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Study shows link between clear lakes and contaminated fish

Person fishing

As spring approaches, thousands of anglers eagerly anticipate the day they can cast their lines into a clear lake and pull out fish for dinner. But at the same time, departments of natural resources in approximately 40 states issue advisories that help fishermen avoid eating a mercury-contaminated catch.

Now, a team of UW-Madison aquatic chemists and limnologists has discovered a link between the amounts of dissolved organic matter (DOM) in bodies of fresh water and increased levels of highly toxic methylmercury in fish.

Methylmercury enters the fresh-water 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. Since DOM is a natural chemical component of aquatic systems that's not consumed by organisms or animals, when methylmercury binds with it, the toxin doesn't enter the food web.

Led by Patrick Gorski, a former graduate student of Civil and Environmental Engineering Professor Emeritus David Armstrong, the group published its findings in the February issue of Environmental Toxicology and Chemistry.

Gorski coupled laboratory experiments with computer modeling that helped him quantify the affinity between methylmercury and green algae — a relationship researchers previously did not know. 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 says.

Person fishing

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,” says Gorski. “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, says Gorski. 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, says Armstrong, would be to determine what characteristics of DOM control methylmercury bioavailability and whether those characteristics differ across various fresh-water 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 says.

He calls the association of methylmercury with natural dissolved organic matter a double-edged sword. “On one hand, binding to DOM reduces bioavailability,” he says. “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.”

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 says.