7 CARIBBEAN SEA EcoSYSTEM ASSESSMENT .. • NORTii Af1ERICAN PLATE I fmtk .. CARIBBEAN PLATE ''"' j COCOS PLATE SOUTH ANERICAN PLATE FIG. 2.1. Tectonic setting of the Caribbean. SoURCE: Molnar and Sykes 1969. distinctive wet and dry seasons. moderate temperature ranges, and persistent trade winds. The wet season is associated with the seasonal northward migration of the Inter-tropical Convergence Zone (ITCZ) and a continuous series of tropical waves that move westward, some developing into depressions, tropical storms, and hurricanes. While local climate may be varied by other factors such as topogruphy or deforestation, the region as a whole experiences a distinctive hurricane season from .June to November. The Caribbean region is also inlluenced periodically by the wider El Nino/Southern Oscillation (ENSO), a multi-ycareyclc involving variations in surface temperatures and salinity, due to the changes in rainfl1ll patterns on the South American continent. Some of the hurricane activity is generated far out to the east in the Atlantic Ocean, but may also originate in the Caribbean Sea. This phenomenon is perhaps the second distinctive and de lining feature of the Caribbean Sea. The third delining feature of the Caribbean Sea is its general surface circulation pattern (Fig 2.3). The pattern is that of waters of the South Equatorial CmTcnt that !lows from a major upwelling of the southem area of Alrica !lowing across the Atlantic and along north-eastern South America into the Caribbean Sea, through the island arc of the Lesser Antilles. In the Caribbean Sea, this stream of water is known as the Antillean CuiTent that llows in a generally westward direction, exiting the Sea via the Yucatan Channel. There is also some inllow of northerly waters into the Caribbean Sea through the inter-island passages in the Greater Antilles, and persistent gyres (circular or spiml movements of water) in the Colombian and Yucatan basins. Water quality, particularly salinity and turbidity of the incoming stream, is much inlluenced seasonally by discharge from the Amazon and Orinoco rivers and those of the Guianas. The Sea is also affected by continental river discharge directly into it. 2.6 River Plume Dynamics The ecology of the Caribbean is greatly afTected by the massive quantities of fresh water and sediments enh:ring the Sea from three great South American river systems: the Amazon, Orinoco, and Magdalena. Although a large part ofthe oulllow orthe Amazon is taken eastward across the Atlantic, a significant quantity flows northward around the coast of the continent into the Eastern Caribbean and, together with the waters of the Orinoco, this creates plumes of buoyant fresh water across wide stretches of the ecosystem (Muller Karger et al. 1988, 1989). In the Western Caribbean, the plume of the Magdalena River extends north and eastward under the inlluence of a cmTent known as the Colombian gyre (Box 2.1). The influx of sediments and nutrients originating from human activities in the Magdalena drainage basin has a -28-