RELEVANT ASPECTS OF STUDYING PERIODICALLY DRYING OUT WATER AREAS IN THE CONTEXT OF THE CARBON CYCLE (THE CASE OF THE LOWER DON BASIN)

Investigations were carried out in the reservoirs of the Don delta and Western Manych to study the biogeochemical cycles of elements, mainly carbon, in the transition zones between land and water. The main physical and geographical processes that determine the supply, transfer and transformation of organic matter and the connectivity of aquatic and terrestrial ecosystems are identified. A decrease of the surface area, based on a comparison of remote sensing data in ArcGIS for the period 1985–2021, of the water surface of reservoirs in average of 15– 16 % was established (in the Don delta, mainly due to the destruction of man-made ponds, in Manych – due to a decrease in the water level). It is possible to slow down biogeochemical processes in drained areas and change one type of process to another as a result of the absence of a hydrological flow. A shift of geochemical barriers towards the river during surges and an increase of mineralized underground runoff during negative surges was recorded under the influence of surge phenomena in the Don Delta. For the system of reservoirs of the Western Manych, the presence of salinity gradients between its individual parts was noted. Investigations have established that the mixing zones of fresh and salt water – biogeochemical “hot spots”, are characterized by a contrasting chemical composition and very different concentrations of organic and suspended matter. A tendency to a significant change in the structure of the upper part of the profile of alluvial soils of the river delta was noted with the continuing low water in the Lower Don. Areas with varying degrees of manifestation of soil-forming processes, as well as the main natural and anthropogenic factors that control these processes, have been identified.