Influence of the Włocławek Reservoir on hydrochemistry and plankton of a large, lowland river (the Lower Vistula River, Poland)

The research on the Włocławek Reservoir (WR), situated on the Lower Vistula River (Poland), was carried out during two periods: between 1986 and 2002, hydrochemistry was studied, and between 1994 and 2000 – plankton. WR is 70 km2 in area, its capacity is 370 × 106 m3 and the retention time with the average flow rate (Q) amounts to 930 m3 s−1 – only 5 days (!). Until 2002, due to cyclic work of a hydroelectric power plant, considerable variability in the daily flow rate was recorded (from 600 to 1800 m3 s−1). Its specific hydrology and fluvial character mean that WR functions in a completely different way compared to dam reservoirs described in textbooks, with significantly longer retention time.WR significantly improves most of the water quality variables in the river. It reduces the concentration of suspended matter (on average by more than 50%), Ptot (by 16%), BOD5 (by 40%) and chlorophyll “a” (by 50–60%) (unpublished data).Phytoplankton is rich in species – altogether 441 taxa, with the count of 7.4 × 106 individuals per dm−3 and biomass of 16 mg dm−3. Throughout the research period, a significant reduction in the abundance of phytoplankton was recorded in WR, sometimes more than 60%. The total number of zooplankton taxa at the site upstream from WR and in the reservoir was almost identical (68 and 67), however in the reservoir itself, a small decrease in the number of Rotifera species and an increase in the number of Crustacea species was observed, as well as a significant, fourfold increase in the total zooplankton biomass. The aforementioned reduction of phytoplankton and enrichment of the river with Crustacea are one of the major biocenotic consequences of the functioning of the Włocławek Reservoir.