Abstract
Mercury (Hg) is a widespread contaminant known to pose severe risks to wildlife and human health. While Hg emissions have declined in recent decades, legacy emissions and stored Hg will continue to impact watershed Hg cycling for the foreseeable future. Boreal forests are a major concern due to their capacity for storing Hg, vulnerability to disturbance, and record of high Hg concentrations in fish. Thus, there is a need to better quantify factors that influence Hg export from boreal forest catchments to inform watershed management decisions regarding Hg. Streamflow measurements, as well as approximately bi-weekly sampling for total mercury (THg), methylmercury (MeHg), and supporting stream chemistry were completed in 19 headwater streams near Dryden, Ontario during the ice-free season of 2019. The results were related to landscape and hydrological indices to elucidate the potential factors governing THg and MeHg export across these catchments. This study shows that while Hg concentrations are relatively low (0.50–20.46 ng l−1 THg; < 0.04–1.21 ng l−1 MeHg) across boreal streams in south central Canada, there are significant differences in Hg export. Catchments within boreal shield landscapes dominated by shallow soils and exposed bedrock export more methylmercury than catchments within glaciolacustrine plain landscapes dominated by thicker sand deposits. Coniferous forest cover is more significant than dissolved organic matter concentrations and more reliable than available % wetland cover data, two metrics commonly included in Hg transport models, for predicting THg and MeHg loads. In the absence of substantial mapped wetland cover, wet forest cover, as defined by the proportion of catchment cover by tree species favoring wet conditions, is shown to be an effective alternative metric. Considering the generally detailed and extensive data on tree species coverage available in Canada’s managed forests, wet forest cover, in addition to coniferous forest cover, could be useful for modelling Hg transport in boreal forest watersheds.
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Data availability
The datasets generated and/or analyzed during the study are available in the Scholars Portal Dataverse repository via https://doi.org/10.5683/SP3/4BA4BL
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Acknowledgements
We thank P. Huang, B. Graff, M. Paz, D. Boulding, C. McCarter, M. Givelas, V. Mangal, and E. Chen for laboratory and fieldwork assistance. D. McCormick provided GIS and mapping support. S. Melles provided helpful comments on an earlier draft of this manuscript.
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This research was supported by a Natural Sciences and Engineering Research Council Strategic Grant to Carl Mitchell.
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CPJM and RWM contributed to the study conception and design. All authors contributed to material preparation, data collection, and analysis. The first draft of the manuscript was written by WYL and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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Lam, W.Y., Mackereth, R.W. & Mitchell, C.P.J. Landscape controls on total mercury and methylmercury export from small boreal forest catchments. Biogeochemistry 160, 89–104 (2022). https://doi.org/10.1007/s10533-022-00941-9
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DOI: https://doi.org/10.1007/s10533-022-00941-9