NUTRIENTS, PLANKTON, AND PRODUCTIVITY IN A WARM-CORE RING IN THE WESTERN GULF OF MEXICO

Argos drift buoy trajectory data showed that a region of anticyclonic circulation about 100 km in diameter was present over the upper continental slope of the NW corner of the Gulf of Mexico in September-October, 1988. Guided by these data, Texas A&M University scientists joined by colleagues from Mexico's Direccion General de Oceanografia Naval surveyed the area from October 17-22 on R/V Gyre cruise 88G-05 with a dense grid of conductivity-temperature-depth and expendable bathythermograph stations. The presense of a subsurface salinity maximum greater than 36.5 psu within the upper 150 m of this anticyclone indicated that it had originated as a warm-core eddy of the Loop Current; however, a maximum of only 36.54 psu at sigma(t) = 25.5 in contrast to as much as 36.88 psu at this density surface in a "fresh" ring indicated that this feature had spent many months in the western gulf since its separation from the Loop Current. Biologically, the warm-core ring was oligotrophic: its surface waters were generally depleted in nitrate to depths of more than 100 m, and chlorophyll standing stocks (< 20 mg m-2), primary productivity (< 0.4 mg C m-3 h-1), and zooplankton biomass (only 4 mL 100 m-3 day and 6 mL 100 m-3 night in the upper 200 m) were all extremely low. Deployments of floating sediment traps within the feature on two consecutive days intercepted fluxes of only 10-20 mg dry weight m-2 d-1 at 50 m and at 100 m. By comparison, at ring periphery where there was measurable nitrate at 100 m (0.2-1-mu-g-at. L-1), chlorophyll standing stocks and primary production in the surface mixed layer were 1.5-2 times higher.