A comparison of winter mercury accumulation at forested and no-canopy sites measured with different snow sampling techniques |
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| Authors: | Sarah J Nelson Kenneth B Johnson Kathleen C Weathers Cynthia S Loftin Ivan J Fernandez Jeffrey S Kahl David P Krabbenhoft |
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| Institution: | 1. Senator George J. Mitchell Center for Environmental and Watershed Research, University of Maine, 5710 Norman Smith Hall, Orono, ME 04469, USA;2. Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA;3. U.S. Geological Survey, 5755 Nutting Hall, University of Maine, Orono, ME 04469-5755, USA;4. Department of Plant, Soil and Environmental Sciences, University of Maine, Orono, ME 04469, USA;5. Center for the Environment, Plymouth State University, MSC#63, 213 Boyd Hall, Plymouth, NH 03264, USA;6. U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562-3581, USA |
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| Abstract: | Atmospheric mercury (Hg) is delivered to ecosystems via rain, snow, cloud/fog, and dry deposition. The importance of snow, especially snow that has passed through the forest canopy (throughfall), in delivering Hg to terrestrial ecosystems has received little attention in the literature. The snowpack is a dynamic system that links atmospheric deposition and ecosystem cycling through deposition and emission of deposited Hg. To examine the magnitude of Hg delivery via snowfall, and to illuminate processes affecting Hg flux to catchments during winter (cold season), Hg in snow in no-canopy areas and under forest canopies measured with four collection methods were compared: (1) Hg in wet precipitation as measured by the Mercury Deposition Network (MDN) for the site in Acadia National Park, Maine, USA, (2) event throughfall (collected after snowfall cessation for accumulations of >8 cm), (3) season-long throughfall collected using the same apparatus for event sampling but deployed for the entire cold season, and (4) snowpack sampling. Estimates (mean ± SE) of Hg deposition using these methods during the 91-day cold season in 2004–2005 at conifer sites showed that season-long throughfall Hg flux (1.80 μg/m2) < snowpack Hg (2.38 ± 0.68 μg/m2) < event throughfall flux (5.63 ± 0.38 μg/m2). Mercury deposition at the MDN site (0.91 μg/m2) was similar to that measured at other no-canopy sites in the area using the other methods, but was 3.4 times less than was measured under conifer canopies using the event sampling regime. This indicates that snow accumulated under the forest canopy received Hg from the overstory or exhibited less re-emission of Hg deposited in snow relative to open areas. The soil surface of field-scale plots were sprayed with a natural rain water sample that contained an Hg tracer (202Hg) just prior to the first snowfall to explore whether some snowpack Hg might be explained from soil emissions. The appearance of the 202Hg tracer in the snowpack (0–64% of the total Hg mass in the snowpack) suggests that movement of Hg from the soil into the snowpack is possible. However, as with any tracer study the 202Hg tracer may not precisely represent the reactivity and mobility of natural Hg in soils. |
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