Special features of the hydrochemical conditions variability in the deep water basins of the Caspian Sea

The features of the long-term variability of the hydrochemical characteristics of deep-water basins of the Caspian Sea are studied on the basis of information obtained during 18- year annual monitoring and supplemented with historical data. The effect of hypoxia and the hydrogen sulfide layer on the position of the nitrate-maximum layer and the content of nitrates in it are shown. Studying the variability of the vertical distribution of the hydrochemical indicators revealed several factors that determine the removal of nutrients from the photic layer and their accumulation in the bottom layer. Among other things, the latest data revealed the unique features of the Caspian coastal upwelling. This will help to avoid underestimation of the productivity of the waters in the Middle Caspian Basin.