Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems

The forest canopy was an important contributor to fluxes of methyl mercury (MeHg) and total mercury (THg) to the forest floor of boreal uplands and we tlands and potentially to downstream lakes, at the Experimental Lakes Area (ELA), northwestern Ontario. The estimated fluxes of MeHg and THg in throug hfall plus litterfall below the forest canopy were 2 and 3 times greater th an annual fluxes by direct wet deposition of MeHg (0.9 mg of MeHg ha(-1)) a nd THg (71 mg of THg ha(-1)). Almost all of the increased flux of MeHg and THg under the forest canopy occurred as litterfall (0.14-1.3 mg of MeHg ha( -1) yr(-1) and 110-220 mg of THg ha(-1) yr(-1)). Throughfall added no MeHg and approximately 9 mg of THg ha(-1) yr(-1) to wet deposition at ELA, unlik e in other regions of the world where atmospheric deposition was more heavi ly contaminated. These data suggest that dry deposition of Hg on foliage as an aerosol or reactive gaseous Hg (RGM) species is low at ELA, a finding s upported by preliminary measurements of RGM there. Annual total deposition from throughfall and litterfall under a fire-regenerated 19-yr-old jack pin e/birch forest was 1.7 mg of MeHg ha(-1) and 200 mg of THg ha(-1). We found that average annual accumulation of MeHg and THg in the surficial litter/f ungal layer of soils since the last forest fire varied between 0.6 and 1.6 mg of MeHg ha(-1) and between 130 and 590 mg of THg ha(-1) among sites diff ering in drainage and soil moisture. When soil Hg accumulation sites were m atched with similar sites where litterfall and throughfall were collected, measured fluxes of THg to the forest floor (sources) were similar to our es timates of longterm soil accumulation rates (sinks), suggesting that the Hg in litterfall and throughfall is a new and not a recycled input of Hg to f orested ecosystems. However, further research is required to determine the proportion of Hg in litterfall that is being biogeochemically recycled with in forest and wetland ecosystems and, thus, does not represent new inputs t o the forest ecosystem.