Trait sorting in Daphnia colonising man‐made lakes

1. We used a zooplankton metacommunity to ask how dispersal, genetic drift and selection act to determine the local and regional distributions of trait variation. Since the formation of the lakes 80 years ago, cladoceran species have sorted into local assemblages that cluster by lake depth. Given this species sorting, we ask whether intraspecific variation in an ecologically important phenotypic trait – body size – has sorted as well. 2. We quantified changes in body size through time by measuring ephippia from D. pulicaria, D. dentifera and D. ambigua recovered from sediment cores from two lakes. We then estimated mean body size of contemporary populations of two competing species, Daphnia pulicaria and D. dentifera, in a laboratory common garden experiment. Finally, we used microsatellite loci to characterise genetic diversity and gene flow among local sites in the metacommunity. 3. Body size was variable both within and among years for the three species of Daphnia examined using sediment cores. For two lakes where we examined body size distributions through time, we observed a significant shift in body size of the first species to arrive after colonisation by other Daphnia species, which suggests selection has occurred historically. 4. Despite heritable variation in body size in the laboratory, evidence for trait sorting was only found for D. pulicaria, which was larger in deeper lakes. Mean body size varied among lakes, but did not sort relative to depth for D. dentifera. 5. Microsatellite data indicated that neutral genetic diversity was low in the region; only 27% of the individuals assayed were unique multi‐locus genotypes. We also found significant patterns of isolation by distance for both species. However, population structure was stronger in D. dentifera than in D. pulicaria. Hence, we conclude that a limited number of colonists have successfully invaded this metacommunity, and those genotypes arriving in this new region have experienced significant dispersal limitation among local sites. 6. Overall, while dispersal and selection have clearly led to the development of predictable community assemblages related to depth in this metacommunity, the distribution of phenotypic traits within species can differ substantially even between two trophically similar species. Our results highlight the complex roles of colonisation history, dispersal, selection and stochasticity in determining inter‐ and intra‐specific patterns in metacommunities.     

Patterns and sources of variation in Daphnia phosphorus content in nature

It has recently been shown that Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation. Daphnia parvula and D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of D. pulex (range 1.54–1.05%) and D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.     

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.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Body size distribution in <Emphasis Type="Italic">Daphnia</Emphasis> populations as an effect of prey selectivity by planktivorous fish

We test the hypothesis that size distribution of a Daphnia population reflects the vulnerability of each size category (instar) to predation by planktivorous fish. We hypothesize that due to the different reaction distances from which separate prey categories can be seen by a foraging fish, each category is preyed upon until its density is reduced and its size-specific apparent density level (number of prey within a hemisphere of radius equal to the reaction distance) or encounter rate (number of prey encountered per time within a tube with a cross section of radius equal to the reaction distance) become equal to those of other size categories. An experiment was performed with populations of Daphnia hyalina and D. pulicaria grown at two Scenedesmus + Chlamydomonas food levels (0.2 and 0.05 mg C per liter) in outdoor mesocosms (1000 l tanks) with predation by invertebrates (phantom midge) prevented by mosquito netting. Once the populations had become established, roach were added to the tanks at dusk each day and allowed to feed for 3 h, while control tanks were kept fish-free. After 20–60 days, while D. pulicaria was at low density level, the densities of D. hyalina in fish tanks were high enough to see that the age structure and size distribution matched those from simulations with the age-structured population model based on size-specific encounter rate. This match, however, remained only up to the point of first reproduction when—in contrast to the size/age distribution predicted by the model—the percentage share of adult instars in the total population decreased rapidly with age. This deviation from the predicted densities of adult instars suggests that neither encounter rates nor apparent densities derived from instar-specific reaction distances are sufficient to explain the instar-specific impact of a visual predator on planktonic prey. This implies that a foraging fish may temporarily change its feeding mode from the typical low-speed harvesting of small but abundant prey from within its visual field volume, to high-speed hunting for more scarce but larger ovigerous females when their abundance allows higher net energy gain. Shifting from one feeding mode to the other may be responsible for damping population density oscillations in Daphnia.     

