Seasonal epilimnetic temperature patterns and trends in a suite of lakes from Wisconsin (USA), Germany and Finland

Epilimnetic temperatures from the early 1980s through 2017 were analyzed for 12 Wisconsin, German and Finnish lakes. Seasonal temperature metrics exhibited large interannual variability with trends differing among regions. In the Wisconsin lakes, only late summer and fall temperatures increased significantly. In the northeastern Germany lakes, temperatures increased in all seasons, but only significantly for some metrics. The Finnish lakes, which spanned the country’s latitude range, exhibited large spring temperature increases influenced by earlier ice-out; summer temperatures also increased significantly, but fall changes were varied. To elucidate longer-term epilimnetic temperature patterns, earlier records from 4 lakes were analyzed. For Lake Mendota (southern Wisconsin), spring and late fall temperatures increased modestly but significantly since 1894; summer temperatures also increased modestly due to a higher frequency of recent summers with warm temperatures and not from new record high temperatures. Trout Lake (northern Wisconsin) exhibited warm temperatures in some summers during the 1930s-1940s similar to warm temperatures in some recent summers. Air-water temperature relationships coupled with long-term regional air temperature data also indicated summer epilimnetic temperatures in the study lakes were likely as warm in the 1930s-1940s as in recent years. Lake data confirmed cooler epilimnetic temperatures occurred in many summers during the 1950s-1980s coincident with intervening cooler air temperatures during this period. Because epilimnetic temperatures have not increased monotonically since 1900, our study supports continued temperature monitoring in lakes with extensive historical data to better understand and project future effects of climate change on lake ecosystems.