||Matti Lepparanta, Kai Myrberg
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The Baltic Sea is a unique basin - it is relatively shallow and small, rather a series of basins connected to the main Atlantic Ocean only via the Danish Straits. The exchange of water through these series of straits is limited, and as a consequence of the positive freshwater balance the Baltic Sea water mass is brackish, with the mean salinity about seven per cent — 1/5 of the salinity of normal ocean waters. This elongated sea lies between maritime temperate and continental sub-Arctic climate zones. In winter it is partly covered by ice and during the most severe winters it is completely frozen over. The variable coastal geomorphology and the wide archipelago areas give the Baltic Sea its individual appearance. There are four major brackish water basins in the World Ocean. Listed from the largest to the smallest they are the Black Sea located between Europe and Asia Minor, the Baltic Sea. The Gulf of Ob in the Kara Sea and Chesapeake Bay, surrounded by Virginia and Maryland on the east coast of the United States of America. They all developed into brackish water basins during the Holocene. During the Weichselian glaciation the Black Sea was a freshwater lake, the Baltic Sea and the Gulf of Ob were under the ice sheet, and Chesapeake Bay was a river valley. The mean depth of the Black Sea is 1,197 m and, due to the strong salinity stratification and extremely slow deepwater renewal, the water masses below 200 m are anoxic. The Sea of Azov in the northeastern part of the Black Sea is often frozen during the winter. The Gulf of Ob is a narrow and long estuary of Ob River in the Kara Sea in the Russian Arctic, ice-covered in winter. Chesapeake Bay is a very shallow and small basin. It is a ria or a drowned valley in a humid subtropical climate zone with hot summers and with ice formation in river mouths in some winters.