Population dynamics of Daphnia spp. and implications for trophic interactions in a small, monomictic lake

In Lake Oglethorpe, Georgia, USA, herbivorous crustacean zooplankton are abundant and dominate zooplankton biomass in winter, but are scarce throughout most of the summer. We used a 3.5 year study of Daphnia population dynamics to infer when food, predators or temperature constrained growth of this population. Transitions between winter and summer consumer assemblages are concurrent with seasonal changes in water temperature, thermal structure (stratification/destratification), resources (autotrophic/heterotrophic-dominated production), and predator abundance and activity (e.g. Lepomis macrochirus and Chaoborus punctipennis). We sampled at weekly or less intervals from April 1992 to September 1995, and determined population abundances for all cladoceran species. For the Daphnia population (Daphnia ambigua + Daphnia parvula), we measured clutch size and length for all individuals. We used average water column temperature (where dissolved oxygen is >1 mg l(-1)) to estimate egg development time from an empirical model. Estimates of Daphnia population birth and death rate were thus generated from abundance, egg ratio and temperature/dissolved oxygen data. We compared observed birth rate (b(obs)) with expected birth rate (b(exp) +/- 95% CI; predicted for food-saturated conditions at ambient temperature). For variable (1-13 week) periods between late November and March, 1992-1995, water temperature was the primary factor constraining Daphnia population growth (b(obs) = b(exp)) From about April to early November, b(obs) < b(exp) suggested food-limited population growth. In spring, summer and early fall (March-October), population densities were several-fold lower than in late fall and winter (November-February). However, all else being equal, egg ratio and population birth rate data would have predicted that Daphnia abundance fluctuates over equivalent ranges in spring and fall. We interpret this discrepancy as evidence for increased rates of extrinsic mortality during the growing season and a seasonal shift in the relative importance of resource and predator regulation. The duration of predator suppression of crustacean population abundance in Lake Oglethorpe and other warm-latitude lakes (similar to 36 degrees N-27 degrees S) is longer (greater than or equal to 3-7 months) than that observed in north temperate lakes (1-2.5 months; 41-52 degrees N).