Field and laboratory studies will be conducted to test the hypothesis that ciliates and rotifers will be more abundant in freshwater zooplandton communities with only small cladoceran species than in those with large cladoceran species. In situ enclosure experiments will test this hypothesis by adding no cladocerans, small cladocerans (Bosmina longirostris), or large cladocerans (Daphnia pulex) to natural communities of ciliates and rotifers. These experiments will be supplemented by: 1) laboratory competition experiments to test the ability of the rotifer Keratella cochlearis to coexist with different-sized cladoceran species; and 2) laboratory experiments and observations on the susceptibilities of a variety of different- sized ciliate species to mechanical interference (damage, predation) from different-sized cladoceran species. The results of this research should be useful in understanding variation in the importance of ciliates and rotifers in natural communities and secondary effects of zooplanktivorous fish, which prey selectively on large cladocerans in natural communities. This study will be the first to extensively investigate interactions between cladocerans and ciliates, and to assess the impact of cladocerans on ciliates in natural communities.
