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

ABSTRACT

Epilimnetic temperatures from early 1980s through 2017 were analyzed for 12 Wisconsin (USA), 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 2 northeastern German lakes, temperatures increased in all seasons (not all significantly). The 3 Finnish lakes exhibited large spring temperature increases influenced by earlier ice-out; summer and fall temperatures also increased except for Konnevesi (central Finland) in early summer and polar Kevojärvi in midsummer. Kevojärvi also exhibited no increase in maximum recorded temperatures. Earlier records from 4 lakes were analyzed to elucidate longer-term epilimnetic temperature patterns. For Lake Mendota (southern Wisconsin), spring and late fall temperatures have increased modestly but significantly since 1894; summer temperatures have also increased modestly because of higher frequency of warm temperatures in recent summers 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 cooler air temperatures. 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 predict future effects of climate change on lake ecosystems.

KEYWORDS:

• climate change
• coherency
• Kevojärvi
• Mendota
• Stechlinsee

Acknowledegments

Our analyses were supported by the University of Wisconsin-Madison Center for Limnology and its North Temperate Lakes LTER Program (NSF Grant #DEB- 1440297), the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and the Pyhäjärvi Institute. We also wish to thank the many staff scientists, students, and technicians who have collected lake temperature data on our study lakes over the course of many decades, sometimes in less than ideal weather conditions. In addition to contributions from personnel at the aforementioned institutions, we acknowledge the contributions from the Wisconsin Dept. Natural Resources, the Finnish Environment Institute SYKE, the Konnevesi Research Station, and the Kevo Subarctic Research Station. Finally, we thank 2 anonymous reviewers for their valuable suggestions to improve the manuscript, as well as suggestions from a US Geological Survey reviewer.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Dale M. Robertson http://orcid.org/0000-0001-6799-0596