Changes in the Trophic Status of Lake Niepruszewskie (Poland)

The paper presents the evaluation of trophic status of Lake Niepruszewskie in the years 1994–2013. This is the first lake in the Samica Stęszewska river, flowing dis-tally through the lakes of the Wielkopolski National Park. Source part of the Samica Stęszewska river basin was, in the years 2004–2012, included to the nitrate vulnerable zone (NVZ), and Niepruszewskie Lake was considered to be threatened by eutrophication. The evaluation of trophic status of the lake was made based on the concentration of nutrients (nitrogen and phosphorus) and indicators of eutrophication (chlorophyll a, water transparency Secchi disk marked) by referring them to the limit values for the eutrophication of surface waters as defined in the Regulation of the Minister of Environment of 23 December 2002 and other criteria used for the classification of the eutrophication of surface waters. Detailed and synthetic indexes of the trophic status (TSI) using the formula Carlson and the ratio of N:P were also made. Concentrations of the nutrients in Lake Niepruszewskie and eutrophication indexes reached high values, indicating a eutrophic or hypertrophic state of the lake. Detailed values of the trophic status index (TSI), on average over the period, corresponded to eutrophic, while in subsequent years they were varied – from mesotrophic to hypertrophic. The worst trophic status was recorded in 2003–2006, after the lowering of the lake dam-ming. The high value of N: P ratio indicates that the lake has huge surplus of nitrogen, and phosphorus is a productivity limiting factor.