Concentrations and Yields of Mercury, Methylmercury, and Dissolved Organic Carbon From Contrasting Catchments in the Discontinuous Permafrost Region, Western Canada.

Climate change and permafrost thaw may impact the mobilization of terrestrial dissolved organic carbon (DOC), mercury (Hg), and neurotoxic methylmercury (MeHg) into aquatic ecosystems; thus, understanding processes that control analyte export in northern catchments is needed. We monitored water chemistry for 3 years (2019–2021) at a peatland catchment (Scotty Creek) and a mixed catchment (Smith Creek) in the Dehcho (Northwest Territories), within the discontinuous permafrost zone of boreal western Canada. The peatland catchment had higher DOC and dissolved MeHg, but lower total Hg concentrations (mean ± standard deviation; 19 ± 2.6 mg DOC L−1; 0.08 ± 0.04 ng DMeHg L−1; 1.1 ± 0.3 ng THg L−1) than the mixed catchment (12 ± 4.4 mg DOC L−1; 0.05 ± 0.01 ng DMeHg L−1; 3.1 ± 2.2 ng THg L−1). Analyte concentrations increased with discharge at the mixed catchment, suggesting transport limitation and the flushing of near‐surface, organic‐rich flow paths during wet periods. In contrast, analyte concentrations in the peatland catchment were not primarily associated with discharge. MeHg concentrations, MeHg:THg, and MeHg:DOC increased with water temperature, suggesting enhanced Hg methylation during warmer periods. Mean open water season DOC and total MeHg yields were greater and more variable from the peatland than the mixed catchment (1.1–6.6 vs. 1.4–2.4 g DOC m−2; 5.2–36 vs. 6.1–10 ng MeHg m−2). Crucial storage thresholds controlling runoff generation likely drove greater inter‐annual variability in analyte yields from the peatland catchment. Our results suggest climate change may influence the production and transport of MeHg from boreal‐Arctic catchments as temperatures increase, peatlands thaw, and runoff generation is altered. Plain Language Summary: Boreal rivers deliver dissolved organic carbon (DOC), mercury (Hg), and its neurotoxic form, methylmercury (MeHg), from contributing landscapes to downstream waters. In northern regions, thawing permafrost (i.e., perennially frozen ground) in peatland environments may release Hg, MeHg, and DOC from soils to rivers. Over 3 years, we measured the concentrations of water chemistry in two boreal creeks with differing contributing landscapes in the discontinuous permafrost region of the Dehcho (Northwest Territories, Canada). Our results showed different patterns in analyte concentrations versus yields (i.e., annual analyte mass delivered per unit area) between the sites. At the peatland‐dominated Scotty Creek, concentrations of MeHg and DOC were consistently high and not primarily controlled by discharge. Still, runoff and analyte yields varied annually as peatlands stored water in a dry year and discharged water in wet years. At Smith Creek, with a mixed landscape of mountains, peatlands, and forests, concentrations of DOC and Hg increased during high‐flow events as runoff contributions from peatlands increased. Over the monitoring period, analyte yields at the mixed catchment varied less due to consistent groundwater inputs and runoff generation from steeper slopes. Changing runoff due to climate change and permafrost thaw will likely alter analyte yields in the region. Key Points: Comparison of mercury, methylmercury, and dissolved organic carbon concentrations and yields for a peatland and a mixed‐landscape catchmentProduction of methylmercury in the peatland catchment was temperature‐dependent, and yields were limited by runoff generation thresholdsAnalyte concentrations in the mixed catchment increased with discharge, and had lower inter‐annual variability in analyte yields [ABSTRACT FROM AUTHOR]