Distribution of nutrient elements and organic carbon in the lower reaches of the Selenga River

Abstract

The content of nutrient elements and organic carbon in the water of the lower course of the Selenga River and its long-term changes under the influence of natural and anthropogenic factors are considered. For analysis, data from field studies carried out in the summer of 2018 and 2019 was used, as well as long-term materials on the chemical composition of water in the closing section of the river (Kabansk gauging station). Today, the hydroclimatic factor largely determines the chemical composition of water in the lower reaches of the Selenga. The aridization of the climate in the region and the associated decrease in the water content of the river led to a decrease in the dilution of diffuse and point pollution. During the period from 1980 to 2017, the average long-term concentrations of total phosphorus increased by 2 times and nitrates by 2.25 times. At the same time, the supply of silicon to Lake Baikal decreased by 1.7 times. Its concentration determines the growth of diatomaceous algae - the main producer of organic matter. Climatic changes largely influence to the transformation of land use on the watershed of the lower reaches of the Selenga River. It was found that agricultural activity on the watershed increases the proportion of suspended form of chemical elements (in particular phosphorus and organic carbon). In the delta of the river, when hydrodynamic conditions change, suspension deposition is observed, the content of dissolved forms of nutrients and organic substances increases. In heated delta areas with low flow rates and intensification of production processes, the organic form of phosphorus prevails over mineral.