Two examples of the interplay between field observations and laboratory experiments from 35 years of research with planktonic organisms

Two studies of complicated ecological phenomena in Lake Maarsseveen (The Netherlands) are presented to illustrate that a combination of field and laboratory analysis might be a successful approach. In the first one, the yearly varying ratio of population abundance of two diatoms, Asterionella formosa and Fragilaria crotonensis during early spring is explained by a combination of abiotic and biotic factors. A fungal parasite blocks population development of the first species at temperatures higher than 3–5 °C . The second species then competes successfully for phosphate and develops a large population. The interaction between Asterionella and the fungus proved to be intricate. Several physiological mechanisms operate together and combined with variable climatic factors makes the prediction of what species will be dominant, impossible. In the second example diel vertical migration of the hybrid Daphnia galeata × hyalina is analysed in the field and the laboratory. Migration is confined to six to seven weeks in June–July when large shoals of juvenile perch are present in the pelagic zone of the lake. These fish exudate a kairomone that enhances the reaction of Daphnia to relative increases in light intensity of dawn. Especially accelerations in rate of relative increases enhance swimming velocity. The extent of enhancement depends, however, on the concentration of the kairomone and on food concentration. A Decision Making Mechanism is introduced and the relation between the response mechanism and the adaptive relevance of diel vertical migration is made.Finally considerations about research strategies and the importance of fundamental research are made. It is concluded that the development of limnology and aquatic ecology might be in danger when the present tendency to give prevalence to applied research continues.     

Differential behaviour and shifts in genotype composition during the beginning of a seasonal period of diel vertical migration

During the first few weeks of a recurring seasonalperiod of diel vertical migration in Lake Maarsseveen(The Netherlands), part of the hybrid Daphniagaleata × hyalina population migrated, whileanother part remained in the epilimnion. In theepilimnion, 0+ perch prey upon daphnids duringdaytime. Gradually, the number of adult Daphniain the epilimnion decrease until the epilimnion isnearly devoid of daphnids. The population as a wholemay decrease, as in 1991, or may increase asin 1992. Genotype composition, as determined byallozyme analysis, changed substantially within afortnight in 1992, and one genotype became dominant.Our data are in agreement with the hypothesis thatpredation on different genotypes (clones)occurs during the beginning of a seasonal period ofdiel vertical migration, though our data do not allowto exclude alternativeexplanations.     

Perch or plankton: top‐down control of Daphnia by yellow perch (Perca flavescens) or Bythotrephes cederstroemi in an inland lake?

1. Seasonal termination of the vernal clear-water phase in Long Lake, Grand Traverse Co., Michigan coincided with severe size-selective predation on juvenile Daphnia pulicaria from 0.8 to 1.8 mm in length. This could be caused by predation by age-0 yellow perch ( Perca flavescens ) or by the exotic predatory zooplankter Bythotrephes cederstroemi .
2. During the initial decline of Daphnia , Ivlev's electivity coefficient for yellow perch from 15.0 to 20.0 mm in length was 0.50 for copepods and −0.75 for D. pulicaria .
3. Bioenergetics modelling of both yellow perch and Bythotrephes demonstrates that, during the initial Daphnia decline, Bythotrephes consumed 1.5–5 times greater total mass than yellow perch. Furthermore, models in which Bythotrephes consumed juvenile Daphnia were more consistent with the timing of the Daphnia decline than those in which yellow perch consumed juvenile Daphnia .
4. The invasion of Bythotrephes into Long Lake seems to be a significant perturbation, introducing effects that propagate throughout the food chain. Bythotrephes created a possible bottleneck for age-0 yellow perch in late June by suppressing Daphnia .     

The effect of accelerations in light increase on the phototactic downward swimming of Daphnia and the relevance to diel vertical migration

The effect of relative increases in light intensity on photobehaviourwas studied in the hybrid Daphnia galeata x hyalina. We firstcarried out a series of experiments to study the influence offish kairomone on several response variables of light-inducedswimming. With fish kairomone present, an increase in the percentageof reacting daphnids to 100% was found at almost all ecologicallyoccurring relative light change rates that were above threshold.The relationship between the relative increase in light intensity(stimulus) and the time expiring between the onset of the stimulusand the start of the downward swimming response was not influencedby fish kairomone, nor did kairomone alter the functional relationshipbetween stimulus strength and downward displacement velocity,although velocity increased. During the previous experiments,various light change rates were applied, but per test run theserates were constant. The natural relative light increase inthe early morning consists of continuously increasing relativelight change rates, turning into decreasing rates after themaximum is reached     

