POPULATION DYNAMICS, PHYSIOLOGY AND THE ADAPTIVE SIGNIFICANCE OF DIEL VERTICAL MIGRATION BY ZOOPLANKTON IN LAKE OGLETHORPE, GEORGIA

This study examines the seasonal population dynamics, physiology and adaptive significance of diel vertical migration of zooplankton in Lake Oglethorpe, a small monomictic lake located in northeast Georgia.;Zooplankton distributions and vertical migration patterns were assessed along with other physical and biological parameters. These data and laboratory life table data were used to evaluate alternative hypotheses thought to explain the adaptive significance of diel vertical migration including, (1) a thermal demographic explanation (McLaren 1974), (2) a photosynthetic metabolic advantage (Enright 1977) and (3) a predator avoidance hypothesis (Zaret and Suffern 1976). Vertical migration patterns were closely correlated with diel light changes (up at dusk and down at dawn) during stratified lake conditions. Field data and laboratory life history experiments suggest that migration activity does not follow the predictions made by the hypotheses of McLaren (1974) or Enright (1977). Vertical migration by Lake Oglethorpe zooplankton appears to be metabolically expensive and follows a set of patterns predicted by a predator-avoidance hypothesis (Zaret and Suffern, 1976).;Laboratory life history experiments were conducted with Daphnia parvula over natural temperatures and food concentrations, and two diurnal fluctuating temperature ranges. There was a distinct interaction between temperature and food concentration. A low food level dampened temperature effects on growth and fitness parameters as compared to a high food level. Diel fluctuating temperatures enhanced developmental rates and realized rates of increase over equivalent average constant temperatures, but caused a reduction in rates of net reproduction. Life table experiments indicate that maximum fitness i.e., highest realized rate of increase for Daphnia parvula, occurs in the warmest food-rich surface waters; and that caution should be taken in making field production or population growth rate estimates from strictly temperature-dictated developmental rates under low resource conditions.;A thirteen month period of sampling rotifers and crustacean zooplankton indicated: (1) rotifers always dominate numerically, (2) crustacean zooplankton dominate in terms of biomass during cool months of the year (October through May) and rotifers dominate the remainder of the year, (3) that rotifers in Lake Oglethorpe maintain higher densities than have been found in other lakes, and (4) population dynamics of Lake Oglethorpe zooplankton show distinct differences from dimictic lakes.