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    Over three years (2012 to 2015), we are comparing MeHg bioaccumulation in three study areas that cover a latitudinal gradient in ecosystem types in the Canadian Arctic, specifically Kuujjuaraapik (sub-Arctic taiga), Iqaluit (tundra) and Resolute Bay (polar desert). In water bodies from each of these study areas, we will investigate two key aspects of MeHg bioaccumulation, specifically MeHg bioavailability to benthic food webs and organism growth rates. Using a cross-ecosystem comparison to test these hypotheses, we will conduct the following in lakes and ponds from each study area: - Characterize the watersheds of study sites (geomorphology and physiography) through satellite image classification and digital terrain analysis in order to examine watershed influences on measured lake physico-chemistry, particularly levels of organic carbon and Hg in sediment and water; - Estimate bioavailable MeHg in sediment pore water using a novel technique (Diffusive Gradient in Thin films, or DGT); - Measure MeHg concentrations in benthic food webs (algae, invertebrates and fish); and - Estimate short-term growth rates in invertebrates and fish using tissue DNA and RNA content. Project results will provide a conceptual model of climate-related environmental processes that affect the exposure of Arctic freshwater fish to MeHg. This information is critical to understand how climate change is affecting temporal and geographic trends of Hg bioaccumulation in Arctic fish monitored by the NCP. Mercury is a priority contaminant of the Northern Contaminants Program (NCP) due to its prevalence in the Arctic and high levels found in some traditional food species. The main objective of this project is to investigate climate-related environmental controls on methylmercury (MeHg) bioaccumulation in Arctic freshwater food webs. Recent evidence indicates that inorganic mercury (Hg) loadings to Arctic lakes decline with latitude; however, MeHg concentrations in benthic invertebrates and fish do not similarly decline along this gradient in Hg loading. These observations suggest that environmental factors may play an important role in ecosystem sensitivity to Hg bioaccumulation in the Canadian Arctic. We hypothesize that climate's dominant control on organic matter production affects two key aspects of MeHg bioaccumulation, specifically MeHg bioavailability to food webs and organism growth rates. In addition, a latitudinal water temperature gradient likely affects organism growth rates.