What goes down must come up: symmetry in light-induced migration behaviour of Daphnia

During a short period of the year, Daphnia may perform a phenotypically induced diel vertical migration. For this to happen, light-induced swimming reactions must be enhanced both at dawn and at dusk. Enhanced swimming in response to light intensity increase can be elicited by fish-associated kairomone in the laboratory, if food is sufficiently available. However, during the light change at dusk the Daphnia are still in the hypolimnion, where no fish kairomone is present and both temperature and food availability is low. Still, what goes down must come up. This raises questions about how Daphnia tunes its light-induced swimming behaviour to prevailing conditions such that a normal diel vertical migration can be performed. We investigated the symmetry in behavioural mechanism underlying these diel vertical migrations in the hybrid Daphnia galeata × hyalina (Cladocera; Crustacea), with special interest for the environmental cues that are known to affect swimming in response to light increase. That is, we tested whether fish- associated kairomone, food availability, and temperature affected both swimming in response to light intensity increase and decrease similarly. We quantified swimming behaviour during a sequentially increased rate of light change. Vertical displacement velocity was measured and proved to be linearly related to the rate of the light change. The slope (PC) of the function depends on the value of the factors kairomone concentration, food availability, and temperature. The changes of the PC with kairomone concentration and with temperature were similar both at light intensity increases and decreases. The PC also increased with food concentration, although during light increases in a different way from during light intensity decreases. Low food availability inhibited swimming in response to light intensity increase, but enhanced swimming in response to light intensity decrease. Hence, ascent from the deep water layers with low food concentration at dusk is facilitated. These causal relations are part of a proximate decision-making mechanism which may help the individual Daphnia to tune migration to predation intensity and food availability.     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

Dynamics of a Boreal Lake Ecosystem during a Long-Term Manipulation of Top Predators

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m⁻² d⁻¹ before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.     

Vertical, lateral and diurnal drift patterns of fish larvae in a large lowland river, the Elbe

A ringnet was used to sample drifting fish quantitatively at distinct depth strata. This method was chosen to examine the lateral, vertical and diurnal drift patterns of fish fry during four sampling campaigns in May 1998 and 1999 in the middle Elbe River, close to the city of Schnackenburg (stream kilometre 475). It became apparent that the fish larvae concentrated mainly at the surface of the right side of the main channel. Fourteen fish species of the four taxonomic families Percidae, Cyprinidae, Osmeridae and Gadidae were caught. Percid fishes (pike‐perch and perch) dominated the caught fish fry assemblage. Several environmental parameters were included in a direct gradient analysis in order to detect influencing variables. The main significant qualities were a temporal aspect and the shore distance. Other important influencing variables were suspended particular matter (SPM), fishing depth, time of the day, current velocity and the filtered volume. Negative correlations of abundance and shore distances and concentrations of SPM were found for several species as well as for the total abundance.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

The food and feeding habits of young perch, Perca fluviatilis, in Windermere

SUMMARY. The food of 0+ perch, Perca fluviatilis L. has been investigated in Windermere, the largest lake in the English Lake District. Whole guts and stomachs from 800 fish were investigated during four 24-h periods between June and September in 1975 and in 1976. Ciliates, algae, rotifers and cyclopoid nauplii constituted the first food of larval perch in Windermere. Vertical samples were taken from the zooplankton, at the same time and in the same location where the fish were sampled, and the concentration of the different food organisms studied. Electivity of the different food organisms was studied from lvlev's index, which showed positive electivity, or selection for some organisms, e.g. Bosmina obtusirostris (Sars.) and copepods (both cyclopoid and calanoid), and negative for others such as Daphnia hyalina var. galeata (Sars.), Leptodora kindti (Focke) and Bythotrephes longimanus (Leydig). In studying predatorprey relationships, Daphnia hyalina was chosen as the 'standard prey', being the most abundant of all food organisms in the diet of young perch, also because its maximum body-depth (carapace width (mm)) was among the largest of the cladocera and copepods in Windermere. Comparison of the maximum carapace width of Daphnia eaten with that of the plankton revealed that smaller specimens were taken by juvenile perch in July, but they tended to select larger organisms during August and September.     

