Influence of the Forest Canopy on Total and Methyl Mercury Deposition in the Boreal Forest

Atmospheric mercury deposition by wet and dry processes contributes mercury to terrestrial and aquatic systems. Factors influencing the amount of mercury deposited to boreal forests were identified in this study. Throughfall and open canopy precipitation samples were collected in 2005 and 2006 using passive precipitation collectors from pristine sites located across the Superior National Forest in northern Minnesota, USA. Samples were collected approximately every 2 weeks and analyzed for total (THg) and methyl mercury (MeHg). Forest canopy type and density were the primary influences on THg and MeHg deposition. Highest THg and MeHg concentrations were measured beneath conifer canopies (THg mean = 19.02 ng L−1; MeHg mean = 0.28 ng L−1) followed by deciduous throughfall (THg mean = 12.53 ng L−1; MeHg mean = 0.19 ng L−1) then open precipitation (THg mean = 8.19 ng L−1; MeHg mean = 0.12 ng L−1). The greater efficiency of conifers at scavenging THg and MeHg from the atmosphere may increase the risk of mercury related water quality issues in conifer-dominated systems.