Seasonal regulation of Daphnia populations by planktivorous fish: Implications for the spring clear‐water phase

The ability of planktivorous fish to regulate the population dynamics of Daphnia gale&a in Lake Mendota, Wisconsin, was examined during spring and summer 1987. Low rates of consumption by yellow perch (Percaflavescens) and cisco (Coregonus artedii) allowed populations of D. galeata to expand rapidly during spring, resulting in a distinct clear-water period that lasted from midMay until early June. A subsequent decline in D. guleatu abundance resulted from low food availability brought about by its exploitation of algal resources. Once populations of D. guleuta were reduced, predation by perch and cisco was sufficient to keep populations of D. g,lleata at low densities during July and August in spite of more abundant algal food resources, Extreme seasonal variation in the degree to which planktivorous fish can regulate daphnid population dynamics may have allowed daphnids to persist in Lake Mendota during the past century in the presence of abundant zooplanktivores.