A study of in situ primary productivity (light- and dark-bottle oxygen experiments) 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 heterotrophic 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 sedimentation are used as an independent approach to estimate phytoplankton primary production. The magnitude of net primary production ranged from approximately 1 to 2 g C m-2d-1 for three direct estimates and from to 3 to 5 g C m-2d-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 resuspension of meroplankton. Our study shows that obtaining precise and accurate direct estimates of phytoplankton photosynthesis and production are more complex than might be anticipated from the application of standard techniques.
- Organic carbon sedimentation