Climate change and catchment controls on the mercury cycle of northern lakes

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Although the cycling of mercury in the environment has been the focus of scientific attention for decades, many governing processes remain poorly described which still prevent us from being able to predict mercury concentrations in fish. Very recent discoveries about the role of vegetation in supplying mercury to catchment soils, and the importance of the delivery of methyl mercury from the catchment to lakes reveal that our understanding about mercury in the environment remains incomplete. The lack of knowledge contributes to our inability to factor climate change into predictions of mercury in biota. Since climate change is already having an impact in Canada's north, understanding the controls on mercury in fish in higher latitude catchments and lakes is extremely urgent for northern residents who rely on fish as a dietary staple. This proposal has three objectives which seek to significantly advance our understanding of the mercury cycle in boreal sub-arctic catchments and lakes. The first objective focusing on the role of vegetation in the transfer of mercury from the atmosphere to soils will take an exciting approach using stable isotopes of mercury in experimental chambers to determine the fractions of mercury in plant leaves that come from the atmosphere and soils. A field study will also evaluate how the shift from spruce to aspen forests in the north after forest fires will change the supply of mercury to soils. The second objective focusses on the role of the catchment on the production of methylmercury, the toxic form of mercury that accumulates in fish. Using a suite of laboratory instruments to fingerprint' the sources of organic matter and stable mercury isotopes to measure biogeochemical processes, we will test the hypotheses that northern lake sediments are dominated by organic matter from the catchment, that the catchment is the most important source of methylmercury to northern lakes, and that thawing permafrost is a specific and growing source of methylmercury in the environment. Once this methylmercury and the dissolved organic matter that it is bound to gets to the lake, it needs to enter the food chain to be accumulated in biota. The final objective of the proposed work will evaluate how future changes in dissolved organic matter quantity and quality will change the way that mercury is assimilated by algae which is at the very base of the aquatic food chain. This proposal addresses three of the most important knowledge gaps in our understanding of mercury cycling in northern catchments: controls on atmospheric deposition; processes governing speciation and transport from catchments, and; controls on bioavailability at the base of aquatic food webs. These advances will dramatically improve our ability to predict current and future patterns of mercury in northern lakes and fish.