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The genus Daphnia (Crustacea: Branchipoda) includes a diversity of species that are generalist grazers on plankton in freshwater lakes. Despite substantial intraspecific variation in body size, explanations of functional differences among Daphnia species continue to emphasize body size. In this study, we ask whether Daphnia in a relatively narrow range of body sizes differ in ability to exploit the array of resources commonly encountered in lakes. We compared juvenile growth rates of several clones and species of Daphnia fed natural planktonic resources that differed in overall dietary richness. We observed, both among and within species, a trade-off between growth rates in rich and poor resource conditions. We compared the ability of different species and clones to exploit natural resources in a lake mesocosm experiment, testing the prediction that clones displaying the greatest growth rates in rich resource conditions would have the highest minimum resource requirements.
Using-L whole water column enclosures, replicate populations of D. rosea and two different morphs of D. pulicaria were established separately in monoculture in a thermally stratified lake. An additional treatment without crustacean zooplankton was also established. We quantified resources in the enclosures using multiple indirect measures (e.g., chlorophyll a) and a growth bioassay. Results supported predictions; rankings of resource levels achieved in the different treatments suggest that Daphnia trade off high maximum growth rates for low minimum resource requirements. This field experiment also documented large differences among the species and clones of Daphnia in their impact on phytoplankton resources, water clarity, and formation of calcite particles (whiting). We suggest that these large ecological effects stem from the trade-off in abilities to use rich vs. poor resources and constitute an important aspect of grazer–resource interactions.
We review the literature on growth and reproductive responses of zooplankton species to resource level and find general agreement with the concept of a trade-off between abilities to exploit rich and poor resource conditions. A reanalysis of data from a comprehensive study on Daphniidae growth confirms our results and suggests that the breadth of species in this family conforms to the trade-off. Furthermore, the trade-off is only weakly, and not significantly, associated with body size. Our study presents a new framework for understanding the functional significance of daphniid diversity to plankton food webs and illustrates how an ecological trade-off provides a means of linking species diversity to ecosystem function.
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