter, attack and capture; during any of these processes prey selection can occur. Developmental changes in copepods, including increases in swimming speed, size and image contrast increase the encounter rate and distance at which they can be detected by predators. Copepods compensate for this increase vulnerability with age through diel vertical migration and improved escape capabilities. This study quantifies the changes in swimming speed and movement pattern with developmental stage of the copepod Acartia tonsa, using a video-computer system for motion analysis. Changes in visible size and image contrast with developmental stage were quantified under simulated natural illumination conditions using a video based image analysis system. The escape responses of the naupliar stages of the copepod Acartia tonsa were quantified in response to a stationary pipette sucking in water at a constant speed. Accurate quantification of the parameters that affect feeding selectivity of planktivorous fish will provide the basis for evaluation of their relative importance in future studies.     

Population regulation and role of mesozooplankton in shaping marine pelagic food webs

Copepods constitute the majority of the mesozooplankton in the oceans.By eating and being eaten copepods have implications for the flow of matterand energy in the pelagic environment. I first consider populationregulation mechanisms in copepods by briefly reviewing estimates of growthand mortality rates and evidence of predation and resource limitation. Theeffects of variations in fecundity and mortality rates for the demography ofcopepod populations are then examined by a simple model, which demonstratesthat population growth rates are much more sensitive to variations inmortality than to variations in fecundity. This is consistent with theobserved tremendous variation in copepod fecundity rates, relatively low andconstant mortality rates and with morphological and behavioralcharacteristics of pelagic copepods (e.g., predator perception and escapecapability, vertical migration), which can all be considered adaptations topredator avoidance. The prey populations of copepods, mainly protozoa(ciliates) and phytoplankton, may be influenced by copepod predation tovarying degrees. The highly variable morphology and the population dynamics(e.g., bloom formation) of the most important phytoplankton prey populations(diatoms, dinoflagellates) suggest that predation plays a secondary role incontrolling their dynamics; availability of light and nutrients as well ascoagulation and sedimentation appear generally to be more important. Thelimited morphological variation of planktonic ciliates, the well developedpredator perception and escape capability of some species, and the oftenresource-unlimited in situ growth rates of ciliates, on the other hand,suggest that copepod predation is important for the dynamics of theirpopulations. I finally examine the implications of mesozooplankton activityfor plankton food webs, particularly their role in retarding vertical fluxesand, thus, the loss of material from the euphotic zone. This revised version was published online in July 2006 with corrections to the Cover Date.     

How plankton copepods avoid fish predation: From individual responses to variations of the life cycle

Analysis of original and published data on predator avoidance by marine and freshwater plankton copepods, a major diet of many young fishes, suggests that individual defense mechanisms play a minor role in copepod anti-predator behavior. Capture success by planktivorous fish depends largely on prey visibility and the ability of the prey to escape. Copepods have almost no chance to avoid relatively large fish when encountered, but they can be ranked according to their ability to escape from larvae and fries. In contrast to small pelagic fish, which are also under heavy predation pressure, copepods rely more on prevention of the threat of predation than on active attempts to escape. Seeking a refuge in habitats non-accessible to predators would be more effective for these small and rather slow animals. Retreat into such refuges is accomplished by vertical and horizontal migrations, either diel, seasonal or ontogenetic. A decrease in activity (feeding, metabolism, reproduction, movement), resulting in diapause in deep water layers in its most pronounced version, is the ultimate attempt by copepods to separate from their predators, both in time and in space.     

Chemical, mechanical and visual cues in the vertical migration behavior of the marine planktonic copepod Acartia hudsonica

Recent experimental evidence in both marine and freshwater systemsindicates that predators can induce vertical migration behaviorin individual zooplankters, yet the specific cues by which zooplankterssense their predators appear to vary. In situ manipulation experimentswere carried out with enclosed populations of the marine planktoniccopepod Acartia hudsonica to re-examine the potential role ofchemical cues in the behavior of A.hudsonica, and to test explicitlyfor the role of mechanical or visual stimuli in triggering verticalmigration behavior in this species. Adult female copepods wereinduced to vertically migrate (descend) when exposed to fishmimics during the day, but no such response occurred when thecopepods were exposed to Fish mimics during the night. Moreover,copepods exhibited no changes in vertical distribution whenexposed to water which, having recently held a natural predator(the threespine stickleback, Gasterosteus aculeatus), was presumedto be laden with predator-produced chemical exudates. Predator-mediatedmechanical or visual cues, or a hierarchy of both, are responsiblefor eliciting vertical migration behavior in adult female A.hudsonica.These results, together with those of other investigations demonstratingthe inducing role of chemical exudates, indicate that the stimulieliciting vertical migration in zooplankton can be expectedto vary between species.     

