Is Everything Lost? Recreating the Surface Water Temperature of Unmonitored Lakes in Poland

One of the fundamental features of lakes is water temperature, which determines the functioning of lake ecosystems. However, the overall range of information related to the monitoring of this parameter is quite limited, both in terms of the number of lakes and the duration of measurements. This study addresses this gap by reconstructing the lake surface water temperature (LSWT) of six lakes in Poland from 1994 to 2023, where direct measurements were discontinued. The reconstruction is based on the Air2Water model, which establishes a statistical relationship between LSWT and air temperature. Model validation using historical observations demonstrated high predictive accuracy, with a Nash–Sutcliffe Efficiency exceeding 0.92 and root mean squared error ranging from 0.97 °C to 2.13 °C across the lakes. A trend analysis using the Mann–Kendall test and Sen’s slope estimator indicated a statistically significant warming trend in all lakes, with an average increase of 0.35 °C per decade. Monthly trends were most pronounced in June, September, and November, exceeding 0.50 °C per decade in some cases. The direction, pace, and scale of these changes are crucial for managing individual lakes, both from an ecological and economic perspective.