MESOSCALE PROCESSES AFFECTING PHYTOPLANKTON ABUNDANCE IN THE SOUTHERN CARIBBEAN 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, sea lever, and Orinoco River discharge series (1979-1982). The information helped outline the spatial extent of upwelling centers and their seasonal variability, and the influence of the Orinoco's discharge on the margin of the southern Caribbean Sea. A seasonal SST cycle was observed 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 to 0.5 degrees C cooler. Along-shore wind stress intensified from November (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) from 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 pigment concentration and coastal sea level, and a direct relationship with zonal wind stress, both at seasonal time scales and at shorter event 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 broad continental shelf, where the horizontal, cross-wind scare of coastal upwelling may be smaller than over a narrow shelf. Pigment concentrations were particularly high near capes and headlands. Concentrations off eastern/central Venezuela and the Orinoco's flow were inversely related. In contrast, pigments near Tobago varied directly with discharge. During the first half of the year, the Orinoco plume was located between Trinidad and Tobago, and pigments around Tobago were low (<0.5 mg m(-3)). During July-November, the plume engulfed Tobago, Grenada and St Vincent, with values >1 mg m(-3). Interannual variation was evident in the series: sea level was unusually high (daily mean sea level up 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 extended periods of unusually strong trade winds and coincided with periods of extremely large patches of high pigments along the coast.