A notable feature of the Black Sea is that oxygen is dissolved (and rich sea life is made possible) only in the upper water levels. Below a depth of about 230 to 330 feet (70 to 100 metres) at the sea's centre and 330 to 500 feet (100 to 150 metres) near its edge, there is no oxygen. This means that its lower levels are, to all intents and purposes, almost biologically dead—not because of pollution but because of continued weak ventilation of the deep layers.
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