Zebra mussels (Dreissena polymorpha) are aquatic invasive species that dramatically change lake ecosystems, with impacts on water quality and food webs throughout the upper Mississippi river watershed. Because of their wide reaching effects on lake ecosystems, zebra mussels may influence economically and culturally important predatory fishes such as walleye (Sander vitreus) through a multitude of complex interactions. These include potential habitat loss or alterations due to increasing water clarity, changes in water quality (nutrients), loss of pelagic food resources supporting larval and adult walleye, and increased mercury exposure due to shifts in diet, foraging locations, and increases in mercury methylation (production of more toxic and bioaccumulative methylmercury). Each of these impacts varies among lakes. Here, we propose to quantify the impacts of zebra mussels on walleye habitat, food webs, and mercury concentrations across a gradient of lakes in the upper Midwest, and to identify factors that confer resilience of lakes and walleye populations to these impacts.
We will use a multi-faceted, multi-disciplinary approach to achieve these goals. First, we will quantify the impacts of zebra mussels on walleye thermal optical habitat using an existing thermodynamic model of lake temperature and light conditions. Second, we will assess the sensitivity of walleye habitat to simulated changes in water clarity across a landscape of thousands of lakes to enable estimation of lake characteristics associated with sensitivity or resilience to water clarity change. Third, we will use stable isotope analysis of Carbon and Nitrogen in walleye, other fishes, and primary consumers in 16 lakes (8 invaded, 8 uninvaded). We will collaborate with state and tribal natural resource agencies to focus on priority lakes for walleye angling, spearing, and consumption. We will compare trophic position and littoral reliance of walleye in invaded and uninvaded lakes. Finally, we will estimate mercury concentrations and use mercury stable isotope analysis of walleye tissues to quantify how exposure and methylmercury production rates are impacted by zebra mussels.
The ultimate goal of this work is to understand the multiple indirect pathways through which invasive zebra mussels impact walleye, and to identify factors that cause lakes to be more or less vulnerable to these impacts. Any zebra mussel-induced changes in walleye habitats, populations, and contamination level of walleye have important implications for harvest, stocking, and consumption of walleye in lakes of the upper Mississippi river basin.