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

Abstract

Gulf Stream-induced upwelling at the shelf break of the South Atlantic Bight (SAB) presents water which, in summer, can intrude onto the continental shelf. In July 1979, an XBT 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/sup 0/C covered 3280 km/sup 2/ and occupied 38 km/sup 3/ shoreward of the 42 m isobath. Using temperature and nitrate distributions and the T/sup 0/C:NO/sup 3/ relationship, the authors determined that 3200 metric tons of nitrate-nitrogen were advected into the study area. Net nitrate-nitrogen fluxes were 32 ..mu..moles x m/sup -2/ x sec/sup -1/ across the 42 m isobath and 30 ..mu..moles x m/sup -2/ x sec/sup -1/ across the 30 m isobath. The advection of nitrate-enrichment 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 the intrusion, production was likely supported by regenerated nutrients. Summertime intrusions supply an estimated 2.9 x 10/sup 4/ mtons NO/sub 3/-N x yr/sup -1/ to the middle shelf area of themore » southern SAB and are thus a major source of nitrogen to that area.« less