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 contrasting 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 24 h in containers with and without zooplankton. Growth rates of heterotrophic flagellates and ciliates were high in the presence of a small assemblage of zooplankton and near zero or negative when either Daphnia pulex or Daphnia galeata was the dominant zooplankton species. Growth 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 comparisons among experiments. In an experiment with equal biomasses but different sizes of D. pulex, flagellate growth rates were lower in treatments with large Daphnia. High mortality in zooplankton communities dominated by larger species of Daphnia appears to be important in determining differences in the abundances of protozoans and rotifers among lakes.