## 4 Discussion
### Biological Conditions
Similarly, there are no significant correlations between the indicators of zooplankton and phytoplankton biomass, as well as any measured hydrophysical and hydrochemical parameters, except water temperature, for which the correlation with different zooplankton indicators ranged from 0.55 to 0.63. Thus, it is currently impossible to identify the main factor determining the development of zooplankton. To understand the processes taking place in the zooplankton community, it is necessary to analyse the complex influence of hydrological and hydrochemical factors. Figure 16: Long-term changes in zooplankton community structure in the Lower Dniester (1949–1952 according to [7], 1976 according to [12], 2016–2020 according to [4,5] and project data). Compared with the period of eutrophication, the saprobity calculated by zooplankton indicators on average was 1.83 (Supplementary Materials Figure S8), the value of the index being slightly better compared with the 1970s, when it was 1.88. Both values correspond to \(\beta\)-mesosaprobic zone (moderately polluted waters) [12; 37; 57]. This also indicates an improvement in the state of the zooplankton community in comparison with the 1970s [12]. A similar characteristic of saprobity (1.38-1.58 in 1962 and 1.74-2.01 in 2008) was given within the study of the Daugava river over the last 50 years [83]. Both examples show that zooplankton communities could keep relative stability for a long period. The quality of historical data on macrozoobenthos of the Lower Dniester was different, and generally, the abundance and biomass were given only as characteristic of a big taxonomic group (family, order, etc.) and the species composition was given separately without qualitative characteristics [3; 6; 9; 10; 11]. Data from the period of natural conditions (before 1954) [3; 9] contain figures available for calculation that made possible a detailed comparison with the modern situation using contemporary methods (Table 3). The \(\alpha\)-diversity was low (usually less than 15 species m\({}^{-2}\)) with a Shannon Index less than 1.5, typical for oligomixed communities. The main shifts in the taxonomic composition of the Lower Dniester took place after the start of Dubossary HPP and Dubossary reservoir exploitation [2; 6]. In the 1950s, [PERSON]. [9] described six community types common for the Dniester river basing on substrates, but without distinguishing them by dominant species. According [3; 9]_Viviparus viviparus, Lithoglyphus anticoides,_ and _Polipedium spp._ were common for rock and aquatic plants biotopes, but currently, they are widely present on silts and sands. This shift in taxon composition and community structure were first described in the 1970s-1980s when the sedimentation characteristic change and the areas of silts increased and currently this tendency is still stable [6; 10; 11]. This illustrates the patch dynamics concept when species composition is stable, but the role and area of each species could change under an environmental or anthropogenic influence [86]. On the other hand, the role of species sensitive to water quality and common in the 1950s, such as _Unio crassus_ ([PERSON], 1788) and _Palingenia longicauda_ (_[PERSON]_, 1791) decreased. These formed their own communities, being extremely important as ecosystem engineers in the Lower Dniester and its delta, playing a significant role in ecosystem production processes [3; 9]. A sharp decrease in the latter species in the 20 th century is a pan-European trend [6; 11; 60]. It is shown that after the close extinction of this species in the 1970s, it recently appeared in the Moldavian Dniester and Prut [87; 88]. We have not found EPT taxa regularly during 2018-2020, but have occasionally. During 2018-2020, only 4 species of crustaceans, mysids (_Katamysis warpadowskyi_ [PERSON], 1893, _Linnomysis benedeni  
Figure 17: Long-term changes in the average \(\pm\) SE seasonal abundance of zooplankton (1949–1952 according to [7], 1976 according to [12], 2016–2020 according to [4; 5] and project data).