Kirkpatrick Marsh looks a picture of environmental health, its waters fringed by waving grass and swarming with fat minnows.
But seeping out of these Anne Arundel wetlands into the Chesapeake Bay is a pollutant - methylmercury - that causes brain damage in people.
"It's quite crazy," says Carl Mitchell, a scientist at the Smithsonian Environmental Research Center in Edgewater. "These areas are so beautiful and pristine looking, but they also produce a lot of methylmercury."
Mitchell is among a growing number of researchers documenting the little-known role that wetlands play in transforming air pollution from coal-burning power plants into a form of mercury that contaminates fish.
As Maryland and other states look to build thousands of acres of wetlands to fight global warming, the research has significant implications. More wetlands would absorb more carbon dioxide, but they also could make mercury health hazards worse.
"Building wetlands has many positive benefits for the ecosystem and society at large," said Mark Marvin-DiPasquale, a microbial ecologist at the U.S. Geological Survey. "But if you increase the amount of wetlands by fivefold, does that mean you get five times more mercury?"
Mitchell is trying to figure this out as he studies marshes outside Edgewater, south of Annapolis.
Mercury isn't created by marshes. The element occurs naturally in rocks, dirt and coal, and it floats into the air when coal is burned in power plants and factories. This airborne form of the metal, called inorganic mercury, doesn't usually get into people, Mitchell said.
The metal becomes a health hazard when it's changed by microorganisms that multiply in the oxygen-deprived muck at the bottoms of rivers, lakes and wetlands. In this mud, bacteria transform inorganic mercury into methylmercury, which is at least 10 times more toxic and collects in the bodies of animals.
Tainted worms are gobbled by small fish, and these are eaten by big fish - such as tuna and shark - that are eventually eaten by humans. The methylmercury accumulates as it moves up the food chain, with the toxicity multiplying 10-fold at each step.
It's ironic that marshes appear to be a vital link in this toxic chain. Wetlands are normally regarded as environmental cornucopias, fountains of life for fish and birds. They clean the Chesapeake Bay by filtering out silt and other pollutants. And marshes absorb the carbon dioxide that is warming the atmosphere.
