The Black Sea is a relatively uniform and deep marine basin with long coasts and almost no islands. It covers an area of 423 000 km2 and has a mean depth of 1 197 m. The western part of the Black Sea has a wide shelf which gradually narrows to the south. The shelf break to the continental slope occurs at a depth of 100–150 m. Around the other Black Sea shores there is either no shelf at all, or, where it occurs, the width does not exceed 10–15 km. The narrow Bosporus Strait, with a maximum depth of 27.5 m connects the Black Sea with the Sea of Marmara and further with the Dardanelles and the Mediterranean Sea. The narrow Kerch Strait, which is only 5 m deep, links the Black Sea with the Sea of Azov.
The lower layers of saltwater (36 ppt) from the Sea of Marmara penetrate into the Black Sea and surface waters of the Black Sea flow into the Sea of Marmara. It is estimated that the lower current carries 202 km3 of water yearly and that the upper one carries 348 km3.
Over 400 km3 of water flow into the Black Sea annually from a number of major river systems. These rivers, however, have great seasonal variations in run-off which, in turn, creates great seasonal variations in the estuaries. The Danube flows into the Black Sea through three branches, the Chilia, Sulina and St. George, which form a delta. The delta covers about 4 340 km2.
As so many large rivers flow into the Black Sea, the surface waters have low salinity. The average salinity of surface water in the central part of the Black Sea is 16 to 18 ppt. At a depth below 120–200 m the salinity increases to 21 to 22.5 ppt. In summer the surface water warms to 25°C, with occasional extremes of up to 28°C at the shores. In winter in the open sea, it cools to 6 to 8°C. The northwestern coast has ice in the winter while the southeastern stays about 9°C.
The relative isolation and restriction of the Black Sea prevents much vertical circulation and keeps the deeper waters low on oxygen. Consequently, organic debris settling from the surface layers decomposes anaerobically. This leads to the formation of hydrogen sulphide (H2S) which is particularly abundant below 180 metres. At depths greater than 137 m there is almost no oxygen and the only marine life found is anaerobic bacteria. The upper waters are quite different and, down to 50 m, have abundant oxygen. The upper circulation is cyclonic with an eddy off the Bulgarian coast.
There is very little diurnal tidal action - only 8 to 9 cm. However, the water level of the Black Sea is subject to seasonal fluctuations averaging about 20 cm. In coastal areas, especially in the northwest, wind causes waves up to 7 m high. Wind waves along the south eastern shore are reported to reach 2 m.
Due to the low salinity, the fauna and flora of the Black Sea are qualitatively poor as compared to the Mediterranean Sea. The Mediterranean has about 7 000 plants and animals, while the Black Sea has only about 1 200. On the other hand, primary productivity, based on nutrients coming from large rivers, is relatively high. Effective potential productivity of phytoplankton in the Black Sea surface waters has been estimated at 106–1 330 mg/m3/24 hrs, while that of the Mediterranean is only 7–157 mg/m3/24 hrs. The phytoplankton includes some 300 species. They are widespread in the open sea to a depth of 100 to 125 m. Near the shore they go down to 200 m. The phytoplankton biomass averages about 0.1 g/m3 of which diatoms make up 79 percent. Zooplankton density is also relatively high. The zooplankton includes more than 70 species and their biomass in the open sea averages about 0.3 g/m3.
Zoobenthos is almost entirely limited to the littoral zone. The lack of oxygen and the presence of H2S prevents the development of benthonic animals in deeper water. Below 50 m the most common benthic form is the pelecypod Modiolus phaseolina. The only benthos below 130 to 180 m is bacteria.
