During recent decades, human impact on lake ecosystems has increased due to intensification of agriculture, irrigation, water consumption, and electrical purposes. Particularly strong changes have been observed in shallow lakes, which are more sensitive to environmental changes and characterized by unfavourable morphometric parameters. The majority of transformations within a lake associated with changes in water level, progressing degradation, or plant succession leading to lake disappearance take place within the littoral zone. The aim of this study was to examine how human pressure, such as water level reduction, affect the surface and water volume of lakes, and estimate the rate and the extent of water loss from lakes during 1940-2008, and predict their potential changes over the next years. The importance of integrating vertical and horizontal considerations in the management of shallow lakes was illustrated. The study was carried out in the freshwater, postglacial lakes Niepruszewskie and Tomickie, located in the central-western agricultural region of Poland. The research comprised analyses of the lakes’ morphometry base on topographic maps, aerial photos, GPS measurements, and data from bathymetric plans and morphometric cards. The study has shown that the observed transformations in the examined lakes are multidirectional and the entire process varies in changing trophic and hydrological conditions. The increase of water retention in the water body increased the lake area, but at the same time accelerated the sedimentation process of organic matter and gradual lake shallowing at the accompanying expansion of the littoral zone area. During almost 70 years, the area of Lake Niepruszewskie shrank by 9%. The shallowing rate, which is not directly visible, was about three times as fast as the contraction of the area itself. In the case of the second of the examined lakes, shallowing and overgrowing processes follow the same course, which will lead to the disappearance of the lake within less than 56 years. The area of Lake Tomickie decreased almost twofold during almost 70 years. Our study shows that analysis of the lake surface alone is insufficient to assess directions of lake development and shrinking.
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