The temporal dynamic of young fish communities in the water bodies of the “Dnipro-Orylskiy” Nature Reserve

The paper identifies regularities of temporary dynamics of the communities of juvenile fish communities in different types of reservoirs of the Dnipro-Orylskiy” Nature Reserve during 1997–2015 and assesses the role of the Reserve as a factor of fish population reproduction within the water area, which is exposed to a significant anthropogenic impact. During the research period, we have identified representatives of 38 fish species. The main trends in the dynamics of synecological characteristics of the juvenile fish communities are explained by the influence of three sources of variation: controlled discharge of the Dnieper River as a result of the creation of a dam cascade, eutrophication, and the reserve regime. The first two sources of influence are negative, while the creation of a protective regime in the Reserve is a positive factor. Eutrophication in the direction of its influence coincides with the negative effects of regulated discharge and significantly enhances them. The creation of a reservoir and regulation of the Dnieper River flow manifests itself differently in different types of reservoirs in the Reserve depending on their typological characteristics. The influence of the eutrophication factor may also manifest itself differently in different types of lake systems. The results we have obtained indicate that for the studied system of reservoirs in general, there is no tendency to change the number of fish communities and this indicator is stationary over time. For the channel part of the Dnieper River and its immediate vicinity, the Orel River mouth has established a trend of increasing the number of fish fry communities, while in other types of reservoirs there is no stable trend, except for reservoirs of Obukhovskie floodplain, where the number of juvenile fish communities has been constantly decreasing. In terms of biomass, there is also an upward trend in these water bodies as well as in the water bodies of the Taromsk ledge. An increase in the number and biomass of communities is accompanied by an increase in the number of species and diversity in Shannon. We explain the decrease in the number of fry communities in the Obukhov floodplain by intensive eutrophication processes, which are most active in these very reservoirs. We attribute the greatest activation of eutrophication processes to the construction of the Orel Canal in the Obukhov floodplain reservoir system. This artificial reservoir drains the lakes of the Obukhovskaya system, eventually contributing to the accumulation of organic matter and the development of oxygen deficiency phenomena in them. In the event of flooding, which is generally less likely due to the regulated flow of the Dnieper River, a fixed level in the Obukhov floodplain is maintained due to the close proximity of the Orel Canal. Water exchange and the associated evacuation of excess organics from the lakes are significantly limited. Stagnation and constant accumulation of organic remains create unfavorable conditions for life and development of young fish. As prospects for further studies, we plan to carry out geobotanical descriptions of aquatic macrophyte communities for phytoindication of ecological regimes of living conditions of fish population in water bodies.