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This paper synthesizes >40 yr of data on phytoplankton abundance in the Chesapeake Bay, USA, spanning the period 1950 to 1990. Long-term changes in the concentrations of surface chlorophyll (B, mg m-3) and integrated water-column chlorophyll (B',,, mg m-2) are assessed in the context of light and nutrient effects on phytoplankton distributions. Significant long-term increases in B were detected from the 1950s to the 1970s in all regions of the Bay. The seaward, polyhaline Bay showed increases in B of 300 to 500%, while mesohaline and oligohaline values increased by between 40 and nearly 400%. Annual means for recent years, 1985-90, showed interannual variations of ca 80%, but trends of the magnitude observed for 1950-70 were not detected. Frequency distributions of B showed shifts in overall distributions to higher values, particularly in the lower, polyhaline Bay where the frequency of B < 2 mg m-3 in the 1950s and 1960s was much higher than in 1985-90. B',,, data showed the apparent lack of a winter-spring maximum of contemporary proportions in the 1960s, in contrast to data from the 1970s, early 1980s and 1985-90 that show a well-developed peak in B',,, for most of these years. Nutrient concentrations and ratios have also changed significantly since the 1960s. Concentrations of dissolved inorganic nitrogen (DIN) in the oligohaline Bay have approximately doubled in the past 20 to 30 yr, wMe orthophosphate (Pod3-) concentrations have generally declined, producing a change in DIN: POA3with rarmfications for nutrient limitation in the Bay. These results are discussed in the context of the regulation of interannual variations m the timing, position and magnitude of the winter-spring phytoplankton bloom in the Bay by freshwater flow from the Susquehanna River.

Long-term trends in the distribution of phytoplankton in Chesapeake Bay: roles of light, nutrients and streamflow