Spatial, temporal and dendrochronological comparisons of mercury in Ontario deciduous trees
Siwik, Eden Isabel Helen
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Mercury (Hg) research in Ontario has centered on aquatic environments since fish can be a significant source of methylmercury for humans. Interactions between terrestrial and aquatic environments in regards to the Hg cycle have received less attention. This study focuses on the role of trees as a source and/or sink of Hg to the environment and the ap- plicability of deciduous trees as biomonitors for Hg. Leaf, bark and wood core samples were taken from a variety of deciduous species including Acer spp. (sugar, silver and red maple), Quercus spp. (red oak), Populus spp. (eastern cottonwood), and Salix spp. (willow) mainly from the Dorset and Kingston, ON regions. Examination of seasonal variation of Hg concentrations ([Hg]) in leaves indicated that average autumn leaf Hg ranged from 17– 45 ng/g and differed significantly among species. Leaf [Hg] increased with the growing season and roughly corresponded to regional atmospheric [Hg]. Microspatial trends in leaf [Hg], such as position on a branch, may increase variability. Tree cores from contami- nated and reference sites in the Kingston area showed that although dendrochronological techniques are not appropriate for estimating historical atmospheric Hg, they are useful for estimating the total Hg burden of a single tree. Populus and Salix, had the greatest bark and wood [Hg] with maximums reaching 18 ng/g. This was significantly higher than more inland trees such as Quercus and Acer, with maximum values of 7 and 1.2 ng/g for bark and wood [Hg]. Individual, average sized trees in this study contained a total of 0.4–2.6 mg of Hg. This study will provide forest and watershed managers with a better understanding of the importance of trees in the Hg cycle and the contribution of leaf and wood Hg to litterfall and waterbodies in southern Ontario.