Nutrients, plankton, and productivity in a warm‐core ring in the western Gulf of Mexico

Abstract

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 σ 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 μg‐at. L −1 ), chlorophyll standing stocks and primary production in the surface mixed layer were 1.5–2 times higher.