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August 26, 2010
DOI: 10.1021/CEN081210135904

Fishing For Methylmercury In Streams

Stream Ecology: Ecologists find novel source of the neurotoxin in aquatic ecosystems

Jeffrey M. Perkel

TEST (RIVER) BED Rivers such as the South Fork Eel River do not need associated wetlands to have methylmercury. Wikipedia
TEST (RIVER) BED Rivers such as the South Fork Eel River do not need associated wetlands to have methylmercury.
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Mountain streams are an angler's paradise: sun-dappled, gurgling, and teeming with fish. Yet those cold, clear waters also contain methylmercury, a neurotoxin that bioaccumulates in living tissues.

Researchers have long recognized that wetlands associated with streams play a role in converting mercury, a common atmospheric pollutant, into its methylated counterpart. But many stream systems don't have attached wetlands and are still contaminated. Now researchers have identified one potential methylmercury source floating in these idyllic streams (Environ. Sci. Technol., 10.1021/es101374y).

A 2009 study by U.S. Geological Survey researchers (Environ. Sci. Technol., 2009, 43, 2720), concluded that "wetland abundance in a river basin is strongly linked to dissolved mercury species," especially methylmercury.  Wetlands promote methylmercury production via their reducing environment and abundant sulfate-reducing bacteria, says ecologist Martin Tsui of the University of Minnesota, St. Paul.

Tsui, his thesis advisor Jacques Finlay, and colleagues study mercury production in the South Fork Eel River in northern California. In previous work (Environ. Sci. Technol., 2009, 43, 7016), the team demonstrated that insect larvae accumulate significant amounts of methylmercury even in the absence of two common sources, feeder wetlands or rainfall runoff.

To find the methylmercury's source in the South Fork Eel River, the team sampled water along the stream channel and its tributaries over the summer of 2009, measuring total mercury, methylmercury, and other inorganic nutrients as a function of water temperature, water level, and algal growth. Initially, methylmercury levels were close to or just above the detection limit (20 pg/L). But as the summer progressed, methylmercury levels rose, peaking in mid-August at about 84 pg/L—almost 14% of the waters' total mercury.

This increase coincided with elevated water temperature and decreased water levels, conditions that promote proliferation of filamentous algae called Cladophora glomerata. When the scientists incubated these algae with fresh stream water in the field, methylmercury levels rose significantly within just a few hours.

The researchers specifically observed that methylmercury levels rose in the stream when the Cladophora mats changed in color from green to "yellow and rusty," which marked the proliferation of diatoms and bacteria on the algae surface. They concluded that these bacteria were the source of the methylmercury, and that they were using the algal mats as a growth surface. 

The study identifies a potentially "significant zone of methylmercury production," says USGS hydrologist Mark Brigham, who was involved in the 2009 USGS study. While wetlands still play a key role in stream methylmercury production, "not all streams are created equally," he says. "There are some types of streams that have the capability to produce significant amounts of methylmercury in the stream itself."

But anglers should not worry about fishing from these streams, Tsui says: The methylmercury levels produced in the South Fork Eel River are lower than in streams with associated wetlands. And even there, the fish are generally safe to eat, albeit in moderation.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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