LONG-TERM TRENDS IN THE DISTRIBUTION OF PHYTOPLANKTON IN CHESAPEAKE BAY - ROLES OF LIGHT, NUTRIENTS AND STREAMFLOW

This paper synthesizes >40 yr of data on phytoplankton abundance in th e Chesapeake Bay, USA, spanning the period 1950 to 1990. Long-term cha nges in the concentrations of surface chlorophyll (B, mg m(-3)) and in tegrated water-column chlorophyll (B-wc' 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 showe d increases in B of 300 to 500 %, while mesohaline and oligohaline val ues increased by between 40 and nearly 400 %. Annual means for recent years, 1985-90, showed interannual variations of ca 80 %, but trends o f the magnitude observed for 1950-70 were not detected. Frequency dist ributions 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- wc' data showed the apparent lack of a winter-spring maximum of contem porary proportions in the 1960s, in contrast to data from the 1970s, e arly 1980s and 1985-90 that show a well-developed peak in B-wc' for mo st of these years. Nutrient concentrations and ratios have also change d significantly since the 1960s. Concentrations of dissolved inorganic nitrogen (DIN) in the oligohaline Bay have approximately doubled in t he past 20 to 30 yr, while orthophosphate (PO43-) concentrations have generally declined, producing a change in DIN:PO43- with ramifications for nutrient limitation in the Bay, These results are discussed in th e context of the regulation of interannual variations in the timing, p osition and magnitude