Lake surface changes of the Osa River catchment, (northern Poland), 1900-2010

The study objective was to prepare a quantitative inventory of lakes and to assess their long-term changes. The research area included the Osa river basin (northern Poland), which is located in a lake district. The research was based on historical cartographic materials published in 1900-1947 (German 1:25,000 'Messtischblatt' topographic maps) and modern topographic maps from 2010 (The 1:50,000 Digital Map of the Hydrographic Division of Poland [MPHP]. The number of lakes increased from 173 to 235, while the total surface area of the lakes fell by 107.55 ha. The largest number of lakes belonged to the 1.0-5.0 ha range on both MPHP and Messtischblatt maps. It was in this range that the largest increase in number of lakes and lake area relative to the historical maps was found. A significant reduction in area of lakes was recorded in the group of lakes with an area from 50 to 100 ha.