Seasonal changes in trophic interactions: physiological mechanisms and consequences for energy and contaminant flow in a Canadian Shield lake

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Winter periods are critical times of the year for aquatic animals. Climate change is already dampening winter conditions in temperate latitudes. Given this, it is surprising that seasonal changes in trophic interactions, especially during winter, are poorly studied. The long term goal of my research is to anticipate how climate change will affect aquatic animals and ecosystems by studying the physiological mechanisms and ecosystem-level consequences (for energy and contaminant flow) of seasonal variation in trophic interactions. Over the next 5 years, I will conduct seasonal sampling in a well-studied lake (Lake Opeongo, Ontario) during summer, autumn, winter and spring to address 3 objectives.In Objective 1, MSc1, 2, 3 and I will explore seasonal changes in fish activity level using multiple physiological indicators (energy use, metabolic enzyme activity, gut structure and function). Fish that reduce their activity level during winter should deplete their energy stores, reduce metabolic enzyme activity and reduce the size and enzyme activity of their digestive system compared to fish that sustain winter activity.For Objective 2, PhD1 and I will apply stable isotopes (d13C, d15N, d34S) and fatty acids of multiple tissues with different turnover rates (plasma, muscle) to identify how winter diet sources contribute to a fish's annual energy and nutrient supply. We will also measure how seasonal changes in trophic interactions drive shifts in food web structure (food chain length and width) and important ecosystem functions (predator biomass and body condition) across multiple seasons and years.In Objective 3, PhD2 and I will quantify how prey mercury (Hg) levels and fish diet change seasonally and affect total fish Hg body burdens. This will provide some of the first data for how Hg uptake and availability change from open water to ice cover periods in a temperate lake.The proposed research will train 5 graduate and 8 undergraduate HQP. Objective 1 will provide the most comprehensive understanding to date of how different fish within a community have adapted their physiology and foraging behavior to cope with existing seasonal conditions. Objective 2 and 3 will identify how differences in physiology and winter activity drive shifts in trophic interactions, food web structures and contaminant dynamics throughout the year and produce new knowledge about critical sources of energy and Hg during poorly-studied winter periods. This work will conceptually advance the field of ecology by fundamentally identifying key mechanisms and processes by which ecosystems respond to variable conditions. It will also lead to breakthroughs in our ability to anticipate which species are most susceptible to warmer, shorter winters, advance conservation and management efforts and provide an unprecedented ability to signal for declines in the function of Canada's freshwater systems under a warming climate.