Morphological responses by Bosmina longirostris and Eubosmina tubicen to changes in copepod predator populations during a whole-lake acidification experiment

Changes in zooplankton populations during the experimental acidilicationof Little Rock Lake provided an opportunity to examine specificmechanisms underlying the morphological responses of bosminidsto changing predation pressure. Two large copepods, Epischuralacustris and Mesocyclops edax, disappeared from the lake'sacidified basin in 1986 and 1989, respectively, while a smallercopepod predator, Tropocyclops extensus, increased during laterstages of acidification. The two bosminid species showed distinctlydifferent responses coinciding with the changes in copepod predation.Bosmina longirostris exhibited a significant decrease in mucrolength with the decline of M.edax and E.lacustris. Its meanbody and antennule length, however, did not change. We suggestthat the decoupling of B.longirostris mucro length and antennulelength may have been related to the persistence of the smallercopepod predator, T.extensus. Eubosmina tubicen showed no apparentresponse to declines in M.edax and E.lacustris abundance ineither mean mucro, antennule or body length. Allometric analysesindicated, however, that mucro length was related to size-dependentcopepod predation for both B.longirostris and E.tubicen.     

Across-shelf predatory effect of Pleurobrachia bachei (Ctenophora) on the small-copepod community in the coastal upwelling zone off northern Chile (23{degrees} S)

To estimate the predation effect of the predominant ctenophorePleurobrachia bachei on the small-copepod community in the upwellingarea off Mejillones (23°S), northern Chile, a series ofoceanographic cruises and predation experiments were conductedin the austral springs 2000, 2001 and 2002. The daily consumptionrates and predatory effect of P. bachei on the small copepods(in terms of % of standing stock and biomass removed daily)were determined at three stations located in relation to theshelf-break (coastal, shelf-break and oceanic) reaching valuesup to 4.5% per day of the <1500 µm copepod standingstock. Our results indicate that the ctenophores were most abundantat the coastal station, that small copepods dominated the copepodcommunity (being more abundant nearshore), and that the relativefrequency of ctenophores with copepods in their guts was alsohigher near the coast. The predatory effect of P. bachei onthe small-copepod community was also higher in the coastal zone.However, the effect of this predation on the copepod biomassin terms of carbon did not decrease steadily seawards, whichmay be due to the larger sized copepods consumed at the offshorestations. Determinations of predatory effect on the secondaryproduction of the more abundant small-copepod populations (i.e26% daily in 2000) suggest that this single species of Pleurobrachiais modulating the population growth rate of the small copepods,the copepod community size structure, and maybe even the alternanceof key species in the Mejillones coastal upwelling zone.     

Environmental factors influencing diurnal distribution of zooplankton and ichthyoplankton

The diurnal vertical distribution of a large number of speciesof zooplankton, icbthyoplankton and micronekton were determinedin the top 150 m in three locations in the Shelf Water, on theNova Scotia Shelf, and Slope and on Georges Bank during springand fall periods. Species were categorized as to their trophiclevel and their type of diurnal migration behaviour. The influenceof temperature, salinity, and water density on the diurnal verticaldistribution of the species was examined. Temperature was foundto have the greatest influence on the distribution of the largestnumber of species. Diurnal migration behavior of the same speciesin Shelf and Slope water and at different times of the yearwas examined. Results showed that species changed their behaviorin the two water masses, while some species changed their migrationbehavior at different times of the year. During the night inApril the most abundant copepod species, Calanus finmarchicus,making up about 80% of the biomass, was found concentrated abovethe thermocline and the main chlorophyll layer. The majorityof the less abundant species of copepods were found below thethermocline and the chlorophyll layer. At night in August thetwo most abundant copepod species, Centropoger typicus and Paracalanusparvus, making up at least 80% of the zooplankton biomass, werealso concentrated above the thermocline and the main chlorophyllLayer. Three species of copepods were concentrated at the depthof the main chlorophyll layer and two species were concentratedbelow the chlorophyll layer and thermocline. The vertical distributionof other zooplankton and ichthyoplankton species was examinedin relation to the thermocline and chlorophyll layer. Relationshipsbetween concentrations of six species of fish larvae and allspecies of copepods in the same samples showed a general increasein the numbers of larvae m^–3 as the numbers of copepodsm^–3 increased in a range of 500–4000 m^–3.However, the concentration of Merluccius bilinearis decreasedas the concentration of copepods exceeded 4000 m^–3 suggestingthat high concentrations of copepods may not be a favourableenvironment for the larvae.     

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.     

