SCALES OF NUTRIENT-LIMITED PHYTOPLANKTON PRODUCTIVITY IN CHESAPEAKE BAY

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton communi ty and on the capacity of consumers to respond to increases in phytopl ankton productivity. Overenrichment and associated declines in water q uality occur when phytoplankton growth rate becomes nutrient-saturated , the production and consumption of phytoplankton biomass become uncou pled in time and space, and phytoplankton biomass becomes high and var ies on scales longer than phytoplankton generation times. In Chesapeak e Bay, phytoplankton growth rates appear to be limited by dissolved in organic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs o f DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N: P and Si:P > 100), seasonal accumulations of phytoplankton biomass wit hin the salt-intruded-reach of the bay appear to be limited by riverin e DIN supply while the magnitude of the spring diatom bloom is governe d by DS1 supply. Seasonal imbalances between biomass production and co nsumption lead to massive accumulations of phytoplankton biomass (ofte n > 1,000 mg Chi-a m(-2)) during spring to spring-sumner oxygen deplet ion (summer bottom water <20% saturation), and to exceptionally high l evels of annual phytoplankton production (>400 g m(-2) yr(-1)). Nitrog en-dependent seasonal accumulations of phytoplankton biomass and annua l production occur as a consequence of differences in the rates and pa thways of nitrogen and phosphorus cycling within the bay and underscor e the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.