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[1] Previous hypotheses had suggested that upwelled intrusions of nutrient-rich Gulf of Mexico slope water onto the West Florida Shelf (WFS) led to formation of red tides of Karenia brevis. However, coupled biophysical models of (1) wind-and buoyancy-driven circulation, (2) three phytoplankton groups (diatoms, K. brevis, and microflagellates), (3) these slope water supplies of nitrate and silicate, and (4) selective grazing stress by copepods and protozoans found that diatoms won in one 1998 case of no light limitation by colored dissolved organic matter (CDOM). The diatoms lost to K. brevis during another CDOM case of the models. In the real world, field data confirmed that diatoms were indeed the dominant phytoplankton after massive upwelling in 1998, when only a small red tide of K. brevis was observed. Over a 7-month period of the CDOM-free scenario the simulated total primary production of the phytoplankton community was $1.8 g C m À2 d À1 along the 40-m isobath of the northern WFS, with the largest accumulation of biomass on the Florida Middle Ground (FMG). Despite such photosynthesis, these models of the WFS yielded a net source of CO 2 to the atmosphere during spring and summer and suggested a small sink in the fall. With diatom losses of 90% of their daily carbon fixation to herbivores the simulation supported earlier impressions of a short, diatom-based food web on the FMG, where organic carbon content of the surficial sediments is tenfold those of the surrounding seabeds. Farther south, the simulated near-bottom pools of ammonium were highest in summer, when silicon regeneration was minimal, leading to temporary Si limitation of the diatoms. Termination of these upwelled pulses of production by diatoms and nonsiliceous microflagellates mainly resulted from nitrate exhaustion in the model, however, mimicking most del 15 PON observations in the field. Yet, the CDOM-free case of the models failed to replicate the observed small red tide in December 1998, tagged with the del 15 N signature of nitrogen fixation. A large red tide of K. brevis did form in the CDOMrich case, when estuarine supplies of CDOM favored the growth of the shade-adapted, ungrazed dinoflagellates. The usual formation of large harmful algal blooms of >1 ug chl L À1(10 5 cells L À1 ) in the southern part of the WFS, between Tampa Bay and Charlotte Harbor, must instead depend upon local aeolian and estuarine supplies of nutrients and CDOM sun screen, not those from the shelf break. In the absence of slope water supplies, local upwelling instead focuses nitrate-poor innocula of co-occurring K. brevis and nitrogen fixers at coastal fronts for both aggregation and transfer of nutrients between these phytoplankton groups.

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...

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