Plankton studies in Buzzards Bay, Massachusetts, USA. VI. Phytoplankton and water quality, 1987 to 1998

Abstract

From October 1987 through September 1998, phytoplankton composition, inorganic nutrients, temperature, salinity, water clarity, and chlorophyll a (chl a) + phaeopigments were moni- tored during 141 monthly cruises in Buzzards Bay, Massachusetts, USA. There were large seasonal and interannual variations in nitrate, silicate and phytoplankton abundance and composition. There were no consistent interannual patterns of recurrent spring blooms, measured as either chl a or phytoplankton cell abundance. Rather, blooms of various taxa were sporadic, particularly diatom blooms in the late summer and fall. Other parameters were similar bay-wide on a given day, and over seasons and years for stations away from New Bedford Harbor, but concentrations of ammonium, phosphate, and chl a were elevated at the New Bedford sewage outfall, prior to its conversion to sec- ondary treatment in September 1996. After conversion, increased water transparency and decreased levels of ammonium indicated improved water quality at the outfall. Bay-wide chl a and nitrate had significant declines over time from the late 1980s to the late 1990s. There were no significant linear trends with time for annual means of temperature, but there was a significant trend of increased warming in spring. Phytoplankton abundance (0.012 to 26.0 × 10 6 cells l -1 , 1988 to 1998) was higher than reported in other coastal waters of New England, due to preservation that did not destroy the delicate microflagellates and phytoflagellates which dominated phytoplankton abundance. Poten- tially harmful algal species included diatoms of the genus Pseudo-nitzschia, and the dinoflagellate Alexandrium fundyense. Buzzards Bay is a favorable habitat for phytoplankton in that it is well- mixed and well-illuminated, and nutrient-replete.