Limnological Review 4 (2004) 115–124 Variations in the trophic state of the mesotrophic Ińsko DuŜe Lake (1970–2002) Jacek Kubiak, Agnieszka Tórz, Arkadiusz Nędzarek, Małgorzata Raczyńska Agricultural University of Szczecin, Kazimierza Królewicza 4, 71–550 Szczecin, Abstract: The study describes changes in the trophic state of Ińsko DuŜe Lake, the cleanest body of water in West Pomerania, between 1970 and 2002. The lake was found to show middle to high resistance to degradation. The moderate vulnerability of the catchment area in terms of nutrient load discharge as well as the high resistance of the lake resulted in a moderate rate of eutrophication of Ińsko DuŜe Lake. The strong resistance of the lake was able to level the adverse catchment are conditions. The studies have revealed that the lake, initially of a mesotrophic status, showed increasing parameters of the trophic state from the 1980s until mid41990s, which was followed by a return of the lake to its previous mesotrophic status at the end of the 1990s. The latest surveys (2002) have shown that the water quality of the lake have improved; the water, previously of the 2 nd purity grade, has gained the characteristics typical for the 1 st grade waters. Some quality indicators (e.g. Secchi depth) have recently reached the values typical for oligotrophic lakes. The improvement in water quality and the return to the lake's mesotrophic status is, on the one hand, due to changes in the use of arable soils located within the catchment are that have undergone for the last 10– 20 years, but, on the other hand, also due to improved waste4water management of the town of Ińsko. Key words: Ińsko DuŜe Lake, water quality, oligotrophic lakes, mesotrophic lake. Introduction and objective In West Pomerania, like in other regions across Poland, excessive run4off loads of nutrients and organic matter have affected surface waters for decades (Tadajewski  ., 1980; Kubiak  ., 1997, 1999; Zdanowski, 1999). Increased level of nutrient concentrations, as well as its consequences, force us to find ways to slow down, stop, or even revert this adverse process or, at least, to find a remedy against its negative outcomes (Lossow, Gawrońska 1998; Lossow, 1998). In order to chose adequate protective measures, a lake should be thoroughly studied for the rate and direction of changes in its trophic state (Kudelska, 1979; Bakiewicz4Grabowska, 1981, 1990; Lossow, 1996). Long4term studies are necessary which would show whether the changes in the lake's water quality are a result of temporary changes in the habitat, arise from local factors related to the catchment area, or reflect wider changes undergoing in the environment (Jańczak, 1995). Considering (1) the necessity to monitor and protect Poland's inland waters, which results from the integration of Poland with the European Union, (2) multiple areas of importance of lakes for West Pomeranian economy, and (3) the effect of eutrophication on the water quality of the lakes, this study was aimed to describe changes in the trophic state of Lake Ińsko DuŜe occurring from 1970 until 2002. The studied body of water is located within Ińsk Lakeland neighbouring with the sandur Drawsko Plain to the east, with moraine Nowogard Plain to the west, and with Drawsko Lakeland to the north4