ANNUAL CYCLE OF AUTOTROPHIC AND HETEROTROPHIC PRODUCTION IN A SMALL, MONOMICTIC PIEDMONT LAKE (LAKE-OGLETHORPE) - ANALOG FOR THE EFFECTS OFCLIMATIC WARMING ON DIMICTIC LAKES

Lake Oglethorpe, Georgia, is a small, monomictic impoundment in the so utheastern U.S. Piedmont region. Seasonal differences in ecological pr ocesses correspond to the two major stratification patterns and result in seasonal differences in pelagic community structure. The winter mi xed period (November-March) provides light and temperatures suitable f or active nutrient regeneration and uptake, population growth of edibl e phytoplankton and crustacean zooplankton, and low bacterial abundanc e 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 ''growin g season.'' The stratified period (April-October) is characterized by rapid development of hypolimnetic anoxia, high bacterial production, c yanobacteria and large algae, heterotrophic flagellates, ciliates, rot ifers, small cyclopoid copepods, and Chaoborus. Heterotrophic activity , HPLC chlorophyll a, and grazer activity were highest at the epilimne tio-metalimnetic boundary. Bacterial abundances (1.5-32 x 10(6) cells ml(-1)) increased in the hypolimnion during the stratified period. Lon g-term (1978-1994) warming within the water column (0.061-0.10 degrees C yr(-1)) follows midwinter (January) air temperature trends. Using L ake Oglethorpe as an analog, we predict as consequences of climatic wa rming in northern dimictic lakes: increases in nutrient cycling rates, winter primary and secondary productivity, summer heterotrophic produ ction, and microbial components of the food web.