Some aspects of the water salinity dynamics in the floodplain ecosystems
Authors:   Institute of Hydrobiology, Ivanova Natalia 
Published: December 2023

Global climate change, often low water content of rivers, as well as changes in the hydrological regime of water bodies as a result of various anthropogenic activities are displayed in the change in the hydrochemical regime of water bodies, namely in the increase in the total dissolved solids and salinity of water. The processes of salinization of freshwater water bodies, their reasons and possible consequences for the environment and humans have being actively researched in recent years. Fluctuations in the water surface level and temporary drying of water bodies can be one of the factors affecting the change in salinity of water in a water body. Significant level fluctuations are characteristic of floodplain ecosystems, for example, for the Stentsivsko-Zhebriyansky Plavni (floodplains) (SZhP), which are the object of research in this work. The purpose of the work is to conduct monitoring studies and analyze the dynamics of water salinity in the floodplain ecosystem of the SZHP. The methods used are expedition research, measurement of salinity and the total dissolved solids using the AZ-8603 multifunctional conductometer, as well as analysis of previous research. Stentsivsko-Zhebriyansky Plavni are especially valuable wetlands of the lower Danube River in terms of biodiversity. Their modern total area is about 78.11 km2, of which 72,34 km2 have been part of the Danube Biosphere Reserve (Ukraine) since 1998. This is a complex of water bodies, watercourses and wetlands, formed approximately 2 000 years ago. These wetlands are floodplains of the lower Danube River, as well as its tributaries - the small steppe rivers Murza, Laptysh, Chatal, Dunaets. Among the reservoirs of the SZhP, it is worth noting the Velykiy and Maly Soloni, Pozhezhny, Grabovsky and Zhebriyansky estuaries. Since 1978, due to the construction of the Danube-Sasyk canal, the SZhP was divided into two parts - Stentsivsky (43 km2) and Zhebriyansky(28 km2). Today, the only combination of parts is a siphon-type duiker with a cross section of 8 m2 under the channel, which is often clogged with reed rhizomes, plant residues, garbage and does not perform its functions. Today, the flow from the floodplains into the sea is regulated, and the inflow of seawater into the wetlands is almost impossible. But at the same time, the total dissolved solids and salinity of water in different parts of the floodplains are higher than the indicators of fresh water – in the winter of 2023, the water salinity ranged from 2.93 g/dm3 near the duiker on the side of the Zhebriyansky plavni to 6.02 g/dm3 near lock № 5 (near the Primorske-Vylkove freeway). In the winter and spring of 2023, the complete drying of reservoirs was observed in some areas. In May 2023, the flow of Danube water into the flood plain was restored. As a result, the water level rose by an average of 0.3 m. At the same time, the salinity of the water near the duiker decreased to 0.81-0.83 g/dm3 (June 2023), and near lock № 5 to 2.12-2.15 g/dm3. At the same time, in different parts of the Zhebriyansky estuary, the indicator fluctuated within the range of 0.82-1.69 g/dm3, while in the bottom layer the indicator increased by 10-110 mg/dm3. In the part of Zhebriyansky floodplains separated by an earth dam, which dried up in winter, the water salinity was 5.77 g/dm3, and in the coastal zone of the Velykiy Soloni Estuary, which was also waterless as early as April 2023, it was 9.99-17.8 g/dm3. Such significant water salinity affects both the hydroecosystem and coastal vegetation, and should be taken into account when choosing to maximize certain ecosystem services during the economic use of these water bodies.