Linking atmospheric mercury to fish advisories
An experiment in lake enclosures links mercury deposition and methylmercury production for the first time.
Down in the mud that lies at the bottom of lakes and wetlands, bacteria convert inorganic mercury into methylmercury, a toxic organic form of the metal that accumulates in fish. But scientists have not been able to make a definitive link between the amount of anthropogenic mercury in air and the levels of methylmercury in fish. Now, a study published today on ES&T’s Research ASAP website (DOI: 10.1021/es060823+) suggests that reducing the amount of mercury that rains down on lakes also decreases methylmercury production.
“This paper is important because it is the first field experiment to show a direct relationship between deposition of inorganic mercury and production of methylmercury,” says Chad Hammerschmidt of the Woods Hole Oceanographic Institution.
In the study, led by Diane Orihel of Fisheries and Oceans Canada, researchers added isotope-enriched mercury to 11 lake enclosures called mesocosms located at the Experimental Lakes Area in northern Ontario (Canada). The extra mercury increased levels of the heavy metal to 2–15× the typical local deposition rate. The boreal lakes of northern Ontario receive relatively low levels of mercury deposition—water bodies in the northeastern U.S. receive ~4× more, according to Orihel.
The researchers found that after 8 weeks, <1% of the isotope-enriched mercury was converted to methylmercury, while a much higher percentage was re-emitted to the atmosphere. However, Orihel emphasizes that even a small percentage of methylmercury is harmful. “It may be a tiny amount, but that is all it takes to drive those fish advisories,” she says.
James Hurley, an aquatic chemist with the University of Wisconsin Sea Grant Institute, says the study shows that decreasing atmospheric loading should lower methylmercury production and, presumably, bioaccumulation in fish.
And Hammerschmidt adds that his own studies bolster this conclusion. In Alaskan lakes, he and colleagues measured loadings of inorganic mercury and the flux of methylmercury out of the sediments. The relationship is linear, he reported earlier this month at the Eighth International Conference on Mercury as a Global Pollutant. Last year, Hammerschmidt found a similar relationship between methylmercury in mosquitoes and average deposition rates of mercury in U.S. lakes.
At the mercury conference, Orihel reported the same linear relationship for small fish. Her team found that isotope-labeled mercury accounted for up to a third of the total concentration of mercury in the muscle tissues of young trout living in the lake mesocosms.
High methylmercury concentrations in North American freshwater fish have prompted health authorities in Canada and most U.S. states to warn against eating too much of the fish. Coal- and oil-fired power plants are the largest sources of mercury emissions in the U.S., according to the U.S. EPA.
But emissions controls are controversial, in part because of the complexity of the mercury cycle. In March 2005, EPA adopted a cap-and-trade rule, which aims to reduce mercury emissions 21% by 2010 and 69% by 2018. The new study suggests that such limits could achieve their intended outcomes rather quickly.
