Phytoplankton community photosynthesis and primary production in a hypereutrophic lake, Lake Apopka, Florida

Abstract

A study of in situ primary productivity (light- and dark-bottle oxygen exper- iments) was conducted biweekly in 1990 and 1991 to measure volumetric rates of phytoplankton photosynthesis and estimate integral phytoplankton photosynthesis and primary production. Concurrently, ambient conditions including water temperature, photosynthetically active radiation (PAR), Secchi disc transparency, dissolved oxygen and phytoplankton biomass (chlorophyll- a) were measured. Results of both light- and dark-bottle experiments are used to 1) demonstrate that Lake Apopka is not hetero- trophic as reported in the literature, 2) provide preliminary estimates of phytoplankton primary production, 3) discuss shortcomings in experimental methodology and 4) evaluate sources of temporal variability in phytoplankton primary production. No strong seasonal patterns were found in ambient physical and chemical conditions with the exception of water temperature. Whole-basin estimates of net organic carbon sedi- mentation are used as an independent approach to estimate phytoplankton primary pro- duction. The magnitude of net primary production ranged from approximately 1 to 2gCm -2 d -1 for three direct estimates and from to 3 to 5gC m -2 d -1 for the indirect method. We conclude that short-term temporal variability in primary productivity is controlled mainly by meteorological forcing of wind-induced turbulence and resuspen- sion of meroplankton. Our study shows that obtaining precise and accurate direct esti- mates of phytoplankton photosynthesis and production are more complex than might be anticipated from the application of standard techniques.