THE IMPORTANCE OF DAPHNIA IN DETERMINING MORTALITY-RATES OF PROTOZOANS AND ROTIFERS IN LAKES

We measured mortality of protozoans and rotifers in three lakes of con trasting zooplankton communities. We also compared protozoan growth in an experiment which controlled Daphnia biomass but varied body size. Mortality was determined as the difference between growth rates over 2 4 h in containers with and without zooplankton. Growth rates of hetero trophic flagellates and ciliates were high in the presence of a small assemblage of zooplankton and near zero or negative when either Daphni a pulex or Daphnia galeata was the dominant zooplankton species. Growt h rates of rotifers were also usually lower in the presence of Daphnia . Mortality rates of heterotrophic flagellates, ciliates, and rotifers were positively related to the mean body size of Daphnia in compariso ns among experiments. In an experiment with equal biomasses but differ ent sizes of D. pulex, flagellate growth rates were lower in treatment s with large Daphnia. High mortality in zooplankton communities domina ted by larger species of Daphnia appears to be important in determinin g differences in the abundances of protozoans and rotifers among lakes .