Midsummer Dynamics of Two Daphnia Species in Wintergreen Lake, Michigan

During 1976, midsummer reductions in densities of Daphnia pulicaria and Daphnia galeata mendotae populations occurred in Wintergreen Lake, Michigan. Previous attempts to determine the causes of such declines have indicated correlations between predator populations, the presence of unpalatable algae or high midsummer water temperatures, and the reduction or disappearance of the daphnid populations. In this study, in situ life table experiments were performed to assess the influence of natural food and temperature conditions on population growth. A weekly sampling program showed that D. pulicaria was dielly concentrated, after early summer, at depths between the anaerobic hypolimnion and the warmer epilimnion, consistent with previous information indicating it to be a cold—water species. However, in situ life table data showed that D. pulicaria was capable of surviving in epilimnetic water up to 27°C, although reproduction was greatly reduced for a 6—day period in mid—July. This reduction in reproduction appeared to be the result of an interaction of high temperatures, declining standing crops of small algae and increasing amounts of Anabaena, Ceratium and Volvox. Daphnia galeata mendotae did not show any adverse response to these mid—July algae—temperature conditions. Analysis of predator gut contents suggested that both daphnids were heavily preyed upon by bluegills (Lepomis macrochirus). Shifts in the size structure of the daphnid populations were also consistent with intense size—selective predation by bluegills, as was the tendency for the age distribution of parthenogenic eggs to become skewed to younger eggs as the midsummer decline proceeded. Differences in body size, ephippial production and habitat preferences between the 2 species are also consistent with the hypothesis that planktivory is a strong selective force in their evolution.