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Mercury and Methylmercury in Decomposing Vegetation of a Pristine and Impounded Wetland

Academy of Natural Sciences, Estuarine Research Center, 10545 Mackall Rd. St. Leonard MD 20685.
Dep. of Geography, McGill Univ., 805 Sherbrooke St. W., Montreal, QC, H3A 2K6 Canada.
Dep. of Fisheries and Oceans, FWI, 501 University Cres., Winnipeg, MB, R3T 2N6 Canada.

^* Corresponding author (heyes{at}acnatsci.org).

ABSTRACT

Mass loss, methylmercury (MeHg), and total mercury (T-Hg) concentrations were measured in sedge, spruce needles, and Sphagnum moss decomposing in a headwater bog and an impounded riparian wetland. No significant differences in the rates of decomposition were apparent for like tissues among sites. Although concentrations of T-Hg increased in some tissues, T-Hg mass was lost from most tissues during decomposition. In the reservoir, approximately 75, 50, and 45% of the initial T-Hg mass was released from the sedge, needles, and moss, respectively, compared to 45, 0, and 40% in the headwater wetland. Therefore, we propose impoundment promotes the release of T-Hg during decomposition. Concentrations, masses, and percentages of MeHg decreased in all tissue types at the headwater-dry site and MeHg concentrations increased in all tissue types at the headwater-wet and impounded sites. At the headwater-wet site, the mass of MeHg only increased in the moss (500%) and T-Hg occurring as MeHg averaged 6% in sedge, 1% in needles, and 7% in moss compared to initial values of 4, 0.5, and 1%. At the impounded sites, the amount of MeHg in the needles and moss increased by approximately 700 and 500%, respectively. Also, T-Hg occurring as MeHg averaged 12% in sedge, 6% in spruce needles, and 17% in moss. We conclude that decomposition of fresh plant tissue under anoxic conditions increases their MeHg mass and impoundment promotes this process. This increase in tissue MeHg mass is most likely the result of in situ methylation of previously accumulated inorganic Hg.

This paper is contribution no. 29 of the Experimental Lakes Area Reservoir Project.

Received for publication January 17, 1997.

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