Healthy lakes to support people and fish

Lakes across the Midwest are facing a number of challenges, from nutrient pollution, to invasive species, to climate change. With the health of our lakes and the people who love them at stake, Gretchen Hansen, faculty member in the Department of Fisheries, Wildlife, and Conservation Biology, is tracking the impacts and researching solutions. 

One challenge boaters in Minnesota are very familiar with is the spread of invasive species, like zebra mussels. Zebra mussels are known to harm native mussel populations, cut the feet of swimmers, and alter the energy flow through aquatic ecosystems, impacting fish populations. But the impacts don’t stop there. Hansen and her team identified two possible impacts to watch — elevated mercury levels in fish and an increase in swimmer’s itch. 

The threat of mercury

Mercury, a potent neurotoxin, poses a serious threat to both aquatic life and human health. Hansen and her team have used a number of approaches to track how mercury in fish is affected by zebra mussels, and understand what might be done about it. In a 2024 study of 21 walleye lakes, they found walleye exhibited 72 percent higher mercury concentrations in lakes invaded by zebra mussels, sometimes exceeding the thresholds related to consumption advisories for human health at small sizes. Yellow perch exhibited even greater increases, with average-sized yellow perch having a 50 times greater risk of exceeding mercury thresholds in zebra mussel invaded lakes. 

Building on this work, Hansen and her team have analyzed statewide patterns in mercury and found similar results across multiple species. Currently, they are working in collaboration with MN DNR and the Leech Lake Band of Ojibwe to dive deeper into understanding the mechanisms behind these observed patterns, and to develop monitoring and fish harvest strategies in response.

“We don’t know the exact mechanism behind these results yet,” explains Hansen. “Early stages of this research focused on the ‘what’ - identifying patterns of increased mercury in fish from lakes containing zebra mussels. We are now focused on the ‘why’, the ‘how’, and perhaps most importantly the ‘so what’ - working with partners to identify strategies to support balancing health risks of mercury with the cultural and health benefits of fish consumption..” Increased mercury concentrations in fish add to a growing list of known negative effects of invasive zebra mussels and highlight how novel ecological stressors like invasive species can interact with existing stressors like heavy metal contamination within inland lakes to affect the environment and human health in unexpected ways. 

An irritating itch

Swimmer’s itch is another human health impact that could be linked to aquatic invasive species. A skin irritation caused by an allergic reaction to parasitic flatworm larvae, anecdotal reports of increasing severity and prevalence are cropping up all around Minnesota’s lakes. The environmental factors driving outbreaks remain poorly understood. However, current research is examining how zebra mussels and other invasive species like mystery snails, may alter ecological conditions and favor parasite prevalence. 

Conserving cold water

Conserving "coldwater" habitat, especially in the face of climate change, has impacts for fish habitat and water quality. Not all lakes are the same or will require the same preservation strategies. Hansen and her team are identifying differences among lakes and their sensitivity to climate change and nutrient loading that influence their capacity to support coldwater fish populations. They hope to identify watershed and shoreline protection strategies that are tailored to Minnesota’s diverse lake types to protect coldwater habitat even as climate change alters temperature and precipitation patterns. Ultimately, this work aims to provide guidance on how to deploy resources effectively and efficiently to protect lakes, fish, and water quality. 

“We want people to get the most bang for their conservation buck...” Hansen explains, “to put their money in places where it is most likely to succeed in conserving coldwater habitat.” 

In a 2022 study, The team used statistical models to estimate the resilience of over 10,000 lakes in the upper Midwest to both climate change and land use. For lakes where land use was predicted to influence coldwater habitat, the researchers identified lake-specific thresholds for protecting or restoring watersheds. Currently, they are expanding on this work to update estimates of protection needs based on increased mega rain events, and to understand why some lakes appear more resilient to watershed development than expected, while others are more sensitive. By studying these outlier lakes, they hope to identify strategies that can be used elsewhere to conserve coldwater fish habitat even as conditions change.

“Sometimes we can feel hopeless when thinking about big challenges like climate change or water quality,” Hansen reflects. “But we really want to focus our research on ways we can make a difference and help tailor our approach to conserving our lakes using the best available science.” 

Explore more ways CFANS researchers are monitoring the health of lakes, fish, and people:  

• Study reveals elevated mercury levels in fish associated with zebra mussel invasions
• Conserving critical habitat in the face of climate change in Midwestern lakes by managing watershed land use
• Understanding the influence of AIS on swimmer’s itch 
• Scientists look for cause of spike in swimmer’s itch plaguing Minnesota lakes