Occurrence of Bunodera sacculata Van Cleave and Mueller, 1932 in Perca flavescens from Silver Creek and Silver Lake,Michigan

Bunodera sacculata (Trematoda: Allocreadiidae) infected 65 (54%) of 120 yellow perch collected in 1997 and 40 (50%) of 80 perch from Silver Creek in 1998. and 17 (30%) of 56 perch in 2000 from Silver Lake in lower Michigan. Gravid B. sacculata occurred in the pyloric ceca and anterior intestine. There were differences in monthly trends (July through August) in prevalences, mean intensities, and mean abundances of B. sacculata in Silver Creek between 1997 and 1998. The mean intensity +/- SD (8.2 +/- 9.1) and mean abundance (4.1 +/- 7.8) of B. sacculata were significantly higher in 1998 than in 1997 and 2000. An additional 20 perch (0+ in age) collected from the creek in July 1998 were all infected and had the highest mean intensity of 21.3 +/- 14.1.     

Interactions Between Predator Kairomone and Food Level Complicate the Ecological Interpretation of Daphnia Laboratory Results

The effect of food concentration on the phenotypic responseof life history traits to two predator kairomones was investigatedin Daphnia. For the experiment, one clone of Daphnia galeatawas used as the prey organism and solutions containing infochemicals(kairomones) of Chaoborus (phantommidge) and Perca (perch) simulatedthe presence of potential predators. The combined effect offood level and predator kairomone on phenotypic plasticity ofDaphnia life history is complex. At low food levels, the responsesin life history traits to kairomones are enhanced. For example,in the Perca treatment, this leads to a younger age at firstreproduction. The presence of Chaoborus kairomone decreasessurvival so that death often occurs before reproduction is reached.This results in a strong decrease in the intrinsic rate of populationincrease and might contribut to the pronounced summer declineof some Daphnia populations during periods of food shortage.     

Occurrence of Sanguinicola occidentalis Van Cleave and Mueller, 1932 in Perca flavescens and Campeloma decisum from a Michigan creek

Sanguinicola occidentalis (Trematoda: Sanguinicolidae) infected 58 (48%) of 120 yellow perch collected in 1997 and 40 (50%) of 80 yellow perch collected in 1998 from Silver Creek in lower Michigan. The mean intensity and mean abundance of this blood fluke were higher in 1998 than in 1997. The fluke was found in the bulbous arteriosus of the perch heart, free in the petri dishes when the gill filaments were teased apart, and in the body cavity washings, and 1 individual was associated with an eye. Of the 269 S. occidentalis examined from perch, none had eggs. Most perch examined and infected were only 1+ yr in age. Spearman's correlation coefficients between S. occidentalis intensity and host length in 1997 and 1998 were not significant. An additional 25 yellow perch (0+ yr in age) collected in 1998 were not infected. Thirty-seven (33%) of 113 snails (Campeloma decisum) examined in July and August 1999 from Silver Creek were infected with S. occidentalis.     

Distribution patterns of fishes in a canyon-shaped reservoir

In August 2004 and 2005, an extensive study of the fish community was carried out in the largest water supply reservoir in the Czech Republic and Central Europe, the canyon‐shaped ?elivka Reservoir, using a fleet of Nordic multimesh gillnets. Fishes were sampled at eight locations along the longitudinal profile of the reservoir and at five benthic depth layers covering depths from the surface down to 18 m (benthic gillnet 1·5 m high), and at three pelagic depth layers down to the depth of 5 m above the bottom (pelagic gillnets 4·5 m high). Catches of both juvenile (age 0+ year) and adult (fishes >1 year) fishes were highest in the upper layers of the water column (i.e. in the epilimnion down to 5 m, and down to 10 m in the benthic habitats). Along the tributary–dam axis in the pelagic habitats, both juvenile and adult fishes preferred the upper part of the reservoir, where the maximum number of species and also the greatest abundance of zooplankton were found. In the benthic habitats, fishes selected location according to factors other than trophic status. More juvenile fishes were recorded in the benthic habitats than in the pelagic habitats. Depth had the largest explanatory power for predicting fish community composition, followed by the affiliation with benthic and pelagic habitats, and location on the longitudinal axis of the reservoir. The fish community was represented mainly by cyprinids and consisted of two distinct groups of species, with bleak Alburnus alburnus, rudd Scardinius erythrophthalmus and asp Aspius aspius dominating the offshore group while perch Perca fluviatilis and ruffe Gymnocephalus cernuus were affiliated with the inshore group of the adult fish community. Roach Rutilus rutilus, bream Abramis brama and pikeperch Sander lucioperca occurred in important proportions in both the inshore and the offshore zones. All species, with the exceptions of adult perch (1+ year and older), 0+ year perch and 0+ year roach, preferred the most eutrophic tributary part of the reservoir. The fish community was relatively stable between the 2 years sampled.     

