Spatial and temporal variation of resource limitation in Chesapeake Bay

We report nutrient addition bioassays at 18 stations in Chesapeake Bay (USA ) to assess resources limiting phytoplankton growth. Data were pooled from several sampling programs conducted from 1989 to 1994. Spatially, light and P limitation declined from low salinity regions to high salinity regions, as N limitation increased. This spatial pattern was driven primarily by fre shwater inflows with high N/P and seawater inflows with low N/P. Seasonally , there was a marked progression of winter light limitation, spring P limit ation, and summer N limitation at mesohaline and polyhaline stations. The s easonal pattern appeared to be caused by temperature, mixing, river dischar ge, and sediment P fluxes. At high salinity stations, we also observed wint er N limitation (caused by DIN depletion prior to spring nitrate delivery), and at lower salinity stations there was fall P limitation (caused by reae ration of bottom sediments). At tidal fresh stations, turbidity and nutrien t concentrations resulted in continuous light limitation, except at some st ations in summer. Interannual decreases in light limitation and increases i n N and P limitation appear to represent improvements in water quality.