## 4 Discussion  
### Long-Term Dynamics of the Runoff  
In official documents, as well as in various publications, it is usual to describe the annual runoff of the Dniester River at about 10.2 km\({}^{3}\)[17]. This value (as will be shown below) is based on its average value, usually obtained from the longest available series of representative observations. However, how true is it today? The data relating to long-term observations on water discharge at the hydrological stations Zalishchyky, Mohyliv-Podilskyi, Maiaky (Ukraine), and Bender (Moldova) were used initially. The choice of these stations was dictated, on the one hand, by the need to correctly identify the impact of the functioning of the Dubossary hydroelectric complex and its reservoirs, as well as climate change on the river flow below the dam, and, on the other hand, by the accessibility of available information. Long-term hydrological data on the Lower Dniester runoff should be distinguished into two main periods: 1946-1981 before the filling of the Dniester reservoir, and 1987-2020 when the Dniester reservoir and HES-1 were put into operation. The influence of Dubossary HPP, put into operation in 1955, was not taken into account, because of its low impact on the total discharge of the river. The main difference between these periods is that the first one is characterised by stable climate, and the other one is characterised by a global warming influence, starting in the middle of 1990s. The combined hydrograph of annual runoff volumes for the period from 1946 to 2020 for each of the four stations is shown in Supplementary Materials Figures S10 and S11. A trend line with a linear regression coefficient \(-\)0.012 (Figure 2) shows a downward trend in the total discharge of the Dniester River at all the stations. We suggest that this trend is due to general climatic changes that began to manifest themselves in the mid-1980s. The degree of the Dniester reservoir impact on the Lower Dniester total discharge, regarding the influence of climatic changes, can be calculated by the analysis of the statistical relationship between the runoff at the Bender HGS and Zalishchyky HGS before and after the reservoir was filled. Figure 8: Autecological preference composition (%) of macrozoobenthos in the Lower Dniester in 2018–2020 calculated using total taxa list with Asterics 4.04. (**a**) Current preference, (**b**) zonation, (**c**) biotope, and (**d**) functional feeding group. The runoff data at the Bender HGS should be divided into two periods:  
1. 1946-1981, the period prior to the filling of the Dniester reservoir (Supplementary Materials Figure S10). The linear regression equation is: \[\mathrm{y}=0.19\mathrm{x}-6.82\] (1)
2. 1987-2020, the period after the filling of the Dniester reservoir (Supplementary Materials Figure S11). The linear regression equation is: \[\mathrm{y}=-0.017\mathrm{x}+8.89\] (2)  
The series of runoff data from 1946 to 1981 (the year when the Dniester reservoir began to fill) may be considered homogeneous, since the main influence on the discharge was exerted by natural factors. The 1986-2020 data series is also homogeneous because of the emergence of a significant factor influencing the Lower Dniester flow. This factor is the reservoir, capable of holding up to 1/3 of the total annual river flow. At first time interval, the trend line has a positive regression coefficient of 0.19 and shows that the water content acquired a local growth trend (1). For the second period, a trend line with a negative regression coefficient of \(-0.017\) shows a decrease in the annual runoff (2). Dependence of the Dniester annual river flow volume at Bender HGS on the flow volume at Zalishchyky HGS, for the periods 1946-1981 and 1986-2020, is shown in the Figure 9.  
The regression line on each of the graphs demonstrates a close linear relationship between the volumes of annual runoff in Bender HGS and Zalishchyky HGS.