## 4 Discussion
### Biological Indicators of HPP Influence in the Lower Dniester  
The effects of dam and HPP construction on biological communities have attracted the attention of researchers and environmental activists throughout the world ([PERSON], 1894). HPP influences multiple aspects of the aquatic ecosystem, primarily changing the current velocity and decrease in general flow of the river, increasing the water temperature, shifting in sedimentation process, and migration of nutrients and other chemical elements. All  
Figure 18: Long-term changes in the average \(\pm\) SE abundance and biomass macrozoobenthos, calculated based on historical ([PERSON], 1894; [PERSON], 1894; [PERSON], 1894; [PERSON], 1894; [PERSON], 1894) and project data. of these effects were observed within the study of the modern environmental state of the Lower Dniester during the 2018-2020 years. It is important to distinguish long- and short-term effects on the ecosystems' biological components, depending on the length of the life cycle of the organisms. Dam construction led to the changes in microalgal communities both in reservoirs and in river reaches and estuaries below the dams [75; 80; 81]. During the study of the immediate effect of release from the Dubossary hydropower station in 2019 [55], it was shown that after the release, the concentration of dissolved silicates, and consequently, the proportion of diatoms, becomes higher. However, in the long term, precipitation of silicates in hydropower reservoirs led to the lack of dissolved silicates, and the dominant phytoplankton functional group shifted from diatoms (Bacillariophyta) to green algae (Chlorophyta) along the flow direction below the dam. The changes in proportion of Bacillariophyta and Chlorophyta may become a primary indicator of HPP impact. An increase of average diatom proportion in the spring may indicate the adaptation of the phytoplankton community in comparison with the first decades after hydropower construction. However, in the summer period, the proportion of Bacillariophyta decreased, together with increasing in cyanobacteria and dinoflagellates, which is characteristic of waters affected by upstream dams [75; 80; 81]. Nevertheless, the proportion of Bacillariophyta in total phytoplankton biomass in the 1970-72 and 2018-2020 remained the same. The reduction in river discharge and consequent current velocity resulted in a change from lotic (rheophilic) communities to lenti (limnophilic) ones. Upper and middle parts of rivers are usually more sensitive to such impacts, based on the River Continuum Concept [63; 66; 75], but in lower reaches, this effect is also visible in long-term studies both in the zooplankton and macrozoobenthos. In the case of the Lower Dniester, the start of Dubossary HPP and Dubossary reservoir exploitation in the 1950s caused the shifts in structure and functioning of invertebrate communities. Since the 1950s, there has been a tendency to decrease the role of running water Rotifera taxa in the zooplankton community and a concomitant increase in that of Copepoda, which may provide an indicator of HPP impact [63; 75; 76; 77]. These structural changes may be a result of increases in average seasonal temperatures, as well as a smoothing in seasonal water fluctuations in the Dniester delta after HPP construction. The last assumption is supported by the work of [PERSON], who noted a change in the ratio towards rotifers in high-water years [12]. However, we have observed no clear correlation between the proportion of rotifers and discharge volumes [16; 57]. The decrease of role of Ponto-Caspian species complexes and reduction of macrozoobenthos community types covering the river bed was observed during the 1970s and 1980s, in comparison with the natural flow period [6; 10; 11]. Due to the changes in sedimentation regime, the area of soft-bottom sediments and water plants increased, favouring Gastropoda, specially _Viviparus ater_, _Viviparus contectus_, and _Viviparus viviparus_.