Annual Cycle of Autotrophic and Heterotrophic Production in a Small, Monomictic Piedmont Lake (Lake Oglethorpe): Analog for the Effects of Climatic Warming on Dimictic Lakes

Lake Oglethorpe, Georgia, is a small, monomictic impoundment in the southeastern U.S. Piedmont region. Seasonal differences in ecological processes correspond to the two major stratification patterns and result in seasonal differences in pelagic community structure. The winter mixed period (November-March) provides light and temperatures suitable for active nutrient regeneration and uptake, population growth of edible phytoplankton and crustacean zooplankton, and low bacterial abundance and production. Monthly estimates of daily primary productivity are in the mesoeutrophic range (40-1,420 mg C m-2 d-1); however, mean annual primary productivity is in the eutrophic range (500 mg C m-2 yr-1) because of greater activity in winter and an extended "growing season." The stratified period (April-October) is characterized by rapid development of hypolimnetic anoxia, high bacterial production, cyanobacteria and large algae, heterotrophic flagellates, ciliates, rotifers, small cyclopoid copepods, and Chaoborus. Heterotrophic activity, HPLC chlorophyll a, and grazer activity were highest at the epilimnetic-metalimnetic boundary. Bacterial abundances (1.5-32×6cells ml-1) increased in the hypolimnion during the stratified period. Long-term (1978-1994) warming within the water column (0.061-0.10⚬ C yr-1) follows midwinter (January) air temperature trends. Using Lake Oglethorpe as an analog, we predict as consequences of climatic warming in northern dimictic lakes: increases in nutrient cycling rates, winter primary and secondary productivity, summer heterotrophic production, and microbial components of the food web.