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

Two Daphnia species exhibited similar dynamic patterns in western Lake Erie. Populations peaked in early summer, declined to minimum abundan ce in mid-July, and then disappeared after August. To determine relati ve importance of food limitation and young-of-year (YOY) fish predatio n in regulating Daphnia dynamics, we examined relationships (1) betwee n edible phytoplankton abundances and Daphnia fecundity and birth rate s, 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 w et wt) and increased consumption by YOY yellow perch caused a midsumme r decline of Daphnia populations. Once the Daphnia populations were re duced, the predation from age-1 and older planktivorous fish in wester n Lake Erie was likely to keep the populations at low densities in spi te of increased food resources in late summer. An increased white perc h (Morone americana) population and the invasion of zebra mussels (Dre issena polymorpha) in western Lake Erie may alter the cycle of Daphnia dynamics, hence influencing yellow perch population in the lake.