Pigment Distribution in the Caribbean Sea: Observations from Space

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The Caribbean is a semi-enclosed tropical sea which is generally considered oligotrophic, but that is influenced by nearly 20% of the annual discharge of the world's rivers (Amazon and Orinoco Rivers) and by seasonal upwelling along the southern margin. To investigate the role of these nutrient sources on the productivity of the region, we mapped the distribution of pigments in the eastern Caribbean (east of 80° W) using a series of Coastal Zone Color Scanner (CZCS) images collected between November 1978 and December 1982. Five additional images were examined for the period 1983-1986. The images revealed a seasonal cycle in the spatial structure of near-surface pigment. During January-May, there were high pigment concentrations (> 0.5 mg m-3) along the continental margin (south of 14°N), where upwelling occurred. Very little pigment (< 0.2 mg m-3) was found in the northern half of the Caribbean at this time. The frequency of upwelling-related blooms decreased after July, but the seasonally-expanding plume of the Orinoco River dispersed pigment over a large area of the Caribbean (> 3 × 105 km2). This plume reached Puerto Rico around September-October and drifted westward, slowly losing its color signature. We estimate that the discharge of the Orinoco contributes 2-12% of the daily nitrogen requirements of the phytoplankton growing in the river plume, and leads to the fixation of 7-29 × 105 tons of carbon per year. The rest of the nitrogen demand appears to be met by nitrogen cycling. The large-scale (> 100 km) pigment distribution patterns in the Caribbean Sea seem to be controlled by wind stress, flux of water through the basin, and river discharge. Westward advection of Atlantic water probably dominates the flow during the first half of the year, restricting the dispersal of blooms to the southern half of the Caribbean while flushing the central and northern portions. As the influx of Atlantic water decreases in the second half of the year, local Ekman transport driven by the trade winds becomes dominant and surface waters drift northwestward throughout the basin. The seasonal sequence of changes in pigment distribution patterns was consistent from year to year except in 1980 and 1984, when wind conditions in the Caribbean were anomalous. Close scrutiny of the 4 years of CZCS images did not reveal any evidence of large-scale (> 300 km) eastward-flowing currents in the central Caribbean. This supports the view that previous observations of countercurrents were based on partial sampling of eddies of 100-250 km diameter.

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Progress in Oceanography, v. 23, issue 1, p. 23-64