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

The mesohaline reach of Chesapeake Bay (USA) recelves most of its allochthonous nutnent input from a single source, the Susquehanna h v e r Seaward of the turb~dity maximum, concentrahons of dissolved inorganic nutrients decrease rapidly as phytoplankton b ~o m a s s Increases along the s a l i ~t y gradient The annual cycle of nverine nutrlent input is In phase with phytoplankton biomass but out of phase with phytoplankton productivity In tfus region kverlne nutnent Input and phytoplankton blomass peak dunng spring but phytoplankton produchvlty peaks dunng summer Seasonal variahons in biomass are correlated rivenne rutrate input whlle seasonal vanations in produchvlty are correlated w t h light and temperature Evldence 1s presented w h ~c h suggests that the spnng flux of nitrogen from the watershed and the summer productivlty maxlmum are coupled via the accumulahon and sedlmentahon of phytoplankton blomass dunng sprlng and subsequent recycling of regenerated n~trogen Into the euphotic zone during summer We conclude that the occurrence of maxlmum productivity dunng summer in the mesohahne reach of the Bay is a consequence of the recycling of nitrogen dehvered to the system durlng the previous spnng Inter-annual variations In the magmtude of the summer productivlty maximum appear to be related to variahons in vertical stratdicahon which influences the vertical flux of regenerated ammonium from the benthos to the euphotlc zone In this context the extent of seasonal oxygen depletion dunng summer appears to be determined by nverlne mtrate input durlng the spring freshet and the strength and variabihty of vertlcal stratification dunng summer 'UMCEES Contnbut~on No.