WRS Masters Defense

Mercury Concentration and Aquatic Food Web Alterations Associated with Zebra Mussel Invasion in Minnesota Lakes

Zebra mussels (Dreissena polymorpha) are an invasive species documented in 299 Minnesota lakes, with 231 of those lakes managed as walleye fisheries. Many ecological changes are associated with zebra mussel invasion, including increased water clarity, depleted pelagic energy resources, expanded littoral zones, deposition of benthic organic matter causing anoxic sediments, and increased benthic energy reliance of food webs. However, the effects of zebra mussels vary among lakes, and few opportunities exist to examine lake ecosystems and food web pre- and post- zebra mussel invasion. Furthermore, recent evidence has suggested that Dressenid mussels may impact contaminant bioaccumulation in higher trophic levels, but these effects have not been well-studied in inland lakes. I took two distinct approaches to understanding zebra mussel impacts on food webs and contaminants in fishes. I combined historic Minnesota statewide mercury monitoring data from 1997-2021 with stable isotope data to provide insight into food web dynamics and mercury concentration alterations with zebra mussel invasion. The probability of exceeding the Minnesota safe threshold for safe eating of average sized northern pike (Esox Lucius) and walleye (Sander vitreus) ranged from 60%-70%, with zebra mussel lakes having a higher chance. Specifically, mercury concentrations analyzed using before-after control-impact (BACI) study design increased by 8.2% in northern pike and 15.4% in walleye for invaded lakes, while uninvaded systems were stagnant or decreased. To quantify resource use and community structure, food webs in Leech Lake were analyzed pre and post invasion using stable isotope analysis of δ13C and δ15N. Fish community response to zebra mussel invasion varied spatially in Leech Lake. Bays on the western shoreline of Leech Lake with heterogeneous habitat increased niche size by 39.81% and fish in those areas relied more heavily on benthic resources following invasion. In contrast to the western bays and to hypothesized effects of zebra mussels, fish from the main basin of Leech Lake, containing mostly homogenous pelagic resources, decreased in niche size by 32.26% and relied more heavily on offshore resources. Taken together, high mercury concentrations in Minnesota northern pike and walleye are exacerbated in zebra mussel lakes. Food web dynamics are variable within the same lake, suggesting preexisting food web structure and access to benthic resources are important to community resilience with zebra mussel invasion.