Grazer—resource interactions in the plankton: Are all daphniids alike

Daphniids have long been considered to be uniquely effective grazers in the planktonic food web of lakes, but whether all daphniid species are equivalent in this functional role is less clear. In particular, a common belief that large‐bodied daphniids are more capable than smaller daphniids at controlling phytoplankton abundance has received limited testing. Using whole water column enclosures in a mesotrophic lake, we compared the ability of four common planktonic daphniids ( Ceriodaphnia reticulata , Daphnia ambigua , Daphnia mendotae , and Daphnia pulicaria ) to exploit a natural assemblage of phytoplankton. We established replicated monocultures of each daphniid species and allowed their populations to reach a carrying capacity determined by resources. We then compared the effects of each daphniid species on phytoplankton biomass, size structure, taxonomic composition and C:N: P stoichiometry. Populations of all four daphniids stabilized at very low birth and death rates, with larger species having a lower density but a higher biomass than smaller species. The seston C: P molar ratio was driven to equally high values (>300) in all treatments; however, daphniid effects on phytoplankton abundance and composition were quite different. The two smaller daphniids were less effective at depressing phytoplankton populations than were the two larger daphniids. This difference was associated with the persistence of a diverse assemblage of digestionresistant green algae in the Ceriodaphnia and D. ambigua treatments but their elimination from the D. mendotae and D. pulicaria treatments. Several lines of evidence, including growth bioassays, that have used juveniles of a clone of D. pulex‐pulicaria , suggest that body size was not an adequate explanation for these differences in daphniid species effects on phytoplankton.