Total Mercury and Methyl-Mercury Concentrations and Pools across 14 U.S. Forest Sites: Factors that Determine Mercury Loads in Remote Terrestrial Ecosystems

Obrist D1*, Johnson D2, Lindberg S3, Luo Y4, Hararuk O4, Bracho R5, Battles J6, Dail B7, Edmonds B8, Monson R9, Ollinger S10, Pallardy S11, Pregitzer K2, Todd D3

The Mercury Deposition Network of the National Atmospheric Deposition Program and various atmospheric monitoring stations across the United States continuously record atmospheric pollution levels of mercury and their significance for deposition loads. Little information, however, is known how these atmospheric pollution measurements relate to ultimate mercury loads observed in terrestrial ecosystems. We performed a systematic investigation of total mercury and methylmercury concentrations and pools in 14 sites in order to determine the main factors that determine large-scale distribution of mercury in remote U.S. forests. Analysis included all major ecosystems compartments of forests, including foliage, branches, bark, bole, different surface litter horizons, and soils at various depths. We also calculated total pools of mercury and methyl-mercury using full biomass, soil mass, or carbon inventories on all sites.

Total Hg concentrations in aboveground biomass (i.e., leaves, bark, bole, and understory) was distributed in a highly random fashion, possibly reflecting previous reports that mercury levels vary greatly among different species, tissue age, or location within canopies. In litter and soils, however, we observed clear spatial patterns of Hg concentrations: four variables, including latitude, annual precipitation, and soil carbon and clay contents (in soils) explained most of the variability in observed Hg concentrations. Observed spatial patterns of mercury concentrations were mostly unrelated to extrapolated values of atmospheric pollution, including atmospheric mercury wet deposition loads, mercury air emissions from EPA's toxics emission inventory, or measured air Hg(0) and Hg(II) levels. Pools of aboveground biomass mercury and litter mercury were directly related to the respective mass of these components, and hence were mainly determined by environmental factors that determined biomass and litter pools. Sites with high concentrations of total Hg also showed higher levels of methyl-mercury. Our results show that ecosystem properties, such as biomass, litter, and soil carbon dynamics, play a key role in determining mercury loads across remote U.S. forests.

1*Corresponding author: Desert Research Institute, Reno, NV (775) 674-7008 ;
2 University of Nevada, Reno
3 Oak Ridge National Laboratory
4 University of Oklahoma, Norman
5 University of Florida, Gainesville
6 University of California, Berkeley
7 University of Maine, Orono
8 University of Washington, Seattle
9 University of Colorado, Boulder
10 University of New Hampshire, Durham
11 University of Missouri, Columbia