New research from the University of Minnesota found that zebra mussel invasions were associated with increased mercury concentrations in several Minnesota fish species.
Using the Minnesota state fish contaminant database, researchers examined mercury concentrations from 357 lakes and six fish species frequently targeted by anglers. Their results were recently published in Fisheries.
The research was funded by the University's Minnesota Aquatic Invasive Species Research Center supported by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).
The researchers found:
- Mercury concentrations increased in harvestable-sized fish in four of six species after zebra mussels became established, relative to their concentrations pre-establishment in the same lake.
- The magnitude of increase varied among lakes and species, but the averages across the lakes studied are as follows:
- Black crappie - mercury increased 21%
- Bluegill - mercury increased 19%
- Northern pike - mercury increased 9%
- Walleye - mercury increased 6%
- Largemouth bass - mercury increased modestly, but the change was not statistically significant
- Yellow perch showed no meaningful change in mercury
- Walleye, northern pike and largemouth bass were more likely to exceed a 0.22 ppm mercury threshold after zebra mussels became established, and some species reached that threshold at smaller body sizes. This threshold is used frequently to distinguish between populations where sensitive populations (children and women of childbearing age) are recommended to eat one meal per month vs one per week.
"The findings show that zebra mussels can alter food webs and contaminant cycling in ways that may affect both aquatic ecosystems and human consumption of fish," said senior author Gretchen Hansen, an associate professor in the College of Food, Agricultural and Natural Resource Sciences. "It also highlights the complexity of separating changes associated with invasive species from those caused by other environmental changes, including climate change, water level and nutrient loading."
Zebra mussels can form dense mats on lake bottoms, and in doing so can deplete oxygen in lake sediments. This makes conditions more favorable for mercury uptake by living things and can lead to higher mercury accumulation in fish.
For people who regularly eat fish, the changes could mean eating slightly fewer meals per month to stay below recommended mercury exposure levels. The impact was relatively small for walleye and northern pike, about 0.3 fewer meals per month, but larger for bluegill and black crappie, at roughly 2.8 and 2.6 fewer meals per month, respectively when looking at an average of the lakes studied. However, it is important to note that not every lake responds the same way and that the magnitude of these changes may depend on preexisting lake conditions. Waterbody specific guidelines are available when applicable.
This study demonstrates that biological invasions can reshape contaminant dynamics at landscape scales, underscoring the global connection between invasion ecology and human health.
About the College of Food, Agricultural and Natural Resource Sciences
The University of Minnesota's College of Food, Agricultural and Natural Resource Sciences strives to inspire minds, nourish people, and sustainably enhance the natural environment. CFANS has a legacy of innovation, bringing discoveries to life through science and educating the next generation of leaders. Every day, students, faculty, and researchers use science to address the grand challenges of the world today and in the future. CFANS offers an unparalleled expanse of experiential learning opportunities for students and the community, with 12 academic departments, 10 research and outreach centers across the state, the Minnesota Landscape Arboretum, the Bell Museum, and dozens of interdisciplinary centers. Learn more at cfans.umn.edu.
