[Research] Invasive Mussels Complicate Efforts to Reduce Mercury in Great Lakes Food Chain
Posted on: December 4, 2019
Zebra Mussels are making it harder for native fish species to get nutrients and researchers at the University of Wisconsin-Madison are finding that this is leading to fish that are enriched in mercury despite efforts by Canada and the U.S. to reduce mercury loading in the Great Lakes. USFWS Photo.

Mercury is a naturally occurring element which poses several challenges when incorporated into fish as a contaminant, methylmercury. When released to the environment from the earth’s crust, either via natural degassing processes or due to anthropogenic activities like burning fossil fuels, mercury enters into a complex cycle, interacting with other elements and forming a variety of compounds. It can remain in the atmosphere for 6-12 months before being deposited in a water soluble form. Water soluble mercury can then be converted to the highly toxic and bioaccumulative methylmercury, which can cause both long-term and short-term damage to kidneys, the brain, and developing foetuses, when consumed in significant amounts by humans.

A new study published in the November 2019 issue of the “Proceedings of the National Academy of Science” shows that Canadian and U.S. reductions of mercury inputs to the Great Lakes (mercury emissions went down 60% between 1990 and 2005 in the U.S., and 85% between 1990 and 2010 in Canada) hasn’t been reflected in reduced mercury in fish to the extent that might have been expected. Through stable isotope ratio analysis in fish and sediment samples from 1978 to 2012, the researchers of Mercury source changes and food web shifts alter contamination signatures of predatory fish from Lake Michigan (Lepak et. al) discovered that an initial decline in mercury concentrations in fish immediately after efforts to reduce domestic mercury inputs did not continue at the expected rate into the 90s and 2000s. An explosive increase of invasive dreissenid mussel populations in the region, zebra mussels in the 90s and quagga mussels in the 2000s, coincided with this stunted rate of mercury reductions in fish. The researchers suggest that this may be due to the impact of invasive species on food web pathways.

Infosuperior spoke with lead author Ryan Lepak, postdoctoral researcher at the UW–Madison Aquatic Sciences Center (ASC), over email to get a better idea of what this research tells us about the Great Lakes ecosystem and shifting mercury sources.

Lead author Ryan Lepak at work, testing Lake Michigan sediment for mercury. Photo courtesy of Ryan Lepak.

Q:  Your research found that reducing the mercury load on the Great Lakes hasn’t been reflected in reduced mercury in fish because of the effect of invasive species on what foods are available to fish. Is it possible to estimate how long it will take for the reduced mercury emissions to be depicted in the food chain if we take these findings into consideration?

Ryan Lepak: As you might know from the many examples worldwide, we cannot begin to predict the interactions invasive species might have on the trophic structure of a lake and the energy pathways fish use to survive. Contaminants like mercury are closely linked to these pathways adding an increased level of complexity. That said, here’s what we’re sure of:

  1. Without reductions to mercury emissions in the past, [mercury levels in] fish today would be even higher (we know this because the air, water and sediment mercury are all lower now than they were 20 years ago.)
  2. Mercury is an element and unlike many contaminants (like organic chemicals) it cannot be destroyed. Human activities have increased the amount of mercury cycling in the world and we’re highly unlikely to ever achieve zero mercury fish.

That said, it is reassuring to find that our strategies to reduce mercury in the environment is actually resulting in quicker declines locally than we’d ever expect. (See: PNAS: Zheng et al., Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions)

Photo courtesy of Ryan Lepak

Q: In the news release from University of Wisconsin Sea Grant, it is explained that dreissenid mussel invasion of the Great Lakes has pushed fish to seek food sources that are more enriched in methylmercury from the lake bottom and nearshore environments. What makes these food sources more enriched in methylmercury?

Ryan Lepak: In many lakes, the bottom is enriched in methylmercury because the microbes that make methylmercury from mercury do their work in the absence of dissolved oxygen. Lake Michigan does not have an appreciable layer of water lacking oxygen. Instead, fish are now eating items from the bottom and nearshore that are nutritionally less valuable and they are working harder to find a meal thus becoming leaner, which in the fish world is bad. The rate of change to these nutritional/diet/energy pathways is changing faster than we are reducing mercury loading to the lake, leading to the false impression that fish [mercury levels] are increasing. In fact, to compare mercury concentrations between fish now and in the past you need to consider how lifestyle changes have impacted the overall well being of the fish. Presently, because of invasive species, lake trout are growing slower and therefore a fish at 400 mm I collect today might be 3-4 years older than a 400 mm fish from 1995. With increased age comes increased time to accumulate mercury.

Q: So fish are having to consume more food that is contaminated with mercury to meet their energy needs and they are becoming leaner because they have to work harder for that energy, which results in higher concentrations of mercury in the fish than would be present if they did not have to adjust their eating habits. Do we know what the concentrations of Mercury in the dreissenid mussels are, relative to what is found in the fish?

Ryan Lepak: They’re actually quite low, maybe 1/10th the amount. 

Q: What is the biggest take-away from your research?

Ryan Lepak: The important story here is that using carbon, nitrogen and mercury fingerprinting (isotopes) we are able to identify shifts in sources of mercury to lake trout following the removal of major mercury sources regionally. Without that aid, we would assume that mercury controls do not work if we solely looked at mercury concentrations in fish through time. In fact what we’ve learned is that the great lakes benefit greatly from local mercury reductions and that invasive species have altered contaminant cycling and bioaccumulation pathways in a considerable way. This highlights the tremendous value of programs focused on monitoring and archiving. Environment Canada and the Swedish hold the only such archives in the world. It’s ironic, collectively we’ve focused intensely on reducing chemicals in the Great Lakes (for good reason) to improve chemical burden on fish for human and wildlife safety while largely ignoring the biological “contaminants” (invasive species). Now we’re finding that due to invasive species, the chemical burden on a similar sized fish is greater, not because we’ve mismanaged chemical loading/inventories but because the biology of the system has changed. 

Ryan Lepak and his co-authors’ research allows us to better understand and monitor Mercury contamination in the Great Lakes food web. They show that we cannot solely focus on the total amount of mercury emissions, but must also look at how these emissions reach the fish we eat through a dynamic system where invasive species are having a major impact on food availability for native fish populations.


Lepak, Ryan F., et al., “Mercury source changes and food web shifts alter contamination signatures of predatory fish from Lake Michigan.” Proceedings of the National Academy of Sciences Nov 2019, 116 (47) 23600-23608; DOI: 10.1073/pnas.1907484116

Zhang, Yanxu, et al., “Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions.” Proceedings of the National Academy of Sciences Jan 2016, 113 (3) 526-531; DOI: 10.1073/pnas.1516312113


University of Wisconsin Sea Grant News Release – Aquatic invasive species are short-circuiting benefits from mercury reduction in the Great Lakes

2005 Water Quality Association Mercury Fact Sheet

EPA Mercury Emissions: The Global Context

IJC 2015 Article – Reducing Great Lakes Mercury Contamination: Regional Efforts May Not Be Enough

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