Transport and transformation of mercury in wetlands.Wet lands are important features of the landscape that influence the supply of different Hg species to adjacent surface waters.Wetlands are typically net sinks of total Hg and sources of MeHg (Grigal 2002, 2003). Rates of total Hg accumulation are greater in wetlands than in upland soils because of the strong association of Hg with organic matter (Grigal 2003). Annual rates of MeHg production in wetlands are approximately 0.1 to 1 µg per m2 per year (Galloway and Branfireun 2004). The factors controlling methylation of Hg in wetlands are not completely understood, but they most likely involve the amounts and types of organic matter, hydrologic flow paths, and rates of microbial activity (Galloway and Branfireun 2004). Wetlands are also a major source of DOC. Organic matter produced in wetlands forms complexes with both ionic Hg and MeHg, enhancing the transport of these Hg species to surface waters but decreasing their bioavailability (Hudson et al. 1994). An elevated supply of DOC to downstream surface