Revealing the dark side of the lake: How winter dynamics control summer conditions in northern prairie lakes

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The high variability of water chemistry and food-web structure in hardwater lakes across the Canadian prairies and their strong dependencies on climate conditions make these lakes ideal systems to evaluate how climate change will impact lake ecosystems. Global circulation models project that temperature increases will be highest in winter, which will shorten the duration of ice cover and change timing and amount of snowmelt (the major supply of water to these lakes). Yet, most research on temperate lakes has been conducted during the ice-free season while winter was considered a dormant period. Hence, our knowledge of lake dynamics and their controls is largely derived from summer conditions. Recently, it has been recognized that under-ice biogeochemical processes can not only be highly dynamic but also significantly influence biological, physical and chemical processes in summer. With climate as a master variable, it will be crucial to carefully characterize current winter dynamics to determine the importance of direct and indirect effects of future milder winters, particularly in hardwater lakes. My previous work indicated that winter oxygen dynamics are controlled by under-ice respiration, both in hardwater and boreal lakes. In contrast, controls of carbon and nitrogen cycles differ strongly from boreal lakes with hydrologic, physical and chemical factors being more influential. Hence, the focus of my proposal will be to partition the importance of metabolic (respiration, primary production) vs. physical (diffusion), chemical (nitrification, C precipitation) and hydrological (e.g., groundwater inputs, snow melt) processes during the fall-winter-spring transitions in prairie lakes, using a combination of laboratory experiments and field observations. ***The proposed work will, for the first time, not only assess the interactions of biogeochemical processes under ice, but also determine the relative importance of metabolic, chemical and physical contributions to C, O and N dynamics. This work will be important for predicting inhabitability of lakes for fishes and invertebrate communities, and better understanding the role of prairie lakes in the global carbon cycle. Subsequently, it will enable us to forecast how future climate change may influence any or all of these processes: Specifically, will the effects of warmer summer temperatures (thermal stress, eutrophication, reduced water levels) and milder winters (lower winterkill risk, increased carbon capture) be synergistic, protagonistic or independent? On a global scale, hardwater lakes (such as those across the Canadian prairies) represent almost 50% of all inland waters. Hence this work will also be highly relevant globally to understand structure and functioning of hardwater lake ecosystems and anticipate future impacts of climate change, population growth and agricultural development.