DAPHNIA POPULATION-DYNAMICS IN WESTERN LAKE ERIE - REGULATION BY FOOD LIMITATION AND YELLOW PERCH PREDATION

Two Daphnia species exhibited similar dynamic patterns in western Lake Erie. Populations peaked in early summer, declined to minimum abundance in mid-July, and then disappeared after August. To determine relative importance of food limitation and young-of-year (YOY) fish predation in regulating Daphnia dynamics, we examined relationships (1) between edible phytoplankton abundances and Daphnia fecundity and birth rates, and (2) between Daphnia biomass consumed by YOY yellow perch (Perca flavescens) and Daphnia death rates. The population peak was created by a burst of parthenogenetic reproduction. Suppression of birth rates (<1 individual d-1) by low edible phytoplankton resources (<4 g.m-3 wet wt) and increased consumption by YOY yellow perch caused a midsummer decline of Daphnia populations. Once the Daphnia populations were reduced, the predation from age-1 and older planktivorous fish in western Lake Erie was likely to keep the populations at low densities in spite of increased food resources in late summer. An increased white perch (Morone americana) population and the invasion of zebra mussels (Dreissena polymorpha) in western Lake Erie may alter the cycle of Daphnia dynamics, hence influencing yellow perch population in the lake.