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The Black Sea is the largest marine anoxic basin in the world. It has an oxygenated surface layer overlying a sulfide-containing (anoxic) deep layer. This condition has evolved because of the strong density stratification on the water column: water with high salinity enters from the Bosporus, while the upper layer water is of riverine origin. This mixture of Bosporus outflow with overlying cold intermediate layer (CIL) water forms the Bosporus plume which ventilates the deep layers of the Black Sea. The rate of CIL formation is variable in response to changing climate. The hydrochemical structure of the Sea is determined by these peculiarities of the hydrophysical regime. Here we will describe the main features of the Black Sea biogeochemical structure from the point of view of the changing of redox conditions. We will also describe the main features of the different scale temporal variability of this structure on the basis of recent expedition data received in 1997-2006. [ABSTRACT FROM AUTHOR]