Year-round water temperature patterns of warming and cooling in Minnesota lakes

Schoenebeck CW, Martin TJ, French WE. 2026. Year-round water temperature patterns of warming and cooling in Minnesota lakes. Lake Reserv Manag. XXX–XXX.Understanding annual water temperature patterns and when and where changes are occurring is essential to understanding the effects on aquatic ecology and identifying workable management strategies. We analyzed continuous epilimnetic water temperature data to identify trends in both annual and weekly water temperature metrics to evaluate if changes were uniform throughout the annual water temperature cycle (when are changes occurring) and if trends were similar across geographic regions (where are changes occurring). Beginning in 2008, the Sentinel Lakes Long Term Monitoring Program has collected year-round water temperature using continuous water temperature loggers in 25 Minnesota lakes. Using the Sentinel Lakes data, we found mean weekly water temperature and growing degree days have changed over time in Minnesota and differed among weeks of the year, geographic regions, and lake mixing type. Most of these trends consisted of warming temperatures during weeks in June and July; however, significant cooling trends in April were also identified. Maximum epilimnetic water temperature increased over time and varied by geographic region, warming the most rapidly within the Northern Lakes and Forest region. Geographically, more significant warming trends occurred in the Northern Lakes and Forests region, while the Canadian Shield region had the lowest amount of warming. Understanding where the slowest rate of change is occurring is important when working to conserve coldwater oxythermal habitat. We encourage managers to consider sampling protocols based on temperature guidelines instead of calendar dates, as changing water temperatures will affect metrics collected during established sampling dates.