The Rise and Fall of Mercury Methylation in an Experimental Reservoir

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• 作者：Vincent L. St.Louis ; John W. M. Rudd ; Carol A. Kelly ; R. A. (Drew) Bodaly ; Michael J. Paterson ; Kenneth G. Beaty ; Raymond H. Hesslein ; Andrew Heyes ; and Andrew R. Majewski
• 刊名：Environmental Science & Technology
• 出版年：2004
• 出版时间：March 1, 2004
• 年：2004
• 卷：38
• 期：5
• 页码：1348 - 1358
• 全文大小：820K
• 年卷期：v.38,no.5(March 1, 2004)
• ISSN：1520-5851

文摘

For the past 9 years, we experimentally flooded a wetlandcomplex (peatland surrounding an open water pond) atthe Experimental Lakes Area (ELA), northwestern Ontario,Canada, to examine the biogeochemical cycling ofmethyl mercury (MeHg) in reservoirs. Using input-outputbudgets, we found that prior to flooding, the wetlandcomplex was a net source of approximately 1.7 mg MeHgha^-1 yr^-1 to downstream ecosystems. In the first yearof flooding, net yields of MeHg from the reservoir increased40-fold to approximately 70 mg MeHg ha^-1 yr^-1.Subsequently, annual net yields of MeHg from the reservoirdeclined (10-50 mg MeHg ha^-1 yr^-1) but have remainedwell above natural levels. The magnitude and timing of Hgmethylation in the flooded peat portion of the wetlandreservoir were very different than in the open water regionof the reservoir. In terms of magnitude, net Hg methylationrates in the peat in the first 2 years of flooding were2700 mg ha^-1 yr^-1, constituting over 97% of the MeHgproduced at the whole-ecosystem level. But in the following3 years, there was a large decrease in the mass ofMeHg in the flooded peat due to microbial demethylation.In contrast, concentrations of MeHg in the open waterregion and in zooplankton, and body burdens of Hg in cyprinidfish, remained high for the full 9 years of this study.Microbial activity in the open water region also remainedhigh, as evidenced by continued high concentrations ofdissolved CO₂ and CH₄. Thus, the large short-termaccumulation of MeHg mass in the peat appeared tohave only a small influence on concentrations of MeHg inthe biota; rather MeHg accumulation in biota wassustained by the comparatively small ongoing net methylationof Hg in the flooded pond where microbial activityremained high. In large reservoirs, where the effects ofwind and fetch are greater than in the small experimentalreservoir we constructed, differences can occur in thetiming and extent of peat and soil erosion, effecting eithertransport of MeHg to the food chain or the fueling ofmicrobial activity in open water sediments, both of whichcould have important long-term implications for MeHgconcentrations in predatory fish.