The effect of summertime shelf break upwelling on nutrient flux in southeastern United States continental shelf waters

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Abstract
Gulf Stream-induced upwelling at the shelf break of he South Atlantic Bight (SAB) presents water which, in summer, can intrude onto the continental shelf. In July 1979, an expendable bathythermograph survey of the continental shelf revealed such an intrusion of cold water off St. Augustine, Florida. From weekly mappings, it was determined that Gulf Stream water > 22.5.degree. C covered 3280 km2 and occupied 38 km3 shoreward of the 42 m isobath. Based on temperature and nitrate distributions and the T.degree.C:NO3 relationship, 3200 metric tons of NO3-N were advected into the study area. Net NO3-N fluxes were 32 .mu.mol .cntdot. m-2 .cntdot. s-1 across the 42 m isobath and 30 .mu.mol .cntdot. m-2 .cntdot. s-1 across the 30 m isobath. The advection of nitrate-enriched water into the photic zone caused a dramatic increase in phytoplankton biomass. The decreasing nitrate concentrations correlated with chlorophyll increases indicating phytoplankton production was mainly at the expense of nitrate advected into the area. Prior to intrusion, production was likely supported by regenerated nutrients. Summertime intrusions supply an estimated 2.9 .times. 104 metric tons NO3-N .cntdot. yr-1 to the middle shelf area of the southern SAB and are thus a major source of N to that area.

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