Food Web Changes Dampen Expected Reductions in Lake Trout Mercury Levels in Lake Michigan—Invasive Species Play Major Role

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Mercury has been recognized as a neurotoxin that can accumulate in fish that might be consumed later by animals and humans. Beginning in the 1970s, numerous factors have resulted in reduced mercury releases in the Great Lakes Region including the Clean Water Act, Mercury Export Ban Act, sulfur and nitrous oxide controls, Mercury and Air Toxics Standards Rule, and changes in energy production from coal to natural gas. However, the connection between the resultant reduced air emissions and fish tissue concentrations has been difficult to establish.

Currently (2019), there are fish consumption advisories for elevated concentrations of mercury in consumable fish from all five Great Lakes, yet natural resource managers in this region are hampered by a poor understanding of how mitigation of sources relates to bioaccumulation responses and effects, which hampers effective strategies to reduce exposure levels for fish and people who consume them.

Therefore, U.S. Geological Survey (USGS) scientists on the Ecologically-Driven Exposure Pathways Science Team studied the combined impact of reduced environmental mercury source loading and food web changes from invasive species on methylmercury bioaccumulation in a predator fish of Lake Michigan. Mercury, carbon, and nitrogen stable isotopes were measured in lake trout tissues archived from 1975 to 2015 by the U.S. Environmental Protection Agency’s (EPA) Great Lakes Fish Monitoring and Surveillance Program, and mercury isotopes were measured in dated sediment cores from Lake Michigan.

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Through combined isotopic analyses, scientists were able to determine that fish tissue concentrations were related to temporal changes in mercury sources and to changes in food web influences owing to increased populations of invasive species. Mercury in fish tissue and bottom sediments decreased rapidly coincident with a reduction of sources (presumably medical and municipal waste incineration) in the late 1980s. However, fish mercury concentration declines were not apparent in the late 1990s when additional actions in the United States lead to sustained reductions in mercury releases. This dampening was coincident with food web shifts in Lake Michigan following the invasive zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis) invasions of the 1990s. This shift dramatically changed carbon and nutrient dynamics in Lake Michigan and resulted in dietary changes of lake trout that presumably lead to consumption of prey items higher in mercury content.

These results reveal why trends in fish mercury concentrations may not mirror declining regional and national mercury emissions in the United States, which implies that mercury concentrations in fish cannot be predicted by emission inventories alone. The results highlight the importance of including ecological factors such as food web changes into trend analyses to consider multiple factors that can be operating simultaneously to affect desired mitigation outcomes.

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The USGS Ecologically-Driven Exposure Pathways Science Team is continuing to provide comprehensive science on sources, movement, transformation, bioaccumulation, and toxicological outcomes of mercury and other toxicants across the United States in collaboration with other Department of Interior bureaus, other Federal agencies (for example, EPA, National Institutes of Health-National Institute of Environmental Health Sciences) academia, State and local agencies, and the private sector.

This study was supported by the Toxic Substances Hydrology and the Contaminant Biology combined programs; EPA Great Lakes National Program Office, Great Lakes Restoration Initiative; EPA Office of Research and Development; University of Wisconsin-Madison; Minnesota Science Museum, St. Croix Watershed Research Station; and the Chinese Academy of Sciences-Institute of Geochemistry.