Lateral variation in the production and fate of phytoplankton in a partially stratified estuary

It is generally believed that the high productivity of many estuaries is a consequence of both allochthonous nutrient inputs and autochthonous recycling of nutrients among producers and decomposers of organic matter. However, the mechanisms by which nutrients are recycled between sources and sinks are not clear We have documented time-dependent variations in density structure along a transect normal to the main axis of Chesapeake Bay which may be important in this regard. These variations influenced lateral distributions of dissolved inorganic nutrients, oxygen, chlorophyll a, and bacterioplankton and appeared to be responsible for high phytoplankton production over the flanks of the main channel relative to production over the channel. Vertical and seasonal variations in bacterial abundance were correlated with phytoplankton biomass, and increases in bacterial abundance were related to the development of phytoplankton bloon~s with lags of 0 to 6 d. Bacterial biomass and production were high throughout the study period, averaging 20 O/ O to over 50 O/ O of phytoplankton biomass and production, respectively. Absolute levels of bacterial abundance were among the highest reported and suggest that bacteria are responsible for a large function of the carbon flux in Chesapeake Bay. Relations among phytoplankton production, bacterial abundance and sediment trap collections indicate that variations in density structure reflect a transverse circulation which may explain how nutrients regenerated below the pycnocline are rapidly recycled into the euphotic zone above. This could be an important mechanism by which river-borne nutrient inputs during spring are coupled to high phytoplankton production during summer.