Food quality influences habitat selection in Daphnia

1. The vertical distribution of Daphnia in stratified lakes strongly depends on the depth profiles of temperature and food resources. However, ecological requirements for these factors are slightly different for juvenile and adult Daphnia. 2. Here, I investigated whether food quality influences the habitat selection of Daphnia pulicaria at night and whether the habitat selection of juvenile and adult D. pulicaria is different. Daphnia were allowed to choose their optimal habitat in large, stratified water columns (plankton towers, Plön) that held either the green alga Scenedesmus obliquus (high quality) in the cold hypolimnion (Hypo‐treatment) or S. obliquus in the warm epi‐ and cold hypolimnion (SCEN‐treatment) or the non‐toxic cyanobacterium Synechococcus elongatus (low quality) in the warm epilimnion and S. obliquus in the cold hypolimnion (SYN treatment). 3. When food (S. obliquus) was present only in the hypolimnion (Hypo‐treatment), juveniles and adults distributed similarly in the water column and spent most of their time in the interface between the warm and the food rich layer. 4. When food was present in the epilimnion and hypolimnion (SCEN‐ and SYN‐treatments), juvenile and adult D. pulicaria moved into the warm and now also food‐rich epilimnion, however, the magnitude of this shift depended on the food type and age class of Daphnia. Adult and juvenile D. pulicaria spent most of their time in the epilimnion when food there was of a high quality (S. obliquus; SCEN‐treatment). However, compared to the juveniles, adult Daphnia spent significantly more time in the colder hypolimnion when epilimnetic food was of a low quality (S. elongatus; SYN‐treament). 5. Therefore, habitat selection of adult D. pulicaria was affected by food quality whereas the habitat selection of juveniles was not. 6. Additional growth and reproduction experiments show that the food quality is likely to be responsible for the different habitat selection of juveniles and adults in the SYN‐treatment. 7. In conclusion, my experiments show that D. pulicaria behaviourally reacts to the quality of its food source.     

Performance trade-off across a natural resource gradient

An important environmental factor determining both phytoplankton and zooplankton community composition is lake depth and thermal stratification. However, there is little information on how the interaction between zooplankton grazers and their phytoplankton food changes along an environmental gradient of lake depth. We contrasted resource availability for daphniid zooplankton populations living in two shallow, unstratified lakes and in two deep, stratified lakes using a novel growth bioassay. Stratified lakes had consistently lower resource richness than shallow unstratified lakes. To test whether resources were important in explaining differences in daphniid composition of shallow and deep lakes, we performed reciprocal transplant experiments. We raised daphniids typical of shallow (Ceriodaphnia reticulata) and deep (Daphnia dentifera) lakes in the resources from replicate shallow and deep lakes and monitored survival and reproduction. The two species exhibited a performance trade-off, measured by life table r and R ₀, across a gradient in natural resource richness. D. dentifera had higher relative fitness than C. reticulata when raised in the poorest resource environment from a deep lake. However, under richer resource conditions typical of shallow lakes, C. reticulata outperformed D. dentifera. We further created a gradient in natural resource quantity (by dilution) to test whether this trade-off in species relative fitness involved aspects of resource quality. No trade-off in species performance was evident across the dilution gradient, indicating that resource quality was important to the trade-off. We conclude that shifts in daphniid species composition along a gradient of lake depth involve an adaptive trade-off in ability to exploit rich versus poor resource quality. Received: 11 May 1998 / Accepted: 15 January 1999     

The role of hybridization in the origin and spread of asexuality in Daphnia

The molecular mechanisms leading to asexuality remain little understood despite their substantial bearing on why sexual reproduction is dominant in nature. Here, we examine the role of hybridization in the origin and spread of obligate asexuality in Daphnia pulex, arguably the best‐documented case of contagious asexuality. Obligately parthenogenetic (OP) clones of D. pulex have traditionally been separated into ‘hybrid’ (Ldh SF) and ‘nonhybrid’ (Ldh SS) forms because the lactase dehydrogenase (Ldh) locus distinguishes the cyclically parthenogenetic (CP) lake dwelling Daphnia pulicaria (Ldh FF) from its ephemeral pond dwelling sister species D. pulex (Ldh SS). The results of our population genetic analyses based on microsatellite loci suggest that both Ldh SS and SF OP individuals can originate from the crossing of CP female F₁ (D. pulex × D. pulicaria) and backcross with males from OP lineages carrying genes that suppress meiosis specifically in female offspring. In previous studies, a suite of diagnostic markers was found to be associated with OP in Ldh SS D. pulex lineages. Our association mapping supports a similar genetic mechanism for the spread of obligate parthenogenesis in Ldh SF OP individuals. Interestingly, our study shows that CP D. pulicaria carry many of the diagnostic microsatellite alleles associated with obligate parthenogenesis. We argue that the assemblage of mutations that suppress meiosis and underlie obligate parthenogenesis in D. pulex originated due to a unique historical hybridization and introgression event between D. pulex and D. pulicaria.     

Larval fish-induced phenotypic plasticity of coexisting Daphnia: an enclosure experiment

1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape. 2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae. 3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation. 4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Mortality dynamics of Daphnia in contrasting habitats and their role in ecological divergence

1. The Daphnia pulex‐pulicaria species complex has been proposed as an example of rapid ecological speciation, associated with divergence along the gradient of waterbody size from temporary ponds to deep, stratified lakes. However, this divergence is incomplete, and thus represents an opportunity to study ecological divergence as it is occurring. 2. Dynamics of twelve populations of Daphnia in the pulex‐pulicaria species complex were monitored over 1 year. Six temporary pond populations and six permanent lake populations were compared to evaluate demographic differences that may contribute to ecological divergence in this complex. 3. Pond populations experienced greater changes in density, which were reflected in more extreme growth rates, higher birth rates and higher mortality rates than those of lake populations. 4. Mitochondrial DNA was isolated from up to three clones from each population, the D‐loop of the control region was sequenced, and a phylogenetic tree was constructed. This tree revealed two strongly supported clades. The clades were not congruent with habitat type and nominal status, indicating that interhabitat gene flow occurs easily and that the nominal taxa are incompletely diverged. 5. Published reports of genetic life history differences in the D. pulex‐pulicaria complex are consistent with the demographic differences reported here. This suggests that ecological differences between the habitats are selectively maintaining trait differences despite the possibility for genetic exchange. Thus, these taxa may be at the inception of ecological speciation.     

