The aim of this study was to evaluate land use in the sub-catchment (i.e. direct catchment) of Lake Gąsawskie in the period of 1945–2011, focusing on its effect on the current ecological status of the lake.The rate of lake overgrowing and changes in its area were assessed over the period of 66 years. The extent of sewerage cover in the Gąsawa commune located in the direct catchment of the lake was taken into consideration. The lake catchment is characterised by the predominance of arable land. Analysis of cartographic materials from 1945, 1991 and 2011 showed an increase in building development in the catchment of the lake at the expense of the percentage of arable land. At the same time the area of this lake was found to decrease from 105.85 ha to 94.98 ha, i.e. by 10.3%. The rate of its depletion is 0.15 ha annually. Studies of the ecological status conducted using macrophytes index ESMI and physico--chemical parameters of water showed a poor condition of the lake. This is also evidenced by a limited number of plant communities, a slight share of immersed macrophytes, particularly hornwort (Ceratophyllum demersum). High concentrations of nutrients promote an intensive development of phytoplankton, as indicated by low water transparency. Results of this study showed that undertaking effective actions in the lake catchment is necessary to limit further degradation of Lake Gąsawskie waters
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