The aim of the study was the long-term observation of changes in the trophic status of the Great Masurian Lakes versus changes in the management of lakes’ watershed. The Great Masurian Lakes (GML), a system of interconnected glacial lakes, were, until the 80s, receivers of untreated and partially treated sewage from cities and diffuse sources, particularly in the tourist season. In the 90s, state farm areas turned into fallow land, which reduced the input of nutrients from diffuse sources. In the late 90s municipal sewage plants began to be systematically built in cities and the larger villages of the region. An analysis of changes in the trophic status of the GML hydro-system during the period of 35 years was based on rotifer indices of lake trophy. Results of the analysis show that in 1976 lakes in the northern and southern part of the GML system were mesotrophic or meso-eutrophic, while in the central part, in the triangle formed by three cities, the beginning of eutrophication was observed. After several years of increased inflow of nutrients due to the impact of tourism, a marked increase in the trophy of lakes was noticed in the central part of the system. In the 90s, after the collapse of large-scale agriculture and the installation of modern sewage treatment plants, the trophic status of nearly all lakes in the central and southern parts of the system ranged between meso-eutrophy and low eutrophy. The exceptions were Lake Nidzkie, whose trophy increased and Lake Niegocin, which became mesotrophic. Changes in the trophic status of the studied lakes indicate high sensitivity of the GML system to anthropogenic changes in their watershed.
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