A semienclosed brackish sea located in a humic zone, with a positive water balance relative to the adjacent ocean (the North Sea and the North Atlantic). The Baltic is connected to the North Sea by the Great Belt (70% of the water exchange), the Øresund (20% of the water exchange), and the Little Belt. The total area of the Baltic is 147,414 mi2 (381,705 km2), its total volume 4982 mi3 (20,764 km3), and its average depth 181 ft (55.2 m). The greatest depth is 1510 ft (459 m), in the Landsort Deep. The topography of the Baltic is characterized by a sequence of basins separated by sills and by two large gulfs, the Gulf of Bothnia (40,100 mi2 or 104,000 km2) and the Gulf of Finland (11,400 mi2 or 29,500 km2). More than 200 rivers discharge an average of 104 mi3 (433 km3) annually from a watershed area of 637,056 mi2 (1,649,550 km2). The largest river is the Newa, with 18.5% of the total fresh-water discharge. From December to May, the northern and eastern parts of the Baltic are frequently covered with ice. On the average, the area of maximum ice coverage is 82,646 km2 (214,000 km2). The mean maximum surface-water temperature in summer is between 59 and 63°F (15 and 17°C). As the Baltic stretches from the boreal to the arctic continental climatic zone, there are large differences between summer and winter temperature in the surface waters, ranging from about 68 to 30°F (20 to −1°C) in the Western Baltic and 57 to 32°F (14 to −0.2°C) in the Gulf of Bothnia and the Gulf of Finland. The salt content of the Baltic waters is characterized by two major water bodies; the brackish surface water and the more saline deep water. Salinities for the surface water range from 8 to 6‰ in the Western and Central Baltic and 6 to 2000 in the Gulf of Bothnia and the Gulf of Finland; salinities for the deep water range from 18 to 13‰ in the Western and Central Baltic and 10 to 4‰ in the Gulf of Bothnia and the Gulf of Finland. The surface currents of the Baltic are dominated by a general counterclockwise movement and by local and regional wind-driven circulations. A complex system of small- and medium-scale gyres develops especially in the central parts of the Baltic. The currents in the Belt Sea are dominated by the topography; they are due to sea-level differences between the Baltic proper and the North Sea. Tides are of minor importance, ranging between 0.8 and 4.7 in. (2 and 12 cm). Water-level changes of more than 6 ft (2 m) occur occasionally as a result of onshore or offshore winds and the passage of cyclones over the Baltic Sea area. The frequency of longitudinal sea-level oscillations is about 13.5 h.Ocean circulation The flora and fauna of the Baltic are those of a typical brackish-water community, with considerably reduced numbers of species compared to an oceanic community. The productivity is relatively low compared to other shelf seas. The major commercially exploited species are cod, herring, sprat, flounder, eel, and salmon, and some fresh-water species such as whitefish, pike, perch, and trout. The total annual catch amounts to about 880,000 tons (800,000 metric tons). The Baltic is completely divided into fishery zones, with exclusive fishing rights belonging to the respective countries. Other than fish the only major resources that have been exploited are sand and gravel in the Western Baltic Sea. It is believed that the deeper layer under the Gotland Basin contains mineral oil, but so far only exploratory drilling has been carried out in the near-coastal regions. Limited amounts of mineral oil have also been located in the Gulf of Kiel. |
