MESOSCALE PROCESSES AFFECTING PHYTOPLANKTON ABUNDANCE IN THE SOUTHERNCARIBBEAN SEA

The variability of phytoplankton biomass in the Caribbean Sea south of 14 degrees N and east of 80 degrees W is examined with a time series of high spatial resolution Coastal Zone Color Scanner (CZCS) satellite images. These synoptic pigment fields are compared with SST, wind, se a lever, and Orinoco River discharge series (1979-1982). The informati on helped outline the spatial extent of upwelling centers and their se asonal variability, and the influence of the Orinoco's discharge on th e margin of the southern Caribbean Sea. A seasonal SST cycle was obser ved throughout the Caribbean, with peaks between August and November ( ca 29 degrees C), and minima between January and June (ca 25.5 degrees C). Waters within 100 km of the continent were consistently similar t o 0.5 degrees C cooler. Along-shore wind stress intensified from Novem ber (ca -0.05 N m(-2)) to maxima in April-May (ca -0.10 N m(-2)), then weakened through June and July, and remained low (-0.04 to -0.07 N m( -2)) until November. Low-frequency (>26 h) filtered coastal sea level was usually below the mean (<-0.04 m) between January and April, near the mean between May and August, and then above the mean (>0.04 m) fro m September to November, The Orinoco discharged a mean 3.9 x 10(4) m(3 ) s(-1), ranging from 1 x 10(4) m(3) s(-1) in March to about 7 x 10(4) m(3) s(-1) in August. The data show an inverse relationship between p igment concentration and coastal sea level, and a direct relationship with zonal wind stress, both at seasonal time scales and at shorter ev ent scales. The frequency and area of blooms was larger during periods of strong wind (January-April), and smaller during June-October. Mean pigment concentration was higher around Margarita (4 year mean of 1.2 mg m(-3)) than off. central Venezuela (4 year mean of 0.45 mg m(-3)), which indicated that more intense upwelling occurs near the former br oad continental shelf, where the horizontal, cross-wind scare of coast al upwelling may be smaller than over a narrow shelf. Pigment concentr ations were particularly high near capes and headlands. Concentrations off eastern/central Venezuela and the Orinoco's flow were inversely r elated. In contrast, pigments near Tobago varied directly with dischar ge. During the first half of the year, the Orinoco plume was located b etween Trinidad and Tobago, and pigments around Tobago were low (<0.5 mg m(-3)). During July-November, the plume engulfed Tobago, Grenada an d St Vincent, with values >1 mg m(-3). Interannual variation was evide nt in the series: sea level was unusually high (daily mean sea level u p to 0.25 m) during the second half of 1979, and unusually low (daily mean as low as -0.20 m) in early 1980. The latter corresponded with ex tended periods of unusually strong trade winds and coincided with peri ods of extremely large patches of high pigments along the coast.