A team of UW-Madison aquatic chemists and limnologists has discovered a missing link between the amounts of dissolved organic matter (DOM) in bodies of fresh water and increased levels of highly toxic methylmercury in fish.
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 more and more often warning signboards at the lake border tell them not to consume the fish because of methylmercury contamination. Interestingly, the relation between the pollution level of the lake and methylmercury in the fish is not obvious. Researchers have named the fact that high levels of mercury can be found in fish from clear lakes not especially heavily contaminated with mercury the "Mercury paradox".
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.
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 combined 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 including algae, DOM and methylmercury and tried to predict the binding of methylmercury.
Starting at relatively low DOM levels, like those found in "clear" northern lakes, and then increasing DOM concentration until it roughly equaled that of a more DOM-rich, brown body of water, the model predicts that more and more methylmercury gets bound 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 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 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 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.
The original study
Patrick R. Gorski, David E. Armstrong, James P. Hurley, Martin M. Shafer, Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum)
, Environ. Toxicol. Chem., 25/2 (2006) 534-540. DOI: 10.1897/04-530R.1
Edenise Garcia, Richard Carignan, Mercury concentrations in fish from forest harvesting and fire-impacted Canadian boreal lakes compared using stable isotopes of nitrogen
, Environ. Toxicol. Chem., 24/3 (2005) 685. doi: 10.1897/04-065R.1