Scientists have, for the first time, demonstrated a direct link between the amount of mercury being loaded into a lake by atmospheric deposition and the amount of toxic methylmercury that accumulates in fish swimming in the lake.
A significant part of mercury in the atmosphere today is generated from human activity such as power plant emissions, and industrial and mining activities. Once deposited to the aquatic environment, bacteria, being present in the sediments of lakes and wetlands are responsible for the conversion of inorganic mercury into methylmercury, an organic form of the metal that accumulates in fish and is and is toxic to humans and fish-eating wildlife. But scientists have not been able to make a definitive link between the amount of anthropogenic mercury in air and the levels of methylmercury reaching the foodweb. A so called "mercury paradoxon" which means that fish swimming in waters with very different levels of mercury contamination can have similar methylmercury concentrations troubles the picture.New research results
Now, a study published in the recent issue of Environmental Science & Technology (Vol. 40/19, pp 5992–6000
) suggests that reducing the amount of mercury that rains down on lakes also decreases methylmercury production. This is an important finding because many areas already have large historical accumulations of mercury in sediments and soils, and it has been difficult to distinguish between the impacts of historical contamination and new deposition.
“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 (see Photo to the left) 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 reemitted 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 in August 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 (Environ. Sci. Technol. 2005, 39, 3034–3039).
At the August meeting, 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 limits could achieve their intended outcomes rather quickly.The original study
Diane M. Orihel, Michael J. Paterson, Cynthia C. Gilmour, R.A. (Drew) Bodaly, Paul J. Blanchfield, Holger Hintelmann, Reed C. Harris, J.W.M. Rudd, Effect of Loading Rate on the Fate of Mercury in Littoral Mesocosms
, Environ. Sci. Technol., 40/19 (2006) 5992-6000. DOI: 10.1021/es060823+Related studies
C.A. Kelly, J.W.M. Rudd, V.L.S. Louis, A. Heyes, Is total mercury concentration a good predictor of methyl mercury concentration in aquatic systems ?
, Water, Air, Soil Pollut., 80/1-4 (1995) 715-724. DOI: 10.1007/BF01189723
R.P. Mason, M.L. Abbott, R.A. Bodaly, O.R. Bullock,
C.T. Driscoll, D. Evers, S.E. Lindberg, M. Murray, E.B. Swain. Monitoring the environmental
response to changing atmospheric mercury deposition
, Environ. Sci. Technol., 39 (2005) 14A-22A.
Chad R. Hammerschmidt, William F. Fitzgerald, Methylmercury in Mosquitoes Related to Atmospheric Mercury Deposition and Contamination,
Environ. Sci. Technol., 39/9 (2005) 3034-3039. DOI: 10.1021/es0485107
B. A. Branfireun, D. P. Krabbenhoft, Holger Hintelmann, R. J. Hunt, J. P. Hurley, J. W. M. Rudd, Speciation and transport of newly deposited mercury in a boreal forest wetland: A stable mercury isotope approach
, Water Resour. Res., 41 (2005) W06016. DOI: 10.1029/2004WR003219
P.C. Pickhardt, C.L. Folt, C.Y. Chen, B. Klaue, J.D. Blum, Impacts of zooplankton composition and algal enrichment and on the accumulation of mercury in an experimental freshwater food web
, Sci. Total Environ., 339/-3 (2005) 89-101. DOI: 10.1016/j.scitotenv.2004.07.025
Michael J. Paterson, Paul J. Blanchfield, Cheryl Podemski, Holger H.
Hintelmann, Cynthia C. Gilmour, Reed Harris, Nives Ogrinc, John W.M.
Rudd, and Ken A. Sandilands, Bioaccumulation of newly deposited mercury by fish and invertebrates: an enclosure study using stable mercury isotopes
, Can. J. Fish. Aquat. Sci., 63/10 (2006) 2213-2224.
J. G. Wiener, B. C. Knights, M. B. Sandheinrich, J. D. Jeremiason, M.
E. Brigham, D. R. Engstrom, L. G. Woodruff, W. F. Cannon, and S. J.
Balogh, Mercury in Soils, Lakes, and Fish in Voyageurs National Park (Minnesota): Importance of Atmospheric Deposition and Ecosystem Factors
, Environ. Sci. Technol., 2006; ASAP Web Release Date: 06-Sep-2006; (Article) DOI: 10.1021/es060822h
Spencer A. Peterson, John Van Sickle, Alan T. Herlihy, Robert M. Hughes, Mercury Concentration in Fish from Streams and Rivers Throughout the Western United States
, Environ. Sci. Technol., 41/1 (2007) 58-65. DOI: 10.1021/es061070uRelated Information University of Manitoba's Home Page for the Experimental Lakes Area, or ELA University of Manitoba: Mercury Loading Experiment at the Experimental lakes Area, northwestern Ontario also known as METAALICUS University of Alberta: Why is METAALICUS Necessary? Brian Branfireun: Presentation on behalf of the METAALICUS Team: Mercury Experiment to Access Atmospheric Loading in Canada and the US USGS: Mercury cycling in Aquatic Ecosystems USGS: Mercury in Aquatic Ecosystems USGS: Mercury-Contaminated Fish--Is it Old or New Mercury? USEPA: Fish advisories USEPA, October 2000, Mercury Transport and Fate in Watersheds Related News
Geotimes, August 2003: In Search of the Mercury Solution (A short introduction to the METAALICUS Study) Science News Focus - Environment, 2 January 2004: Uncertain Science Underlies New Mercury Standards EVISA
News, April 27, 2004: FDA/EPA recommends pregnant women to restrict
their fish consumption because of methylmercury content EVISA News, April 27, 2004: New kind of mercury found in fish EVISA News, November 23, 2004: Is the methylmercury paradox real ? EVISA
News, March 20, 2005: New results on the distribution of mercury in the
USA is fueling the discussion on the necessity of the reduction of its
emission EVISA News, April 3, 2005: Dissension on the best way to fight mercury pollution EVISA
News, August 29, 2005: Is methyl mercury limiting the delight of
seafood ? - To answer this question is a challenge for elemental
speciation analysis EVISA News, September 9, 2005: Regulating Mercury Emissions from Power Plants: Will It Protect Our Health? EVISA News, February, 17, 2006: Study shows link between clear lakes and methylmercury contamination in fish EVISA News, July 16, 2006: Sulfur fuels the methylation of mercury EVISA News, August 16, 2006: Mercury pollution threatens health worldwide, scientists say EVISA News, September 23, 2006: Report Finds Mercury Contamination Permeates Wildlife Systems
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