SPRINGTIME NUTRIENT AND CHLOROPHYLL-A CONCENTRATIONS IN THE SOUTHWESTERN GULF OF MAINE

We report nutrient and phytoplankton pigment concentrations during thr ee hydrographic surveys in the southwestern Gulf of Maine (GOM) during the South Channel Ocean Productivity Experiment (SCOPEX). During the first survey in mid-March 1988, winter conditions prevailed in the dee per central waters of the survey area, with a deep mixed layer, high s urface nutrient concentrations (NO3 > 8 mu M), and low Chl a concentra tions (<0.5 mu g l(-1)). In shallower areas along the western flank of the southwestern GOM and over Georges Bank, the spring bloom had begu n with Chl a concentrations > 2 mu g l(-1). During the second survey i n late April 1988, the spring bloom was declining in the southwestern GOM, and considerable spatial heterogeneity in the stage of the bloom and the levels of nutrients occurred. Near the sill of the Great South Channel, and east of Cape Cod where a low salinity surface plume had stabilized the water column, nutrient and phytoplankton concentrations were low (NO3 < 0.5 mu g M, chl a 1-2 mu g l(-1)). In the deeper offs hore waters in some areas, the bloom still continued (NO3 1-2 mu M, Ch l a > 4 mu g l(-1)), while in the northewastern part of the study area it had just begun (NO3 > 5 mu M, Chl a 1-2 mu g l(-1)). The spring bl oom had terminated throughout the southwestern GOM during the third su rvey in early June 1989. A low salinity surface plume that entered the study area along the western flank extended east from Cape Cod to Geo rges Bank, resulting in a strong pycnocline at 15-20 m. Nutrient and p hytoplankton concentrations in the surface plume were low in the deepe r central region of the survey area (NO3 <0.25 mu M, Chl a < 1 mu g l( -1)). Beneath this surface plume, subsurface maxima of ammonia and Chl a occurred (NH3 > 2 mu M, Chl a > 1 mu g l(-1)). Over Nantucket Shoal s and Georges Bank, the water column was well mixed due to enhanced ti dal dissipation, resulting in higher nutrient and phytoplankton concen trations. The nutrient and phytoplankton data collected in these hydro graphic surveys suggest that phytoplankton abundance during spring is controlled by seasonal hydrographic changes. These observations do not support the SCOPEX hypothesis that dense surface aggregations of the copepod Calanus finmarchicus in this region in late spring are due to unusually high phytoplankton productivity.