Daphniids act as keystone species in lake ecosystems by controlling phytoplankton biomass and experiencingintense fish predation. However, the importance of single daphniid species as trophic links betweenphytoplankton and fish remains unclear, especially compared with other zooplankton taxa. To disentangle the roleof individual zooplankton taxa in the food web of a large lake, we performed an in-situ mesocosm experiment withnatural phytoplankton and zooplankton communities in three treatments with native, invasive or no fish predators,respectively. A large daphniid, Daphnia longispina, was the zooplankter most strongly predated by both fish species,and also had the highest top-down effects on phytoplankton. All other zooplankton taxa, including a smalldaphniid species, had minor roles in terms of both predation by fish and grazing on phytoplankton. We suggest thatdaphniid species with large body sizes can strongly link higher and lower trophic

Cultural oligotrophication is expected to shift lake zooplankton to become dominated by calanoid copepods. Hence, understanding the influence of calanoids on the taxonomic and size structure of the lower plankton food web is crucial for predicting the effects of oligotrophication on energy fluxes in these systems. We studied the effect of an omnivorous calanoid, Eudiaptomus gracilis, on the lower planktonic food web using an in situ incubation approach in large and deep Lake Constance. We show that E. gracilis significantly reduced ciliate, phytoplankton, rotifer, but increased bacteria biovolume. Highest clearance rates were observed for ciliates whose biovolume declines may have caused a release of predation pressure on bacteria. E. gracilis grazing shifted the size structure of the phytoplankton community by reducing mean phytoplankton cell size (directional selection) and simultaneously increasing cell size variance (disruptive selection). Ciliate cell sizes experienced a similar selective regime in one of the experiments, whereas in the other two experiments, no change of size structure was detected. Results suggest strong influences of E. gracilis grazing on the lower plankton food web and a significant shift in phytoplankton size structure. For evaluating detailed effects of omnivorous consumers on plankton size structure, cascading interactions need to be considered.