Control of phytoplankton production and biomass in a river-dominated estuary: Apalachicola Bay, Florida, USA

Physical, chemical, and biological processes interact in complex patterns through time to control estuarine phytoplankton productivity and biomass. Hydrodynamic model results, together with biological, nutrient, and physical data acquired from Apalachicola Bay from May 1993 through May 1996, were used to elucidate factors that control phytoplankton productivity and chlorophyll (chl a). The estuary receives freshwater, which originates in the Apalachicola, Chattahoochee, and Flint River watersheds, with maximum river flow occurring in late winter and early spring. Maximum chl a (mean ± 1 SE: 5.38 ± 0.40 μg chl a l–1) values occurred during winter, while primary productivity maxima occurred in late spring and summer months when temperature and photosynthetically active radiation (PAR) reached their annual maxima. Approximately 75% of annual primary production (255 ± 78 g C m–2 yr–1) occurred from May through November of each year. During this period, however, river dissolved inorganic nitrogen and soluble reactive phosphorus input accounted for 40% of the annual inputs (30.9 ± 5.1 g N m–2 yr–1 and 0.60 ± 0.15 g P m–2 yr–1, respectively). Approximately 25% of annual primary production occurred from December to April, concurrent with low PAR values and low water temperature. Low chl a concentrations in summer months were concurrent with high phytoplankton productivity, high zooplankton abundance, low river flow, and low nutrient input to the estuary. In 2 of 3 years, export from Apalachicola Bay provided a significant control for phytoplankton biomass magnitude during winter. However, on an annual basis grazing accounted for 80% of the chl a loss from the estuary.