Bottom reliefThe Baltic Sea lies within the continental shelf. The average depth of the sea is 51 meter. In the areas of shallows, cans, near the islands, there are small depths (up to 12 meters). There are several basins in which the depths reach 200 meters. The deepest basin is Landsertskaya with the maximum depth of the sea - 470 meters. In the Gulf of Bothnia, the maximum depth is 254 meters, in the Gotland Basin - 249 meters. The bottom in the southern part of the sea is flat, in the north - uneven, rocky. In the coastal areas sand is distributed among the bottom sediments, but most of the sea floor is covered with deposits of clay silt of green, black or brown glacial origin.
Hydrological regimeA feature of the hydrological regime of the Baltic Sea is a large excess of fresh water, formed due to precipitation and river flow. The brackish surface waters of the Baltic Sea through the Danish straits go to the North Sea, and the salt waters of the North Sea enter the Baltic Sea with a deep current. During the storms, when the water in the straits is mixed up to the very bottom, the water exchange between the seas changes - throughout the straits section, water can flow both into the North and into the Baltic Sea. The Baltic Sea in March 2000 (NASA) The circulation of the surface waters of the sea is directed counter-clockwise, although strong winds can disrupt the nature of the circulation. The tides in the Baltic Sea are semi-diurnal and diurnal, but their magnitude does not exceed 20 centimeters. More important are the overtaking phenomena - sea level fluctuations, which can reach 50 centimeters offshore, and at the tops of bays and gulfs - 2 meters. In the top of the Gulf of Finland in some meteorological situations, the level can rise up to 5 meters. The annual amplitude of sea level fluctuations can reach 3,6 meters in Kronstadt, Ventspils has 1,5 meters. The amplitude of the seismic oscillations usually does not exceed 50 centimeters.
Compared with other seas, the excitement on the Baltic Sea is insignificant. In the center of the sea there are waves up to 3,5 meters, sometimes above 4 meters. In the shallow bays, the height of the waves does not exceed 3 meters, but they are steeper. However, it is not uncommon for large waves to form more than 10 meters in height, when storm winds form waves from deep water to shallow water. For example, in the area of the banks of Elands-Sedra-Grunt, the height of the wave is 11 meters. A small salinity of the surface layer contributes to a rapid change in the state of the sea. In winter, ships are threatened with icing. These features of the Baltic Sea along with a high level of navigation, a large number of navigational hazards turn navigation in this sea into a difficult occupation. Transparency of water decreases from the center of the sea to its shores. The most transparent water in the center of the sea and the Gulf of Bothnia, where the water has a bluish-green color. In coastal areas, the color of the water is yellow-green, sometimes a brownish hue. The lowest transparency is observed in the summer due to the development of plankton. Sea ice appears first in the bays in October-November. The coast of Bothnicheskiy and a significant part of the coast (except for the southern coast) of the Gulf of Finland are covered with fast ice thickness of up to 65 centimeters. The central and southern parts of the sea are usually not covered by ice. Ice melts in April, although in the north of the Gulf of Bothnia, drifting ice may also occur in June. Often there is a floating bottom ice.
The temperature of the surface layers of water in summer in the Gulf of Finland is 15-17 ° C, in the Gulf of Bothnia - 9-13 ° C, in the center of the sea - 14-17 ° C. With increasing depth, the temperature slowly decreases to the depth of the thermocline (20-40 meters), where a sharp jump to 0,2-0,5 ° C occurs, then the temperature rises, reaching the bottom of 4-5 ° C.
SalinitySalinity of sea water decreases from the Danish straits connecting the Baltic Sea with the salty North, to the east. In the Danish straits, salinity is 20 per mille at the sea surface and 30 per mille at the bottom. To the center of the sea, salinity decreases to 6-8 per mille at the sea surface, in the north of the Gulf of Bothnia, dropping to 2-3 per mille, in the Gulf of Finland to 2 per mille. With depth, salinity increases, reaching in the center of the sea at the bottom of 13 ppm.
The Baltic Sea is rich in seafood, in addition, there are oil reserves, in particular, the development of the D-6 field in the exclusive economic zone of Russia within the Kaliningrad region is being carried out, iron manganese concretions and amber deposits have been discovered.