Influences of river flow on the dynamics of phytoplankton production in a partially stratified estuary

The mesohaline reach of Chesapeake Bay (USA) receives most of its allochthonous nutrient input from a single source, the Susquehanna River. Seaward of the turbidity maximum, concentrations of dissolved inorganic nutrients decrease rapidly as phytoplankton biomass increases along the salinity gradient. The annual cycle of riverine nutrient input is in phase with phytoplankton biomass but out of phase with phytoplankton productivity in this region. Riverine nutrient input and phytoplankton biomass peak during spring, but phytoplankton productivity peaks during summer. Seasonal variations in biomass are correlated with riverine nitrate input while seasonal variations in productivity are correlated with light and temperature. Evidence is presented which suggests that the spring flux of nitrogen from the watershed and the summer productivity maximum are coupled via the accumulation and sedimentation of phytoplankton biomass during spring and subsequent recycling of regenerated nitrogen into the euphotic zone during summer. We conclude that the occurrence of maximum productivity during summer in the mesohaline reach of the Bay is a consequence of the recycling of nitrogen delivered to the system during the previous spring. Inter-annual variations in the magnitude of the summer productivity maximum appear to be related to variations in vertical stratification which influences the vertical flux of regenerated ammonium from the benthos to the euphotic zone. In this context, the extent of seasonal oxygen depletion during summer appears to be determined by riverine nitrate input during the spring freshet and the strength and variability of vertical stratification during summer.