According to a new study published today in the Proceedings of the National Academy of Sciences 40 years of reduced mercury use, emissions and recharge in the Great Lakes region have largely not led to equivalent declines in mercury content, accumulates in large game fish.
Researchers, including those at the University of Wisconsin-Madison, argue that this is largely due to invasive aquatic species in Lake Michigan – most notably quasi and zebra mussels – that have stopped the food web and forced fish to look atypical mercury-rich food sources.
Mercury or methyl mercury, as it exists in fish, is a neurotoxin that can cause damage to the nervous system if consumed by humans or animals. The study has implications for health officials and natural resource managers who need the best science possible to inform their decisions, says lead author Ryan Lepak, a doctoral researcher at the Center for Aquatic Sciences at the University of Madison (ASC).
"Our work emphasizes that mercury concentrations in fish cannot be predicted solely by emission inventories and other factors such as changes in the food web are needed to get the full picture," says Lepak, who is based in US Environmental Protection Agency, Great Lakes Toxicology and Ecology in Duluth, Minnesota. "All five of the Great Lakes have fish consumption tips in place because mercury poses a disproportionate risk to the health of children and pregnant women."
Lepak and co-author James Hurley, ASC director, are interested in what is has led to increased concentrations of mercury in larger fish, such as lake trout, despite policies that have greatly reduced mercury emissions in the Great Lakes waterway.
To understand the history of mercury in the Great Lakes fish and to identify the sources of the pollutant, the research team performed a combination analysis of mercury, nitrogen and carbon isotope – what Lepak calls "fingerprint" – on lake trout samples, archived from 1978 to 2012.
From the same period, Lepak also examined archived sediment samples taken from the bottom of the lake to compare tendencies of mercury sources with sediment and fish.
The years of study cover the period after which hospitals and municipalities stop burning waste they save. Great lakes from additional mercury contamination. Researchers expect the drop to reduce methylmercury in fish.
However, Hurley says that although the unique fingerprinting technique shows measurable changes in mercury concentrations in archival fish and from lake sediment samples since the 1980s, "there has been no apparent decrease in mercury concentrations in these fish afterwards, although sludge recording revealed reduced mercury loading. "
Hurley notes that this is despite the fact that after about 1990 emissions and mercury use were significantly reduced and led to reducing the mercury load in the Big One era.
The culprits for this, according to the analysis, are the invasive Dresenid mussels, zebras and quagus that exploded in Lake Michigan in the 1990s. Estimates indicate that there are now trillions of mussels that have led to significant changes in the eating habits of lake trout
Because the Dresenids filtered and consumed free-floating phytoplankton and cleared Michigan lakes, fish were forced to search for food. deep waters in the offshore area of the lake and at the bottom of the lake in coastal waters. The food the fish find here provides them with less energy but is enriched with mercury.
"Humans enjoy fishing for lake trout, but the larger the fish, the more mercury has accumulated in the fillets of the species," explains Lepak. "Unfortunately, people have to take into account pollutant levels when choosing to serve fish for dinner."
Mercury levels in fish fluctuate with water levels in lakes
Ryan F. Lepak el, "Changes in mercury source and relocation of food webs change the signatures of predatory fish contamination from Lake Michigan" PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1907484116
University of Wisconsin-Madison
Benefits of short-circuiting invasive species from mercury reduction in the Great Lakes (2019, November 4)
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