Seasonal regulation of Daphniapopulations by planktivorous fish: Implications for the spring clear-water phase

The ability of planktivorous fish to regulate the population dynamics of Daphnia galeata in Lake Mendota, Wisconsin, was examined during spring and summer 1987. Low rates of consumption by yellow perch (Perca flavescens) and cisco (Coregonus artedii) allowed populations of D. galeata to expand rapidly during spring, resulting in a distinct clear-water period that lasted from mid-May until early June. A subsequent decline in D. galeata abundance resulted from low food availability brought about by its exploitation of algal resources. Once populations of D. galeata were reduced, predation by perch and cisco was sufficient to keep populations of D. galeata 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