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Web Date: December 3, 2012

Alder Swamps Eat Up Mercury

Metals Pollution: Less methylmercury flows out of swamps with alder trees than flows in, researchers find, unlike other wetlands
Department: Science & Technology
News Channels: Environmental SCENE
Keywords: mercury, methylmercury, swamps, alders, forestry management, sulfate-reducing microorganisms
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Unexpected Sink
Methylmercury, a toxic form of mercury, has long been thought to form in all types of wetlands, but some swamps, including this one in southern Sweden, seem to consume it instead.
Credit: Rose-Marie Kronberg
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Unexpected Sink
Methylmercury, a toxic form of mercury, has long been thought to form in all types of wetlands, but some swamps, including this one in southern Sweden, seem to consume it instead.
Credit: Rose-Marie Kronberg

A particularly toxic form of mercury may get destroyed in a place where researchers previously thought it was generated. A recent study of wetlands in Sweden has found that some, instead of creating methylmercury, serve as a sink for it (Environ. Sci. Technol., DOI: 10.1021/es303543k).

Past research in wetlands in Canada, the Florida Everglades, and elsewhere has pinpointed wetlands such as bogs, fens, and swamps as a source of methylmercury. This form of the metal is neurotoxic and can be readily taken up by fish and passed along to birds and humans. The mucky freshwater bodies receive inorganic mercury from the air, rain, and stream runoff. In the wetlands, sulfate-reducing microorganisms transform the mercury by adding a methyl group, making the metal bioavailable to fish and other creatures.

But a study of swamps in southern Sweden shows that wetlands that are home to black alder trees (Alnus glutinosa) may treat incoming mercury differently. Researchers led by Ulf Skyllberg of the Swedish University of Agricultural Sciences in Umeå measured various forms of mercury in the water flowing into the wetlands from upstream sources, in the wetland soils, and in water flowing downstream of the sites.

They found that one alder swamp released one- to two-thirds as much methylmercury as flowed into it, according to water samples taken from 2006 to 2010. They also collected soil samples five times over that period, which showed methylmercury was not accumulating there. The researchers also looked at an hour or so of flow at eight other alder-dominated wetlands during a May rainstorm and determined that more methylmercury flowed in than out.

But over the same four years, seven other wetland sites without alders released more methylmercury downstream than had flowed in. The team does not yet know what caused the difference, but they suspect that the nutrients present in alder swamps influence the growth of microbes that break down methylmercury.

Skyllberg and his colleagues suggest that people could restore alder swamps that have been drained across northern climes to prevent methylmercury from flowing downstream into freshwater streams and lakes. Such a buffer zone would be particularly useful, Skyllberg says, in logged areas or regions with peatlands or bogs, which tend to have high levels of methylmercury.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society