Thesis (M.S.)--Eastern Illinois University, 1997. / Includes bibliographical references (leaves 55-60).

Predation by northern pike is believed to have initiated a divergence in whitefish into several different morphs, differing in size, habitat use and growth rate.  The development of a small pelagic zooplankton feeding morph is expected to have large impacts on the zooplankton community. In this study the effect of a changing predation regime on Bosmina, before and after introduction of pike in Valsjön, was investigated.  By looking at the change in carapace length (and indication of the level of predation pressure from fish) and mucro index (an indication of the level of invertebrate predation) of Bosmina remains in lake sediment the changing predation pressure from invertebrates and fish could be investigated. These features proved to be good proxys for the level of defense against fish and invertebrate predation.  However, other species than whitefish, and unknown interactions seems to have affected the zooplankton community. This makes it hard to tell which effect is due to diversification in whitefish and which is not. Also it is not clear that it is pike that has induced the divergence in the whitefish population. Other species like brown trout might also have been involved.

4 ABSTRACT Characteristic high helmets in Daphnia cucullata, which increase resistance of Daphnia against various invertebrate predators, can be induced by chemical cues released by these invertebrates but also by small-scale turbulence. I evaluated the response of D. cucullata to both factors in several clones originating from two riverine pools differing in their predator regime. In a large pool, the dominant invertebrate predator is the cladoceran Leptodora kindtii, and the Daphnia population there shows a typical cyclomorphosis. In the second (small) pool, Daphnia do not produce helmets, despite the presence of the phantom midge larvae Chaoborus flavicans. I compared phenotypic changes of Daphnia cucullata clones from these pools to Chaoborus kairomones and to a small-scale turbulence in laboratory experiments. Kairomones induced significantly longer helmets in clones from both pools; however, only clones form the large pool reacted also to turbulence. As all daphnids in the experiments responded to chemical cues from Chaoborus, I assume that either the kairomone dose under natural conditions in the small pool is too low to induce helmets or the phenotypic response of the local population is mediated by other factors. On the other hand, the variation of response to small-scale turbulence suggests that...