Fluvial Lakes Affect on Phosphorus and Potassium Concentrations in the Samica Stęszewska River

The aim of the study was to evaluate the effect of shallow lakes on reactive, total phosphorus and potassium concentrations in the river water and analyse the effect of changes in water retention in lake on nutrient concentrations in river waters. The study was carried out in the Samica Steszewska River. This is lowland river, which flows through two polymictic lakes. The study site is located in the Wielkopolska Lowland, Central-west Poland. Water samples were collected at depths of 0.5 m below the water surface. Reactive, total phosphorus and potassium were analysed monthly by standard methods. The study was carried out from January to December 1999–2002 (period with high water retention), 2005–2008 (period with low water retention), at four control points. Sites were located at the inflow and outflow of the Samica Stęszewska River into and out of Niepruszewskie and Tomickie Lakes. From 1974 to 2002, Lake Niepruszewskie was regulated at its outlet by a weir. In 2002, the water level was reduced. Changes of water retention in Niepruszewskie Lake influenced water discharge of the Samica Stęszewska River. The results indicate that changes in water retention have significantly influenced water quality in the river, particularly total and reactive phosphorus concentrations; however, this influence was not observed in respect to potassium concentrations. Nutrient concentrations in river strongly depend on water quality of the lake ecosystem and their buffering capacity. Additionally, by improving water quality, increased oxygen concentrations, and decreasing dissolved nutrient concentrations, as well as increased amplitude of water level fluctuations in lakes may increase P-fixation rates in outlet streams.