Seasonal variations in copepod size: effects of temperature, food abundance, and vertebrate predation

Seasonal dry weights of female and male Cyclops bicuspidatusthomasi, Diaptomus ashlandi and Diaptomus minutus were studiedin southeastern Lake Michigan during 1975–1981. Smallestanimals occurred during summer and early fall, and largest animalsin winter and spring, a pattern reported for other copepods.The range of weights for the species and sexes decreased fromwinter to summer, and converged in summer to a value of approximately2 µg/animal. Surface water temperature and abundancesof young-of-the-year (YOY) fish were inversely correlated withweights of males and females of the three copepod species. Standingstocks of important phytoplankton groups, especially pennatediatoms and flagellates, were positively correlated with copepodweight. Seasonal change of copepod body size appears to be morethan simply a function of temperature; seasonal predation byYOY fish may be a factor influencing sizes of adult copepods.     

Zooplankton diversity and the predatory impact by larval and small juvenile fish at the Fisher Banks in the North Sea

The biomass and diversity of the mesozooplankton and fish larvaecommunity were investigated across a frontal zone in the centralNorth Sea in the early summer, to investigate whether larvalfish predation is a regulator of mesozooplankton production.Pronounced changes in the mesozooplankton community were observedacross the front off the Jutlandic coast. The biomass and thediversity of copepods changed across the front as the populationof Calanus finmarchicus became abundant in the deeper water.The crustaceans (Acartia spp. and Evadne spp.) and polychaetelarvae dominated the mesozooplankton in the coastal water. Thebiomass of fish larvae was dominated by gadoid larvae. As inthe copepods, a shift in fish diversity was observed in thefrontal zone. On the coastal side of the front, whiting (Merlangiusmerlangus) dominated the biomass, while offshore from the frontwhiting were absent and cod (Gadus morhua) was the dominantlarval fish species on the deeper stations. The present investigationdemonstrates two different trophic pathways related to hydrographyin the central North Sea. First, in the shallow coastal water,the abundant small neritic copepods are predominately predatedby whiting, while in the offshore region the larger oceaniccopepods are predated by cod larvae. However, the predationpressure by the fish larvae was in general low (<10%) relativeto copepod production per day. Consequently, in the early summer,the copepod production potentially results in a build-up ofcopepod biomass on both sides of the front.     

Selective predation by herring and mysids, and zooplankton community structure in a Baltic Sea coastal area

In lakes, fish and invertebrate predation are recognized asstrong structuring forces on zooplankton communities. The objectof this investigation was to study whether predation has a similarpotential in a coastal area of the brackish Baltic Sea and ifit could explain observed differences in zooplankton communitystructure between a reference area and an eutrophied area. Speciescomposition and daily vertical migration of zooplankton andzooplanktivores, and the diets of the latter, were studied inJuly and August at two 30 m deep stations differing in primaryproductivity. The biomass of zooplankton >35 µm wasdominated by copepods, but cladocerans and rotifers also occurredin significant numbers. The dominating zooplanktivores wereherring (Clupea harengus) and the mysid shrimp Mysis mixta.They fed almost exclusively on zooplankton, mainly copepods,and their estimated food consumption equalled or exceeded thesummer copepodite production. The structure of the zooplanktoncommunity cannot be explained by effects of predation or byfeeding conditions alone. Increases in rotifer and cladoceranabundances with increased primary production suggests effectsof food supply. However, a generally rapid decline in the annualsummer peak of cladocerans may be caused by predation. The totalabundance of copepods did not increase with improved feedingconditions, but there was a shift in species dominance. Thecopepod Ewytemora affinis hirundoides, which was intensivelypreyed upon, increased with increased phytoplankton production,while Acartia bifilosa and/or A.longiremis, which was less eaten,decreased. Predation may explain a pronounced daily verticalmigration of the most predated copepods. They occurred in deeperwater during the day, when the visually feeding herring wereactive, and moved closer to the surface at night when M.mixtaleft the bottom, to forage in the water column.     

Top-down control in pelagic systems: a role for invertebrate predation

Limnologists have long recognized the importance of predation in freshwater communities. The majority of study of predator effects has involved vertebrate predators, with emphasis on planktivorous fish. Documented effects of planktivorous fish have been so dramatic that manipulations of their populations are seen by many as potential tools in lake management. However, the success of such manipulations is often less than desired due to the ubiquitous complexity of food webs and the pervasiveness of compensatory responses to food web manipulation. Recently, enormous effort has been applied to the Lake Kinneret pelagic food web in effort to reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing water clarity. We compared potential predation pressure on Lake Kinneret herbivorous zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We found that, despite having much lower biomass, cyclopoid copepods accounted for a greater portion of the predation mortality on herbivorous zooplankton than bleak. Our results suggest that reductions in predation pressure by bleak will not yield subsequent increases in herbivorous zooplankton biomass. Rather, reductions in bleak predation pressure may allow for increases in cyclopoid copepod abundance and thereby a net increase in predation pressure on herbivorous zooplankton.