| Summary: | Arctic mercury (Hg) contamination is an ongoing threat to human populations and ecosystems alike. Compared to preindustrial levels, elevated concentrations of Hg in air and water samples have been observed across the high latitudes with recent estimates demonstrating substantial Hg storage in Arctic soils. Climate change is expected to have a strong impact on the stability of permafrost landscapes potentially releasing large quantities of Hg from thawed soils to surface waters. This study investigates multi-year, seasonal, and diel dynamics of total mercury (THg) and methyl mercury (MeHg) concentrations, particulate partitioning, and flux from two small High Arctic rivers, subject to permafrost disturbance, at the Cape Bounty Arctic Watershed Observatory, Nunavut, Canada. Water samples were collected from the outlet of the rivers during the melt season (2007-2017; excluding 2011, 2013-2015), and from several small hillslope tributaries with various levels of permafrost disturbance (2009, 2016, 2017). Results indicate that there are large diel and inter-annual variations in THg concentration and flux, in part due to climate-driven changes in discharge and the physical disturbance of active layer and permafrost soils. A large proportion of THg (30.5-72.5%) was particulate bound, and significant positive relationships were observed between THg, suspended sediment, discharge, and organic carbon concentrations in both rivers. MeHg concentrations were low for both rivers (0.05 ng L-1) and were poorly correlated with discharge, suspended sediment concentration or organic carbon. The timing and intensity of runoff was a dominant driver of THg flux in all years with the majority of discharge and peak THg concentrations occurring during either the brief nival freshet or uncommon late season rainfall events. These results provide a critical link between Hg stored in Arctic soils, permafrost disturbance and fluvial Hg export, with important implications for Hg cycling in a changing Arctic. M.Sc.
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