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APPENDIX E 327 Gulf of Mexico The Gulf of Mexico forms the northeastern component of the Wider Caribbean. It is surrounded by the Yucatan Peninsula, Cuba, and the Florida coast and exhibits a wide continental shelf. Its northern shoreline consists mostly of sedimentary material derived from Mississippi River Basin. The western and southern coasts are characterized by large lagoons separated from the sea by barrier beaches. Residence times for water within lagoons varies widely. In the Great Barrier Reef, times of 0.5-4 days have been estimated for lagoons of 2-10 km in diameter; for Bikini Atoll, 40-80 days, and for the very shallow Fanning Atoll (18 km long but only a few meters deep), periods of up to 11 months have been estimated (Pickard and Emery, 1990). The salinity of lagoons varies with tidal action, evaporation, and freshwater input from rain and runoff from land (Elder and Pernetta, 1991). The lagoons serve as an important habitat. The Gulf of Mexico and the Caribbean Sea are connected by the Yucatan Channel (sill depth about 1,600 m). The topography is rugged, with great contrasts between ridges and troughs. This is an area of particular interest for tectonophysical and geophysical studies, due to the presence of pronounced gravity anomalies, volcanism, and strong seismic activity (Neumann and Pierson, 1966). The Gulf of Mexico may be divided into two halves, based on the character of its circulation. The eastern part is dominated by the Loop Current, whose water originates in the northwestern Caribbean Sea as the Yucatan Current and flows into the central eastern gulf. The Yucatan Current flows over a sill between the Yucatan and western Cuba and deepens to 1,800 m in the Gulf (Pinet, 1992). From there, it veers eastward and exits to the south of Florida. (Water in the eastern Gulf that is deeper than 600 m remains in the basin, trapped by a shallow sill south of Florida [Pinet, 1992].) This current rotates clockwise and has surface speeds of 50-200 cm per second (Pinet, 1992). In contrast, circulation is weak and variable in the western half of the Gulf of Mexico where the clockwise surface flow averages less than 50 cm per second (Pinet, 1992). Primary productivity in the Gulf of Mexico is generally low, averaging about 25 grams (g) carbon per m3 per year; however, some areas are much more productive due to upwelling and an inflow of nutrients from the Mississippi River. In the northern Gulf, primary productivity ranges from 250-350 g carbon per m3 per year. Almost two-thirds of the United States contributes to freshwater runoff into the Gulf of Mexico, greatly stressing the environment. Most pollutants discharged by U.S. rivers are dispersed in the western Gulf, where levels may build up due to the weakly circulating water of the area (Pinet, 1992).
