Ecosystems are characterized by their storage and transfer of nutrients and energy. Therefore, understanding ecosystems requires an understanding of the functional role of each of its members. The water flea DAPHNIA occupies a central position in freshwater ecosystems, transferring energy and organic matter from primary producers (algae) to higher consumers (invertebrates and fish). Since swimming behavior affects DAPHNIAšs ability to find food and avoid predation, it is a vital part of DAPHNIAšs ecology. In this thesis, I investigate the role swimming behavior plays in structuring DAPHNIAšs functional role in aquatic ecosystems.

I investigated the ability of DAPHNIA to alter its swimming behavior in response to changes in temperature, food concentration and light intensity using two scale-independent measures, the fractal dimension and the integrated turning angle. Of the two measures, the integrated turning angle was the most diagnostic measure, revealing several differences among treatments. The differences among treatments indicate that DAPHNIA indeed alter their swimming behavior in response to different environmental conditions.

I devised the Virtual Plankton system to test the hypothesis that small-scale zooplankton swimming behaviors affect DAPHNIA predation risk. By observing fish ŗpreying˛ on computer-generated prey images whose movement was precisely controlled, I found that fish preferentially choose individuals that hop faster than neighbors. This indicates that small-scale swimming behavior, by increasing conspicuousness, plays an important part in determining DAPHNIAšs overall vulnerability to predation.

Finally, I investigated the effectiveness of DAPHNIA escape swimming in four DAPHNIA clones from different predation regimes. The results of simulated and real predation experiments indicate that DAPHNIAšs escape response is both efficient and effective in that (1) escape behavior is only enhanced under environmental conditions in which visual predation is a significant threat, i.e. in the presence of both light and predator chemical cues (kairomones), (2) escapes are usually performed only in response to hydromechanical stimuli which are strong enough to signify a potential predator and, (3) under realistic lighting conditions, their escape behavior is sufficient to escape predation from small fish.