PLANKTON STUDIES IN BUZZARDS BAY, MASSACHUSETTS, USA .2. NUTRIENTS, CHLOROPHYLL-A AND PHEOPIGMENTS, 1987 TO 1990

As part of a larger study of hydrography, water chemistry and plankton in Buzzards Bay, Massachusetts, USA, we quantified levels of nutrients (ammonium, nitrate, nitrite, orthophosphate, silicate), chlorophyll a and phaeopigments from 1 October 1987 to 11 September 1990 (n = 1080 for each parameter). Although there were considerable monthly, seasonal and interannual fluctuations of some variables, certain trends were apparent. Ammonium comprised up to 99.79 % (mean = 75.79 %) of total dissolved inorganic nitrogen (DIN = NH4+ + NO3- + NO2-). This was due to nutrient loading at one station from a sewage outfall, and to resuspension of benthic nutrients due to shallow depth and frequent mixing throughout the entire bay. Nitrate levels were maximal in winter, whereas those of orthophosphate were higher during the warmer months. Both variables exhibited interannual variation. Silicate exhibited sustained bay-wide summer increases in all 3 years, followed by precipitous declines in autumn. Inverse relationships between silicate and diatoms during 1987 to 1988 suggest that silicate fluctuations were biologically related, in that the summer increase was a rebound from heavy utilization during the spring diatom bloom, and the autumn silicate decline was due to utilization by a fall diatom bloom. Chlorophyll a levels were comparatively high (>5.0 mg 1-1) and relatively uniform throughout the bay except for higher concentrations near the sewage outfall, or within an enclosed harbor where a hurricane dike appears to increase residence times of phytoplankton blooms by reducing circulation. Buzzards Bay is a habitat favorable to high phytoplankton production because shallow depth and frequent mixing result in a water column that is holomictic and euphotic throughout most of the year, and the sediments act as a nutrient pump injecting ammonium and other remineralized nutrients into the water column.