Inland sea in southeast Europe, linked with the seas of Azov and Marmara, and via the Dardanelles strait with the Mediterranean; area 423,000 sq km/163,320 sq mi; maximum depth 2,245 m/7,365 ft, decreasing in the Sea of Azov to only 13.5 m/44 ft. It is bounded by Ukraine, Russia, Georgia, Turkey, Bulgaria, and Romania, and the rivers Danube, Volga, Bug, Dniester and Dnieper flow into it, keeping salinity levels low. Uranium deposits beneath it are among the world's largest. About 90% of the water is polluted, mainly by agricultural fertilizers.
The Black Sea is Europe's most polluted sea; rivers feeding it bring raw sewage, nitrates, phosphates, pesticides, heavy metals, and other pollutants from 13 countries. This is accelerating the natural accumulation at the bottom of the sea of a layer of hydrogen sulphide, ammonia, methane, and ethane, which is rising by 2 m/7 ft a year. Below a depth of 150 m/500 ft 90% of the water is anoxic (lacking in oxygen), mixing little with the surface water, and supporting little life. It constitutes the world's largest mass of anoxic water and is caused by bacterial action on rotting vegetation.
In addition to pollution, the comb jellyfish Mnemiopsis leidyi has contributed significantly to a reduction in marine biodiversity. It entered the Black Sea in the early 1980s in ballast water from a US ship. Without predators it thrived, feeding on fish eggs and larvae. Fish catches were reduced by 90% in only six years. By 1990 the estimated biomass of the jellyfish peaked at 900 million tonnes.
Research shows that the building of the Iron Gates Dam (1970–72) across the River Danube in Serbia and Romania, about 1,000 km/620 mi from the Black Sea, has had a serious effect on marine life. Sediment that would have been washed downstream into the sea is now deposited behind the dam, reducing the flow of silicates by two-thirds. The reduction in silicates has led to a change in the types of algae that live in the sea, favouring species that form toxic blooms; these can kill fish and other marine life.
The Black Sea is a major trade artery for Eastern Europe; the chief ports are Odessa and Mykolayiv (Ukraine), Novorossiysk (Russia), Batumi (Georgia), Constanţa (Romania), Varna and Burgas (Bulgaria), and Trabzon (Turkey). Currents, storms, and occasional floating ice can make navigation hazardous in winter. There is a strong surface flow out through the Bosphorus, with a deep-water inward flow.
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