Long-term dynamics of the zooplanktivorous fish community in Lake Mendota, Wisconsin
Zooplanktivorous fishes selectively prey on larger species of zooplankton which typically alters zooplankton community composition. However, interspecific differences between zooplanktivores may affect the timing and intensity of predation, and therefore their ability to regulate zooplankton population and community dynamics. The objectives of this research were: (1) to characterize the diet and distribution of the major zooplanktivorous fishes in Lake Mendota; (2) to develop an energetic model for white bass (Marone chrysops) that could be used with existing models for cisco (Coregonus artedii) and yellow perch (Perea flavescens) to investigate patterns in zooplanktivory; (3) to examine recruitment variability in the dominant zooplanktivores; and (4) to describe the relationship between zooplanktivorous fishes and zooplankton in Lake Mendota between 1880 and the present day. Historically, the zooplanktivorous fish community has been dominated by cisco, yellow perch and white bass. Intensive sampling indicated that all three species are zooplanktivorous, with little evidence of die!, seasonal or ontogenetic shifts in diet. All three species showed a strong preference for Daphnia pulicaria; however, interspecific differences in physiology and life history affect their ability to regulate D. pulicaria population dynamics. Predation by cisco is intense in the spring when D. pulicaria populations are becoming established, while predation by yellow perch and white bass does not peak until later in the summer. In addition, individual cisco consume 10 times more zooplankton in their lifetime than do either yellow perch or white bass. As a result, cisco are the only fish species able to control zooplankton species composition in Lake Mendota. Interannual variation in abundance of fishes across life stages was related to zooplankton biomass and summer growth temperatures. A large stochastic component to recruitment prevented development of models to forecast year-class strength. Variable recruitment and aperiodic die-offs produced large shifts in the relative composition of the zooplanktivorous fish community. These changes are evidenced in the zooplankton record. When mean annual total zooplanktivory exceeded 0.9 g Daphnia·m2 ·d·1, D. pulicaria were rare. Only when cisco were abundant was this level of zooplanktivory realized. Species composition is crucial in determining the ability of zooplanktivorous fishes to regulate zooplankton population dynamics.