Surface water temperatures are warming in many lakes across the globe, and this is widely attributed to warming air temperatures. Yet two lakes in Pennsylvania (USA) have shown long-term increases in surface water temperatures over the past 27 summers during a period with no significant increase in regional air temperature. We examined the relationship between long-term trends in seven metrics of whole-lake thermal structure in two lakes and several potential driver variables. Driver variables included water transparency, lake pH, and meteorological variables. Both lakes exhibited significant surface warming and hypolimnetic cooling, resulting in stronger thermal stratification that further reduced mixing and heat transfer to deep waters. During this time period, there were no long-term trends in solar radiation or in thawing degree days, but annual precipitation and lake pH increased. Water transparency greatly decreased due to increased dissolved organic matter quantity and color, most likely due to increased precipitation and recovery from anthropogenic acidification. In both lakes, the changes in lake thermal structure and heat distribution were strongly related to the decreases in water transparency and increases in dissolved organic matter. This transparency-mediated mechanism may augment the effects of air temperature-driven lake warming in other regions where decreasing transparency is also prevalent, further enhancing increases in surface water temperature and thermal stratification. These results have important ecological and biogeochemical implications, highlighting the need for investigations of multiple drivers to fully understand how lakes will respond to future climate change.
Bibliographical noteFunding Information:
We thank the Lacawac Sanctuary and Field Station for access to Lake Lacawac and use of research facilities. This work benefited from participation in and travel support from the Global Lake Ecological Observatory Network. We thank Bill Renwick and Ben Felzer for GIS support, climate analysis, and helpful feedback on previous versions of this manuscript. We thank Bruce Hargreaves for providing wind data and both Bruce Hargreaves and Nihar Samal for their helpful comments and suggestions on previous versions of this manuscript. John Melack and Sally McIntyre provided extensive discussions that have substantially improved the manuscript. We also thank Erin Overholt and the Global Change Limnology Research Laboratory at Miami University for their logistical support and Miami University’s Center for Analytics and Data Science for their financial support. Data used in this study are available through the Environmental Data Initiative (EDI; https://environmentalda-tainitiative.org). We acknowledge support from Miami University and NSF Grants DEB-1754265 and DEB-1754276. The authors declare that they have no conflict of interest.
- climate change
- lake browning
- thermal stratification
- thermal structure
- water transparency