We analyzed spatio-temporal changes in concentrations and elemental compositions of dissolved organic matter in the waters of the Caspian Sea over the period 2010–2015. These studies showed that over the past 5 years of lowering of sea levels, mean concentrations of Corg and Norg remained virtually unchanged, whereas concentrations of Porg decreased more than three-fold, suggesting a qualitative change in organic matter. Based on data averages, concentrations of Corg, Norg, and Porg in Southern waters were slightly higher than in the North and Middle-Caspian waters during the monitoring period. In deep areas at the border region of hydrogen sulfide occurrence, concentrations of Corg, Norg, and Porg increase. Hence, the presence of hydrogen sulfide in the deep waters of the Derbent and South Caspian basins, and the corresponding increase in organic matter contents, indicates no radical improvement of vertical mixing of the sea in these areas during the eight years of our regression model. Moreover, according to the carbohydrate distribution, organic substances of petroleum hydrocarbons contribute considerably to a common pool.
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 dis tribution 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 produc tivity of the waters in the Middle Caspian Basin.
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