RII Track-4: Deciphering sources and accumulation pathways of mercury in the tributaries and lake of a forested watershed using stable mercury isotopes

Dartmouth College, Hanover NH

Investigators

Abstract

Non-technical Description Concentrations of mercury, a potent neurotoxin, frequently exceed levels safe for consumption by humans and wildlife in fish from pristine, freshwater lakes. The most toxic form of mercury is methylmercury, which is also the form of mercury that accumulates in wildlife and humans. The environmental factors that control the transformation of mercury to methylmercury in the environment are complex, and the sources of mercury and the environmental conditions involved in production and movement of methylmercury throughout the forests of the northeastern United States are not well understood. This project aims to use an emerging analytical technique developed by collaborators at the University of Michigan that relies on discrimination between subtle differences in the masses of mercury isotopes to identify the sources of mercury in a northern New England lake, with the goals of advancing the application of this cutting-edge new method. Importantly, application of this method will significantly improve current understanding of the sources of mercury in soil and water, the potential for production of methylmercury, and the accumulation of mercury in wildlife. Development of the collaboration with the University of Michigan will provide training for the PI and will significantly expand her research capacity at Dartmouth University. Technical Description Natural variability in mercury isotope ratios have been used to trace inputs and identify processes that regulate levels of methylmercury, the toxic form of mercury that is taken up by the biota. Biota in forested catchments of the northeastern United States are subject to high concentrations of methylmercury, but the production and transport of methylmercury in watersheds are mediated in complex ways by landscape characteristics (e.g., percent wetland and open water connectivity) and water chemistry (especially dissolved organic carbon levels). In this study, stable isotopes of mercury will be analyzed to decipher the sources of methylmercury to the tributaries of a pristine, oligotrophic lake, and to trace the transfer of mercury in the watershed from these tributaries to the lake. Isotopic mercury signatures in organisms can fingerprint methylmercury accumulated from the ecosystem over long time periods. These signatures reflect transformations (i.e., mercury methylation and demethylation) that occurred prior to biological uptake, allowing them to be used to distinguish sources of methylmercury (e.g., watershed, in-lake production) and pathways controlling methylmercury formation (e.g., biotic methylation, photodegradation) in the ecosystem. Through collaboration with the University of Michigan, and access to the collaborator's analytical instrumentation and facilities, mercury isotopic signatures will be determined in biota from the tributaries and the lake to: 1) explain sources and pathways controlling methylmercury levels across streams with varying landscape characteristics; 2) determine the effect of mercury complexation to organic carbon on mercury isotope signatures; and 3) identify major sources of methylmercury to fish in a pristine lake. This work will establish the use of mercury isotopic fingerprints to determine sources and production pathways of methylmercury in a watershed with no point-sources of mercury contamination, and provide a valuable new application of stable mercury isotope signatures.