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APPENDIX E 325 wich Trench, in the Antarctic Ocean, on the east of the South Sandwich Island and adjoining the Scotia Ridge. The trench extends 600 miles and reaches a maximum depth of 8,260 meters (m), located between latitude 55Â°S longitude 32Â°W and latitude 61Â°S longitude 27Â°W (Fairbridge, 1966). Two to three thousand tourists each year visit the Antarctic. Palmer Station, a U.S. research base on the peninsula, has become such a popular destination that a quota has been introduced (Elder and Pernetta, 1991). It is estimated that total annual production of plant matter in surface waters south of the Antarctic Convergence is 610 million tonnes. The Baltic Sea The Baltic Sea, including the Gulf of Bothnia and the Gulf of Finland, is the largest area of brackish water in the ocean system (Pickard and Emery, 1990). It is brackish because precipitation and runoff greatly exceed evaporation (Sverdrup et al., 1942). Its bottom topography is irregular, with a mean depth of 57 m and a number of basins, the deepest of which is 459 m deep (Pickard and Emery, 1990). The Baltic is connected to the Atlantic Ocean at its southwest end through intricate passages. Its sill depth in the narrows between Gedser and the Darss is about 18 m, leading to the Kattegat and the North Sea (Pickard and Emery, 1990). Evaporation and precipitation each are estimated at about 47 centimeters (cm) per year, thus canceling one another. Annual river runoff is equivalent to 130 cm of water over the entire sea; however, there are significant year-to-year variations (Pickard and Emery, 1990). Overall general circulation is weak (approximately 1 cm per second) (Pickard and Emery, 1990), as there are no tidal currents to disturb the stratification (Dietrich, 1963). Because of shallow sill depth, a rejuvenation of the deep water occurs only when large-scale meteorological conditions can override the estuarine circulation. These conditions are not rare: Significant vertical mixing can occur because the Baltic basin is so shallow and broad (Gross, 1967). Thus, it is possible for the residence time of the Baltic to be less than one year, although this is variable. The area's humid climate aids the development of a density discontinuity layer, thus greatly Preventing a thermohaline convection. The Baltic is a two- layer system with a well-mixed upper layer 30-50 m deep in the south, increasing to 60-70 m in the central Baltic (Pickard and Emery, 1990). Dissolved oxygen may reach saturation levels in the surface layers but is relatively low in deep water. Changes may occur on a decadal time scale and are related to variations of inflowing water to the south. There has been a general trend toward decreasing dissolved oxygen values since the beginning of the twentieth century (Pickard and Emery, 1990). In many of the deep basins which have a residence time of several years, anoxic conditions occur (Pickard and Emery, 1990).