Finding the optimal vertical distribution: behavioural responses of Daphnia pulicaria to gradients of environmental factors and the presence of fish

1. The vertical distribution of zooplankton results from active habitat choice aiming to optimise fitness gain in a system of trade‐offs. 2. Using large, controlled indoor mesocosms (Plön Plankton Towers), we monitored the behavioural response of Daphnia pulicaria to vertical gradients of temperature, food, oxygen and light, in the presence and absence of fish predation. 3. In the absence of fish, Daphnia distributed as predicted by an ideal ‘free distribution with costs’. If the food was distributed homogeneously, they stayed in the warm epilimnion, while they balanced their time dwelling in epi‐ and hypolimnion if the food was concentrated in a deep‐water maximum. 4. However, oxygen depletion in the hypolimnion, representing an additional cost, prevented Daphnia from completely exploiting the hypolimnetic food maximum. Consequently, the proportion dwelling in the hypolimnion was larger if oxygen was not limiting. 5. Fish predation had an overwhelming effect, driving Daphnia into the hypolimnion under all experimental conditions. If permitted by oxygen availability, Daphnia used the whole hypolimnion, but oxygen depletion reduced their possible habitat to the upper hypolimnion with oxygen concentrations above c. 0.7 mg L^?1. As fish were less tolerant of low oxygen, the layer below the thermocline formed a predation refuge for Daphnia.     

Patterns of Intraspecific DNA Variation in the Daphnia Nuclear Genome

Understanding nucleotide variation in natural populations has been a subject of great interest for decades. However, many taxonomic groups, especially those with atypical life history attributes remain unstudied, and Drosophila is the only arthropod genus for which DNA polymorphism data are presently abundant. As a result of the recent release of the complete genome sequence and a wide variety of new genomic resources, the Daphnia system is quickly becoming a promising new avenue for expanding our knowledge of nucleotide variation in natural populations. Here, we examine nucleotide variation in six protein-coding loci for Daphnia pulex and its congeners with particular emphasis on D. pulicaria, the closest extant relative of D. pulex. Levels of synonymous intraspecific variation, π_s, averaged 0.0136 for species in the Daphnia genus, and are slightly lower than most prior estimates in invertebrates. Tests of neutrality indicated that segregating variation conforms to neutral model expectations for the loci that we examined in most species, while K_a/K_s ratios revealed strong purifying selection. Using a full maximum-likelihood coalescent-based method, the ratio of the recombination rate to the mutation rate (c/u), averaged 0.5255 for species of the Daphnia genus. Lastly, a divergence population-genetics approach was used to investigate gene flow and divergence between D. pulex and D. pulicaria.     

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free‐swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.     

Seasonal dynamics and interspecific competition in Oneida Lake Daphnia

I investigated the population dynamics and competitive interactions of two species of the suspension-feeding crustacean Daphnia in Oneida Lake, N.Y. Both species have persisted in the lake for decades, but their water-column densities are negatively correlated. The larger Daphnia pulicaria dominates in some years, the smaller D. galeata mendotae in others, and in some years one species replaces the other seasonally. Although this pattern results in part from annual variation in vertebrate predation pressure, predation alone cannot explain the irregular daphnid dynamics. In 1992–1995, I examined the water-column abundances, birth and death rates of both species. D. pulicaria dominated in two years, D. galeata mendotae was replaced by D. pulicaria in one year and in 1994, both species persisted in low numbers. To test the effect of temporal changes on the strength of intra- and interspecific competition on both juvenile and adult daphnids, I manipulated a series of field enclosures in 1994 and 1995. The outcome of competition varied within and between years, and its effects were most evident at the highest densities and lowest resource levels. For adults of both species, the effects of interspecific competition were detected more often than those of intraspecific competition. Lipid reserves (a metric of fitness) among juveniles were generally low, with those of D. galeata mendotae often being less than those of D. pulicaria. Contrary to the results of other studies examining competition in daphnids, spatial segregation and predictable within-year reversals in competitive dominance most likely do not play a large role in fostering coexistence of the Oneida Lake daphnids. Instead, coexistence of these competitors is promoted by interannual variation and long-lived diapausing eggs. Received: 20 July 1997 / Accepted: 21 November 1997     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.