## 2 Materials and Methods  
### Study Area and Sampling Sites  
During the research period from 2018 to 2020, water and bottom sediment and biological samples were taken in the Lower Dniester at two stations: near Palanca village (51 km highway or 27 km of the Dniester from the confluence of the Dniester estuary; 46.419 N 30.17389 E), at a distance of 324 km from the Dubossary hydropower plant and near the Maiaky village (46.4128 N 30.2627 E) at a distance 15 km from the Dniester estuary (Figure 1).  
The main difference between the selected stations is that Palanca is the furthest upstream point on the border of Ukraine with Moldova, and the Maiaky station is located below the confluence of the Turunchuk and Driester rivers. The width of the river in the Palanca site is up to 80 m, and the average depth is 3.2 m. In 2018-2020, the average annual runoff volume reached about 3.32 km\({}^{3}\), the average annual discharge was 105 m\({}^{3}\) s\({}^{-1}\), and the average flow velocity was 0.32 m s\({}^{-1}\), varying from 1.1-1.2 m s\({}^{-1}\) during floods down to 0.15-0.25 m s\({}^{-1}\) during the autumn-summer low-water period. The river width at the Maiaky site reaches up to 180 m, the average depth is 3.4 m, the average annual runoff volume is 7.71 km\({}^{3}\), and the average annual discharge is 245 m s\({}^{-1}\). Within the study period, the average flow velocity was 0.35 m s\({}^{-1}\), varying from 1.25-1.35 m s\({}^{-1}\) during floods down to 0.15-0.25 m s\({}^{-1}\) during the autumn-summer low-water. As mentioned, the Palanca site is the furthest upstream point on the border between Moldova and Ukraine (reference point of observation on the Driester by the Institute of Zoology of Moldova), the Maiaky site is a hydrological, hydrochemical, and hydrobiological long-term research point, located in the Ukraine territory. The choice of these stations made it possible to make a comparison of the obtained data with published historical data [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. The right bank of the Driester, where samples were taken, is raised above the water level, to 10 m in the Palanca area and up to 20 m in the Maiaky area, with trees and shrubs on the bank and grassy-mass soil cover. Downstream of the Maiaky station, there are flooded areas and the shore is lowered. The bottom substrates of the river are represented by petal (black and grey silts and sandy silts), particulate organic matter, psamal and akal mixture substrates (sands with granite gravel), and rare woody detritus increases in the section of the river in the Maiaky area. The high water plants are represented by a coastal strip of _Phragmites_ Adams., _Myriophyllum spicatum_ L., etc. [11].  
Sampling for plankton and hydrochemical data was carried out in flowing sections; samples were taken from the central part of the river by boat, and in the absence of a boat, they were taken by entering the river and using a throw-in container, at a distance of at least 4 m from the shoreline. Water column samples for biological (phyto- and zooplankton) and hydrochemical analyses were taken monthly. Bottom sediments for zoobenthos and pollution were sampled in the coastal part at depths of no more than  
Figure 1: Map of the investigated area of the Lower Driester (HPP = hydropower plants; HGS = hydrological gauging stations).  
0.8-1.5 m. Zoobenthos samples and samples of water and bottom sediments for metal concentration were taken seasonally.