AQUATIC HEATWAVES INCREASE PRIMARY PRODUCTION IN THREE NORTH TEMPERATE LAKES

Widespread eutrophication of inland waters is interacting with changing temperatures globally, potentially altering algal production and resulting in harmful blooms. While increases in mean water temperature are linked to enhanced primary production, it is unclear how discrete weather extremes like heatwaves alter phytoplankton dynamics in lakes. Heatwaves may increase phytoplankton production by enhancing photosynthesis and growth, but could drive declines in phytoplankton if extreme temperatures cause damage to individual cells or disproportionately increase respiration. We investigated changes in phytoplankton biomass following 27 summer heatwaves between 2008-2019 in three north temperate lakes located in Michigan, United States using daily summer measurements of surface chlorophyll a. These lakes vary in water color and two of the lakes were experimentally enriched with nutrients in some years of the study period, offering a unique opportunity to study the effects of heatwaves on phytoplankton under a variety of nutrient and light conditions. On average, aquatic heatwaves in the study lakes increased phytoplankton biomass by 57%, but the magnitude of change due to the heatwaves varied by year, lake, and event. For example, the low-nutrient reference lake had a lower mean increase in chlorophyll following heatwaves (31%) than either of the nutrient addition lakes (69%, 97%). Our results suggest that the interacting effects of other global change drivers such as eutrophication and brownification will help shape the response of phytoplankton to extreme heat events.

Primary Presenter: Daniel Szydlowski, Center for Limnology (dszydlowski@wisc.edu)

Authors:

Daniel Szydlowski, Center for Limnology, University of Wisconsin-Madison  (dszydlowski@wisc.edu)

Katie Bollini, Center for Limnology, University of Wisconsin-Madison  (bollini@wisc.edu)

Michael Pace, Department of Environmental Sciences, University of Virginia  (mlp5fy@virginia.edu)

Grace Wilkinson, Center for Limnology, University of Wisconsin-Madison  (gwilkinson@wisc.edu)