Natural Chemistry of Caspian Sea Waters

Abstract

Principal features of the natural chemistry of Caspian Sea waters are assessed: the salt(ionic) composition, dissolved oxygen, active reaction (pH), and nutrients. Their climatic regime and interannual variability are considered on the basis of historical data of ship measurements and expeditionary studies of recent years. It is shown that the natural hydrochemical regime of Caspian Sea waters (especially of its near-mouth areas) is characterized by significant spatial inhomogeneity and interannual variability closely related to interannual variability in the volumes and chemical composition of riverine runoff. An alternation of two types of the natural hydrochemical water regime was established manifested in the multiannual variability in all the characteristics considered, especially in the deep and near-bottom layers of the Caspian Sea. One of these types existed under the conditions of large river discharge at the beginning and at the end of the twentieth century. It was characterized by reduced concentrations of dissolved oxygen (down to the values typical of hypoxy) and elevated nutrient contents. The peak development of the other type was confined to the 1960s–1970s under the lowest river discharge. It was characterized by opposite deep and near-bottom anomalies of the dissolved oxygen (positive anomalies) and nutrient contents (negative anomalies).