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 \mth 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

Seasonal variations in the distribution of dissolved inorganic nitrogen, sihcon, and phosphorus along the salinity gradient of Chesapeake Bay from 1984 to 1988 suggest that dissolved silicate (DSi) controls the magnitude of diatom production during the spring bloom, causes the collapse of the spring bloom, and leads to changes in floristic composition. High sedimentation rates of chlorophyll biomass observed during this per~od could be due to Si-deficiency, suggesting that the supply of DSi may also control the flux of phytoplankton biomass to the benthos, an important parameter of seasonal oxygen depletion in the Bay.

ABSTRACT-Although it is increasingly clear that picoplankton play a major role in the oceanic carbon cycle, relatively little is known concerning the significance of picoplankton in coastal systems subject to significant evironmental variance on tidal to interannual scales. Here w e report on seasonal and interannual patterns of variability in the productivity and biomass of phototrophic and heterotrophic picoplankton (P-and H-PICO, respectively) and on the flow of carbon from phytoplankton to H-PICO. Annual cycles in the biomass and productivity of both picoplankton trophic levels exhibit winter-spring minima and summer maxima but do not appear to be directly coupled in terms of carbon flow from P-to H-PICO. H-PICO exceeds P-PICO durlng sprlng when picoplankton productivity is 1 0~1 , and P-PICO exceeds H-PICO during summer when productivity is high. P-PICO productlvity and biomass increase rapidly each year to an early summer peak (20",, of total phytoplankton productlvity on average) immediately following the collapse of the spring diatom bloom. In contrast, H-PICO productivity and the abundance of bacterioplankton (the predominant group of H-PICO) increase slowly to a late summer peak (equivalent to 16 % of phytoplankton productivity on average). Recently released phytoplankton exudates are a major source of dissolved organic carbon with H-PICO taking u p an average of 54 % during spring and 8 3 ?h during summer Variations in H-PICO are closely coupled to the release of DOC, most of which is produced by phytoplankton > 2 pm. New nutrient input to the Bay appears to be coupled to H-PICO productivity \rla increases in the productlvlty and DOC release of phytoplankton > 2 pm in slze.

A mesocosm experiment was conducted in which estuarine plankton population dynamics and rates of production, consumption, and nitrogen uptake and release were compared for 13 d in 400 1 vats with and without additions of the diatom Thalassiosira weissflogii and the copepod Acartia tonsa. In the vats with copepods, both phytoplankton biomass and production were reduced by grazing whereas in the vats without copepods, nutrient limitation reduced both phytoplankton production and standing stock. Bacteria dominated both estimated heterotrophic carbon biomass and production early in the experiment. However toward the end of the experiment, the biomass of zooplankton was often greater than the combined carbon pools of phytoplankton, bacteria and heterotrophic flagellates. Highest release rates of ammonium and dissolved free amino acid nitrogen occurred in the vats with copepods. Highest bacterial growth rates also occurred in the presence of copepods suggesting that grazing activity and copepod excretion stimulated the growth of bactena. An Inverse relation between the biomass of copepods and flagellates suggests that the copepods consumed significant quantities of heterotrophic nanoflagellates. Estimated growth rates of flagellates were highest in the vats with copepods. Our results demonstrate that the presence oi copepoas can result in more eiiicient nutrient recycling, higher primary production/chlorophy11 and greater production by bacteria and heterotrophlc nanoflagellates.

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