Contemporary Evolution and Water Quality of Lakes Rewetted After 19th Century Drainage in the Olsztyn Lake District (Poland)

Rewetting of drained wetlands is practiced as a method to enhance biodiversity, improve water and nutrient retention, and counteract climate change. While rewetting efforts are most commonly directed toward various types of wetlands, there are relatively few reports on the restoration of lakes drained in the past. The Olsztyn Lake District is a region where extensive, organized drainage works were carried out in the 19th century, leading to the disappearance of numerous lakes. This paper examines the changes that have occurred since the early 19th century in a group of 143 lakes identified as the complete set of fully drained lakes in the region. An analysis of cartographic materials revealed that the total area of these lakes, originally about 3000 hectares, was reduced to nearly zero by the early 20th century. However, a gradual restoration of the former lakes is now being observed, primarily as a result of spontaneous processes but also through planned interventions. The study of water quality and trophic status in 25 fully restored lakes revealed that such water bodies typically exhibit unfavorable conditions, primarily due to excessive phosphorus levels. A similar state was observed in 14 examined wetlands, which were partially rewetted. In the absence of organized restoration programs in the study region, the slow trend of passive rewetting of such water bodies is expected to continue. However, this process may be hindered by adverse hydroclimatic changes associated with ongoing climate warming.