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An aim of the research was to test a hypothesis that impact of a shallow hypertrophic reservoir on riverine zooplankton is limited to a short section of the outflowing river because of a rapid conversion of the lentic habitat into the lotic one. The second hypothesis assumed that species richness of zooplankton is increasing along the river due to washing out littoral and benthic species. The study was carried out in the years 2009–2010, during summer-autumn strong dominance of toxic filamentous Cyanobacteria in a shallow hypertrophic humic Siemianówka Dam Reservoir (northeast Poland) and the outflowing Narew River. During research samples were collected on three consecutive days because water requires approximately as much time to cover the distance from the dam down to 118 km of the river. Results of two-year studies confirmed both hypotheses. The hypertrophic reservoir was the huge source of rotifers and crustaceans for the outflowing river. However, a rapid decline was observed in zooplankton abundance below the dam at the distance of 9.1 km. With decreasing share of reservoir dominants along river, an increasing importance of littoral and benthic rotifers constituted ca 0.8% at the dam and 37.5% at 118 km of the river and crustacean ca 4% and 60% respectively. The dam reservoirs similar as fluvial lakes disrupt river continuity by changing the conditions of the zooplankton development. The strongest influence of reservoir on potamozooplankton communities is observed below outlets.
The effect of two different reservoirs on some parameters of river water quality has been studied. The Antoninek Reservoir (wetland-type, mean depth 0.4 m, area 7.2 ha, mean residence time 0.5 day) and the Maltański Reservoir (also not stratified but plankton-type, mean depth 3.1 m, area 64 ha, mean residence time 34 days) are situated on the same river (mean discharge 0.67 m³ s⁻¹). Water of this river, which leaves the hypertrophic Swarzędzkie Lake upstream from the reservoirs, is rich in nutrients and plankton. As it flows through the Antoninek Reservoir it loses most of the phytoplanktonic organisms, measured as chlorophyll-a (70%), mostly due to the shading effect of emergent vegetation. The fraction autotrophic picoplankton (APP) also decreased considerably, both in numbers and biomass (about 68% and 54% on average, respectively). A similar reduction was recorded for the numbers of bacterioplankton (39% on average) and suspended solids (66%). Phytoplankton growth was observed again in the Maltański Reservoir, due to the longer residence time of water and lack of macrophytes. Chlorophyll-a increased by 37%, APP numbers by 120%, their biomass by 154%, while the numbers of bacterioplankton only by 31%. Changes in plankton abundance and biomass as a result of retention of water in a shallow reservoir influenced the quality of outflowing water. Their range strongly depended on the presence of macrophytes within the reservoir.
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