Long-term studies of water chemistry and zooplankton interactions in a submontane dam reservoir in variable hydrological years (dry, wet, average)

The differences in environmental parameters and zooplankton community were studied in the epilimnion of the Carpathian Dobczyce Dam Reservoir on the Raba River (southern Poland) between hydrologically dry (HD), average (HA) and wet (HW) years distinguished on the basis of mean annual flow for 2000-2017. We found significant differences in water chemistry (conductivity, nutrients: P-tot, PO₄³⁻, NH₄⁺) in the epilimnion between the studied hydrological years. The total density and dry weight of zooplankton and the density of Rotifera: Keratella quadrata (Müller, 1786), Copepoda: Copepoda n. det. (immature stages), Cyclops strenuus Fischer, 1851, Eudiaptomus gracilis (Sars G.O., 1863) and Cladocera: Bosmina longirostris (O.F.Müller, 1776), Daphnia cucullata G.O. Sars, 1862, and D. longispina (O.F.Müller, 1776) were significantly higher in HA and HW years than in HD years. The highest densities of many Rotifera, Cladocera and Copepoda species were found in HA and HW years with high flows or floods in the river in spring or early summer. At these times, the epilimnion was rich in P-tot (diffuse pollution), considered as a limiting factor for algal growth in Carpathian reservoirs. The best Generalized Linear Models (GLM) for zooplankton density included 4-6 factors, among which P-tot, NO₃⁻, and pH were always present. We found that river flows prevailing in a given year determine water chemistry and eutrophication processes, which has a significant impact on densities and dry weight of zooplankton communities in HD, HA and HW years. The results obtained have important implications for proper management in mountain catchments.