Grazing and nutrient influences of Daphnia and Eudiaptomus on phytoplankton in laboratory microcosms

In two experiments, top-down and bottom-up influences of the herbivorous crustaceans Daphnia pulicaria and Eudiaptomus graciloides on phytoplankton were compared in laboratory microcosms. In a long-term experiment (63 days), both grazers were able to establish populations. The Daphnia population exerted stronger grazing pressure, whereas Eudiaptomus fed more selectively. Daphnia retained relatively more phosphorus (P) and thus caused algal P limitation; with Eudiaptomus as a grazer, both nitrogen (N) and P remained limiting. In a short-term experiment (1 day), N and P release rates and algal-specific grazing rates by both consumers were measured. In this experiment, the increase in concentrations of soluble mineral nutrients (Delta NH4:Delta SRP) also supported the notion that Daphnia mobilized relatively less P than Eudiaptomus. Dry weight-specific grazing patterns did not differ greatly in this experiment. The present experiments and data from the literature suggest that Daphnia populations influence phytoplankton communities not only by the strong grazing pressure they are able to exert, but also by immobilizing large proportions of mineral nutrients (mainly P) within their biomas