Phytoplankton Biomass and Composition in a River-Dominated Estuary During Two Summers of Contrasting River Discharge

Estuaries located in the northern Gulf of Mexico are expected to experience reduced river discharge due to increasing demand for freshwater and predicted periods of declining precipitation. Changes in freshwater and nutrient input might impact estuarine higher trophic level productivity through changes in phytoplankton quantity and quality. Phytoplankton biomass and composition were examined in Apalachicola Bay, Florida during two summers of contrasting river discharge. The 20 μm diatoms were an increasingly (20 to 70 %) larger component of phytoplankton biomass. Lower summer river discharges that lead to an areal contraction of lower (5–25 psu) salinity waters composed of higher phytoplankton biomass dominated by small ( 25 psu) salinity waters composed of relatively lower phytoplankton biomass and a higher percent contribution by >20 μm diatoms. A reduction in summer river discharge that leads to such a change in quantity and quality of estuarine phytoplankton available will result in a reduction in estuarine zooplankton productivity and possibly the productivity of higher trophic levels.