The Great Masurian Lakes: Hydrological Regime and Summer Phytoplankton

Abstract

The lakes of the Great Masurian Lakes System create the largest lake complex in Poland hydrologically divided into three basins – Northern, Central, and Southern. The water flow direction depends on the position of the watershed between Vistula and Pregola Rivers which can vary depending on the water level and the magnitude of the water outflow from the Pisa and Wegorapa Rivers. The studies aimed to analyze the changes in the hydrological regime and the ecological state of these lakes in 2008 and 2010–2012. In June 2010, the bifurcation zone occurred in the central part of the system due to the high quantity of precipitation. A positive heterograde oxygen curve in summer was in less eutrophic lakes, while a negative heterograde oxygen curve in more eutrophic lakes and a clinograde oxygen curve in highly eutrophic lakes. Phosphorus content was typical of medium or highly eutrophic lakes; therefore classifications based on physicochemical elements indicated below good ecological potential and status of all lakes. The least intensive phytoplankton growth was in the Northern Basin whereas the most intensive in the Southern Basin. Biological assessments based on phytoplankton indicated maximum potential in two lakes and good potential and status in five lakes, whereas the remaining lakes had moderate, poor, and bad potential and status. In 2010, when oxygen conditions improved significantly, the classification based on PMPL indicated significant deterioration up to bad ecological potential and status, in the lakes within the bifurcation zone and in several lakes in the Southern and Northern Basins.