Researchers Find Link Between Clear Lakes, Mercury Contamination

A team of University of Wisconsin-Madison aquatic chemists and limnologists (limnology is the study of inland waters (both saline and fresh)) announced on Feb. 15 they have discovered a link between the amounts of dissolved organic matter (DOM) in bodies of fresh water and levels of highly toxic methylmercury in fish.

Methylmercury enters the freshwater food chain by binding with microscopic organisms like green algae, which other organisms subsequently eat. But, using a computer model, the researchers discovered that when the water also contains high concentrations of dissolved organic matter, there is a greater tendency for methylmercury to chemically bind with DOM, rather than green algae.

DOM is a natural chemical component of aquatic systems and is not consumed by organisms or animals. As a consequence, when methylmercury binds with DOM, the toxin doesn't enter the food web, the researchers said.

Led by Patrick Gorski, a former graduate student of civil and environmental engineering, and professor emeritus David Armstrong, the group published its findings in this month's issue of Environmental Chemistry and Toxicology.

Gorski coupled laboratory experiments with computer modeling that helped him quantify the previously unknown relationship between methylmercury and green algae. Then he applied the results to a natural system. "I came up with a model and had algae present, methylmercury present and DOM present, and tried to predict at what concentrations they would outcompete each other," he said.

He began at relatively low DOM levels, like those found in "clear" northern lakes, and increased DOM concentration until it roughly equaled that of a more DOM-rich, brown body of water. "And as you start ramping up the DOM concentration, it starts outcompeting the algae for the methylmercury, and then more and more methylmercury gets bound to the DOM," Gorski said. "So the model predicts, at really high DOM concentrations, that methylmercury will competitively bind to the DOM instead of the algae."

The research may help explain why so many mercury warnings are issued for fish from clear lakes, Gorski said. But he stresses that it's an initial step in being able to predict how methylmercury enters the bottom of the food chain.

The next step, Armstrong said, would be to determine what characteristics of DOM control methylmercury bioavailability and whether those characteristics differ across various freshwater systems. "If so, we would like to identify relatively simple methods to measure these differences so that these measures could be used in surveillance programs to help identify systems most vulnerable to methylmercury bioaccumulation," he said.

The researchers called the association of methylmercury with natural dissolved organic matter a double-edged sword. On one hand, binding to DOM reduces bioavailability, but on the other hand, association with DOM also can carry mercury from surrounding uplands and wetlands into lakes, meaning that higher DOM inputs into lakes is not necessarily a "good thing" with respect to mercury levels in lake food webs, the researchers said.

Researchers need to understand better the resulting balance between these two effects of mercury association with DOM, says Armstrong. In a broader context, they also must learn more about how quickly mercury levels in aquatic food webs would decline if mercury emissions into the atmosphere-and their subsequent deposition onto watersheds-were reduced. "The interaction of mercury with DOM is one part of the puzzle," he said.

Patrick Gorski:

David Armstrong: