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Web Date: February 13, 2012

Less Acid Rain Could Mean More Mercury In Fish

Pollution: Norwegian team proposes connection between increasing pH in lake waters and bioavailability of mercury
Department: Science & Technology
News Channels: Environmental SCENE
Keywords: mercury, acid rain, organic matter, bioaccumulation, methylmercury, food webs, Norway
Good And Bad News
Lakes, including this one in Norway, have seen less acid rain but also higher levels of mercury in fish.
Credit: Gunnhild Riise
Good And Bad News
Lakes, including this one in Norway, have seen less acid rain but also higher levels of mercury in fish.
Credit: Gunnhild Riise

Regulations on mercury emissions in Europe have led to a steady decline in mercury entering the environment since the 1990s. So government scientists in Norway were surprised to discover in 2009 that mercury levels in lake fish had strongly increased since 1991. Researchers now propose that this could be the result of another environmental success story: reduced acid rain (Environ. Sci. Technol., DOI: 10.1021/es3002629).

When the government’s mercury data in fish came out, Ståle Haaland, a limnologist at Bioforsk, the Norwegian Institute for Agricultural and Environmental Research, and his colleagues were studying how reductions in acid rain changed lake chemistry. As rain’s acidity drops, the pH of lake waters increases, which they found makes organic material from soil surrounding the lakes more soluble (Environ. Sci. Technol., DOI: 10.1021/es903179j).

Other researchers had shown that dissolved organic carbon can react with relatively inert forms of mercury—including those stored in lake sediments from times when mercury inputs were higher—to produce methylmercury. This form of the metal is the most toxic for humans and readily moves up through lake food webs, eventually accumulating in fish that people eat, such as trout and perch. The recent government report described trends in methylmercury levels in these fish.

So Haaland’s team decided to compare lake concentrations of organic carbon and mercury in hopes of understanding the counterintuitive mercury increases in fish. When the researchers examined government data on organic carbon levels in three southern Norwegian lakes where regulators had detected increased mercury levels in fish, they saw a strong correlation between organic carbon levels and the fish mercury levels. The researchers point out that because the data are limited, they cannot draw definitive conclusions about the connection. But they think that their hypothesized mechanism could be at play in other regions in Europe as well as in the U.S.

Mark Marvin-DiPasquale, a microbial ecologist with the U.S. Geological Survey (USGS) in Menlo Park, Calif., who focuses on mercury research, says the researchers have an interesting hypothesis, but he’s eager to see further research. “There’s a whole long laundry list of other things that could explain this trend,” he says, such as the increase in pH leading to more iron entering the lake. Some research suggests iron also can play an important role in mercury methylation.

Collin Eagles-Smith, an ecologist with USGS in Corvallis, Ore., who also studies mercury, is similarly intrigued. If true, this connection presents environmental scientists with a real conundrum, he says, “where amelioration of one environmental threat exacerbates another.”

Chemical & Engineering News
ISSN 0009-2347
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Doug Burns (Mon Feb 13 13:31:33 EST 2012)
This hypothesis is intriguing because we have also seen increasing DOC concentrations in many streams and lakes in the US that seems to be related to decreases in acid deposition. However, there are also some reasons to believe that decreasing levels of acid deposition may play a role in decreasing Hg levels in biota because the formation of methyl Hg is related to bacterial sulfate reduction, which is dependent in many ecosystems on the supply of sulfate from acid deposition. Not to mention effects of increasing pH and increasing aquatic productivity that may also act to lower Hg levels. Maybe the effect of increasing DOC trumps all other factors? An important topic for future research.
Kendra Zamzow (Sat Mar 10 12:22:12 EST 2012)
Doug: I agree with you. Methylation is all about the organisms that can methylate, which are primarily sulfate reducing bacteria. Conditions that make the environment more amenable to them -- high sulfate, relatively neutral pH, high organic carbon, warm temperatures -- are almost certain to increase methylation of mercury in a system. There also needs to be an oxygen gradient -- because the sulfate reducers do not directly use most organic carbon, they can only use small hydrocarbons like alcohols and depend on aerobes to chomp up the larger chains. This seems so obvious to me, that I welcome any explanation that can show me how DOC by itself influences methylation, without the other factors that support sulfate reducers. It is also the reason I think we will see higher methylation in Alaska, where I live, as more permafrost melts and as coastal estuaries stay ice-free longer. Estuaries in particular are wonderful supporters of sulfate reducers.
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