Diel vertical and horizontal distribution of crustacean zooplankton and young of the year fish in a sub-alpine lake: an approach based on high frequency sampling

Understanding the spatial dynamics of predators and their preyis one of the most important goals in aquatic ecology. We studiedspatial and temporal onshore–offshore distribution patternsin young of the year (YOY) Eurasian perch (Perca fluviatilis)and crustacean zooplankton (Daphnia hyalina, Cyclops prealpinus)along a transect in Lake Annecy (France). Our study representsa first attempt at coupling hydroacoustic fish survey and highfrequency zooplankton recording to assess simultaneously thelarge-scale distribution patterns of YOY fish and their zooplanktonprey over a diel cycle (day, dusk and night sampling). We hypothesizedthat the spatial distribution of zooplankton could be shapedby both anti-predator behaviour (horizontal and vertical migrations)and predation losses. Fish biomass, size structure and dietwere assessed from split-beam echosounding and net trawlingsamples, whereas crustacean abundances were estimated with asmall modified Longhurst–Hardy continuous plankton recorder.We evaluated the diel changes in the spatial distribution patternsof fish and zooplankton and determined the overlap between theirdistributions. Fish biomass was dominated by YOY perch in upperwarmer layers and salmonids (Coregonus lavaretus and Salvelinusalpinus) in the colder and oxygenated deep layers. YOY perchwere aggregated in dense schools in the epilimnion during theday and dispersed at night. Fish biomass was distributed alonga strong increasing onshore–offshore gradient at night,whereas crustacean prey showed a decreasing gradient. This onshore–offshorenegative gradient in crustacean distribution, expressed on ashorter scale during the day, shifted toward the surface watersat night. A distinct kinetic of diel vertical migration (DVM)patterns was exhibited by daphnid and cyclopoid populationsand resulted in distinct vulnerability to perch predation. Spatio-temporaldistribution of crustaceans in Lake Annecy during the diel cyclestudy was probably shaped both by predation loss to YOY perchand by anti-predator behaviour (DVM, DHM) by zooplankton. Theimplications for fine-scale studies of fish-zooplankton interactionsare discussed.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

Evidence for negative impact of plerocercoid infection of Triaenophorus nodulosus on Perca fluviatilis L. stock in Upper Lake Constance, a water body undergoing rapid reoligotrophication

The macro-pathological impact of plerocercoid infection of perch Perca fluviatilis livers was quantitatively determined by an index of liver lesion ( I _LL) in fish from Upper Lake Constance (ULC). These results were compared to the I _LL values of perch from two other lakes. From February to November 1999, 2071 perch were examined from the three sites. Prevalence of infection in ULC was 13% in actively growing first year (0+ year) perch and 94% in older fish, whereas in the two lakes used for comparison the figures for 0+ year perch were 12 and 20%, and in older fish 62 and 60%, respectively. Infected livers from perch caught in ULC revealed significantly higher I _LL values than those of fish from the other two lakes. Perch livers from ULC containing more than three Triaenophorus nodulosus larvae also showed significantly more serious pathological alterations than uninfected livers or those with normal infection levels of one to three T. nodulosus . In all three lakes, excluding heavily infected perch from ULC, 0+ year perch livers revealed little or no damage, whereas in livers of older perch pathological alterations were apparent. In addition, analysis of age and total length data showed that perch infected with one to three parasites in ULC grew significantly slower than uninfected ones, and severely infected perch (greater than three parasites) grew significantly slower than those with a normal parasite load. With respect to mass, growth of ULC fish suffering normal infection (49% of all adults) was reduced by 9% and in those with severe infections (45% of all adults) growth was 16% below expected at normal harvesting age.     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g × h — hyalina and the multilocus allozyme genotypes of the hybrid D. g × h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g x h – hyalina and the multilocus allozyme genotypes of the hybrid D. g x h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.