Importance of the Forest Canopy to Fluxes of Methyl Mercury and Total Mercury to Boreal Ecosystems

Abstract

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 wetlands and potentially to downstream lakes, at the Experimental Lakes Area (ELA), northwestern Ontario. The estimated fluxes of MeHg and THg in throughfall plus litterfall below the forest canopy were 2 and 3 times greater than annual fluxes by direct wet deposition of MeHg (0.9 mg of MeHg ha(-1)) and 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, unlike in other regions of the world where atmospheric deposition was more heavily 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 supported by preliminary measurements of RGM there. Annual total deposition from throughfall and litterfall under a fire-regenerated 19-yr-old jack pine/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/fungal 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 differing in drainage and soil moisture. When soil Hg accumulation sites were matched with similar sites where litterfall and throughfall were collected, measured fluxes of THg to the forest floor (sources) were similar to our estimates 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 forested ecosystems. However, further research is required to determine the proportion of Hg in litterfall that is being biogeochemically recycled within forest and wetland ecosystems and, thus, does not represent new inputs to the forest ecosystem.

Atmospheric wet and litterfall mercury deposition at urban and rural sites in China

“…Litterfall samples were collected using litterfall collectors (0.25 m 2 at MLG and 1.0 m 2 at MCB, MDM, and MAL) at the sampling sites (St. Louis et al, 2001;Fisher and Wolfe, 2012;. The collectors were constructed using an untreated wood frame and a Nylon net bottom with a pore size of 1.0 mm × 1.0 mm.…”

“…Given the measured atmospheric PBM and GOM concentrations (Fu et al, 2015b;Yu et al, 2015), the dry deposition fluxes of PBM and GOM at the MCB, MDM, and MAL sites were estimated to be 3.0, 9.6, and 2.3 µg m −2 yr −1 , respectively, using the average dry deposition velocities of PBM and GOM over forests modeled by L. . At the MLG site, annual dry deposition flux of PBM and GOM was estimated to be 4.4 µg m −2 yr −1 using the comparison of precipitation and throughfall data collected side by side (St. Louis et al, 2001;Fu et al, 2010a;. The importance of litterfall in the total deposition of Hg has been highlighted by many previous studies (St. Louis et al, 2001;Lindberg et al, 2007;.…”

“…However, dry deposition of Hg is difficult to be quantified accurately because of the lack of proper measurement techniques. Recently, an increasing number of studies have suggested that measurements of litterfall (uptake of total gaseous mercury -TGM) and throughfall (washout of GOM and PBM deposited to foliage surfaces) could be a good indicator of Hg dry deposition to forest ecosystems (Johnson and Lindberg, 1995;St. Louis et al, 2001;.…”

Dynamic modelling of the long term behaviour of cadmium, lead and mercury in Swiss forest soils using CHUM-AM

“…the dominant tree species (De Vries and Bakker, 1998;Driscoll et al, 1994;Gandois et al, 2010;Lindberg et al, 2007;Perez-Suarez et al, 2008;St Louis et al, 2001). Therefore, we increased depositions values from literature by a factor of 2 (deciduous forests) and 2.5 (conifer forest) for Cd and Hg, and 2 (deciduous forests) and 3 (conifer forest)…”

“…Historical HM inputs were assessed by peat and ice core studies conducted in Switzerland Ross-Barraclough and Shotyk, 2003;Shotyk et al, 2002;Weiss et al, 1999). In addition, the tree species also influences the HM inputs (Perez-Suarez et al, 2008;St Louis et al, 2001) which has been taken into account in our estimations with different corrections factors for conifer and deciduous forests. Uncertainties are also related to the sampling of the soil matrix and the soil solution at the same spot.…”