The importance of a plume of tidally-mixed water to the biological oceanography of the Gulf of Maine

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Abstract
During the warmer months of the year the eastern Gulf of Maine features a plume of cold water which extends from the tidally well-mixed area adjacent to Grand Manan Island at the mouth of the Bay of Fundy southwest along the Maine coast to well beyond Penobscot Bay. Near Grand Manan Island the plume waters are cold (ca. 10°C), nearly vertically isothermal and carry high concentrations of dissolved inorganic nutrients. The analysis of temperature-salinity diagrams and hydrographic vertical section plots indicate that the deeper waters in Jordan Basin, of slope water origin, upwell and contribute a significant fraction of the high nutrient concentrations. The plume waters become increasingly stratified as they flow to the southwest which leads to a phytoplankton bloom downstream. Nitrate concentrations within the euphotic zone of the plume decrease approximately linearly (ca. 194 mg-at NO3-N · m−2) along a distance of about 130 km travelled after its exit from the Grand Manan area (ca 7.5 days transit time). Cross frontal mixing and tidal flushing along the south and north sides of the plume could account for ca. 18% of this decrease. Total chlorophyll concentrations increase nearly linearly with distance along the first 80 km and then decrease in the following 50 km, presumably the result of phytoplankton being grazed by zooplankton which apparently propagate in response to the increasing chlorophyll levels. Over the distance of increasing chlorophyll concentrations (80 km) the nitrate decrease, corrected for physical losses laterally, would support a "new". primary production of 1.46 gm C m−2 d−1. Our analyses suggest that as much as 44% of the new nitrate which enters the Gulf of Maine at depth through the Northeast Channel upwells in the eastern Gulf becoming part of the plume, and hence this feature appears to be very important to the nutrient budget and general biological oceanography of the inner Gulf of Maine.