Springtime nutrient and chlorophyll a concentrations in the southwestern Gulf of Maine

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We report nutrient and phytoplankton pigment concentrations during three 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 deeper central waters of the survey area, with a deep mixed layer, high surface nutrient concentrations (NO3 > 8 μM), and low Chl a concentrations (<0.5 μg l-1). In shallower areas along the western flank of the southwestern GOM and over Georges Bank, the spring bloom had begun with Chl a concentrations >2 μg 1-1. During the second survey in 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 μM, Chl a 1-2 μg 1-1. In the deeper offshore waters in some areas, the bloom still continued (NO3 1-2 μM, Chl a > 4 μg 1-1), while in the northeastern part of the study area it had just begun (NO3 > 5 μM, Chl a 1-2 μg l-1 The spring bloom had terminated throughout the southwestern GOM during the third survey in early June 1989. A low salinity surface plume that entered the study area along the western flank extended east from Cape Cod to Georges Bank, resulting in a strong pycnocline at 15-20 m. Nutrient and phytoplankton concentrations in this surface plume were low in the deeper central region of the survey area (NO3 <0.25 μM, Chl a < 1 μg 1-1 Beneath this surface plume, subsurface maxima of ammonia and Chl a occurred (NH3 > 2 μM, Chl a > 1 μg 1-1). Over Nantucket Shoals and Georges Bank, the water column was well mixed due to enhanced tidal dissipation, resulting in higher nutrient and phytoplankton concentrations. The nutrient and phytoplankton data collected in these hydrographic 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. © 1994 Elsevier Science Ltd.

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Continental Shelf Research