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 Direecion General de Oeeanografia 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 a 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'l), 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 •tg-at. L'l), chlorophyll standing stocks and primary production in the surface mixed layer were 1.5-2 times higher. 1979; Vukovich and Crissman, 1986; Vukovich and Hamilton, 1989]. Unfortunately, the ability to locate Gulf of Mexico warm-core rings by their sea surface temperature anomaly is usually limited seasonally to the period November-May, for during the rest of the year surface waters are so uniformly warm that anticyclonic rings cannot be readily distinguished from the surrounding NW continental slope water. Naval (SEDEMAR). It was the second autumn cruise in a continuing initiative "Analysis Multidisciplinario de Investigaciones en Golfo Occidental" (AMIGO) which was begun by TAMU-SEDEMAR in the fall of 1987 [see Biggs et al., 1988]. A few weeks before the scheduled sailing date, the Science Applications International Corporation (SAIC) joined the planning process, sharing with TAMU their own plans to use R/V Pelican and Naval Ocean Research and Development Activity (NORDA) aircraft to survey a warm ring which had separated from the Loop Current in summer 1988 and which by fall 1988 had moved into the western gulf.Argos drifter trajectory data processed by SAIC showed that two drifters (3345 and 3353) which had been deployed earlier in 1988 indicated that three regions of anticyclonic circulation were present west of 90øW. One of these was the warm ring formed in summer 1988 which had been seeded with drifter 3345; since this was the sixth ring to be drifter tracked by SAIC, SAIC data reports refer to it as edd...

On the use of a simple primary productivity model to assess the skill of a physical ocean model

“…The intruding LC and the eddy it sheds can be easily seen in IR and ocean color imagery as well as in altimetric sea surface height anomalies (Elliott 1982). The intrinsic mesoscale variability that characterizes these eddies has a profound influence on determining both the heat (Sturges et al 1993, Vukovich and Crissman 1986, Vukovich and Maul 1985 and nutrient (Biggs 1992) budgets of the region.…”

“…The Loop Current (LC) transports relatively warm, nutrient-poor subtropical waters from the Atlantic into the Gulf. The Loop Current intrudes deep into the Gulf (Biggs 1992) before shedding an anticyclonic eddy and exiting the Gulf (e.g. Elliott 1982).…”

Fish Resources of the Gulf of Mexico

“…There is large spatiotemporal variability in oceanographic conditions, with the Gulf of Mexico influenced greatly by inflows and discharges from rivers and other land-based sources, including the Mississippi River, and by large-scale oceanographic features, such as the Loop Current and associated core eddies of different thermal conditions (Govoni and Grimes 1992; Sturges and Leben 2000). Combined, these factors result in large spatiotemporal variability in physicochemical conditions, causing primary production to vary markedly within and across areas of different oceanographic conditions in the Gulf of Mexico (Grimes and Finucane 1991;Biggs 1992). Physical-chemical variability affects the distribution, growth, and mortality of pelagic larvae of many fish species in the Gulf of Mexico (Govoni et al 1989;DeVries et al 1990;Lang et al 1994).…”

Foraging strategy switch of a top marine predator according to seasonal resource differences

“…This is consistent with our results which showed seals foraging in winter were more likely to spend greater time foraging at depth and increase foraging success in response to elevated plankton densities. Strong meandering meso-scale eddies created by the energetic ACC (Chelton et al, 2007) are thought to facilitate plankton accumulation (Godo et al, 2012) and retention times long enough to transfer energy to different trophic levels (Biggs, 1992;Riandey et al, 2005;Benitez-Nelson and McGillicuddy, 2008), including fish (e.g., Nishimoto and Washburn, 2002;Zainuddin et al, 2006) and apex predators such as SESs (d 'Ovidio et al, 2013). These advected water parcels are thought to sustain the pelagic ecosystem east of its origin and could explain the progressively eastward displacement of winter foraging seals.…”