Summary: Nearly 87 % of the Black Sea is entirely anoxic (without oxygen) and contains high levels of hydrogen sulphide. This is the result of past geological events, its shape and its specific water balance (high degree of isolation from the world ocean, deep water depression with a maximum depth of 2 212 metres (m) in the centre of the sea, the extensive drainage basin and the large number of incoming rivers). x01x02 Deep pelagic and benthic organisms are largely absent. x01x02 The wide diversity of biotopes provides favourable conditions for invasion of alien species to the Black Sea. x01x02 The composition and structure of the marine communities is constantly changing with the decline of certain species and the expansion of others. x01x02 Generally, in undisturbed natural conditions species diversity in Black Sea fauna is approximately three times less than that in the Mediterranean. x01x02 Increasing salinity due to inappropriate water management and regulation, and pollution of brackish coastal lakes and estuaries represents a threat to relics and endemic species, especially in the Sea of Azov. x01x02 As a result of eutrophication, primary production has increased and the number of species has declined. These changes, as well as inappropriate fishing practices, have affected the fish stocks. A change towards small pelagic fish, i.e. anchovy and sprat, has been observed. x01x02 Deterioration of some marine habitats and a lack of laws and technology for regulating the introduction of alien species, for example via ballast waters, have allowed the invasion of such species. These have produced mass populations, which have changed the equilibrium of the native marine ecosystems. Growing concern for the fate of the Azov-Black Sea basin is stimulating a search for efficient countermeasures to combat unwanted alien settlers, since they are now the dominant species across the food web. x01x02 Overexploitation of fish stocks in combination with the invasion of the comb jellyfish Mnemiopsis leidyi has caused the collapse of
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