Fire, drought and permafrost thaw: interactive effects on dissolved organic carbon, mercury, and nutrients in peatland-rich watersheds of boreal western Canada.

Permafrost thaw, wildfires, and increased amplitude of floods and droughts are all aspects of climate change that risk affecting the downstream mobilization of dissolved organic carbon (DOC), the neurotoxin methylmercury (MeHg), and various nutrients from boreal peatlands. In this study, we monitored water chemistry at the outflow of eighteen peatland-rich catchments (125 – 1700 km2) across a climate gradient in boreal western Canada. Our study included a wetter year (2022) and two extreme drought years (2023 and 2024) which also resulted in extensive fires that affected nine of the watersheds in 2023. Flood conditions in 2022 resulted in high hydrological connectivity, particularly in comparison to 2023 and 2024, where drought conditions coupled with extensive wildfires limited connectivity and combusted existing carbon stores. This reduced the mobilization of the co-transported MeHg in streams during the drought years, as well as resulting in no observable effects of the wildfires overall on water chemistry due to this limited connectivity within the peatlands.

Peatlands, covering over 30% of the Dehcho Region and Hay River watershed, are likely the primary source of these deleterious solutes due to the biogeochemistry of anoxic waterlogged soils. The goals of this monitoring program were developed collaboratively with the Dene Tha’, K'atl'odeeche, and Dehcho First Nations; their perspectives have been critical for this project. Community members expressed concerns about declining water quality potentially related to climate change, and the associated impacts on ecosystem health and fish resources. To assess the temporal variability of water quality, we studied four creeks with similar catchment sizes and peatland extent along a permafrost gradient using in-situ field sensors and grab samples. Additionally, a Before-After-Control-Impact methodology was used to evaluate the impacts of wildfires on water quality in eighteen streams along a permafrost gradient to assess northern vulnerability. Previous research has observed that peatlands in the discontinuous permafrost zone have poorer hydrological connection to the stream network compared to peatlands south of the permafrost boundary, resulting in higher solute concentrations in the southern region extent. Higher flow conditions during the spring resulted in minor peaks in MeHg and DOC concentrations, with decreasing concentrations as freshet ended. However, warmer temperatures that accelerate microbially associated DOC and MeHg generation resulted in the highest annual concentrations, despite extreme drought conditions that should’ve limited surface water sourcing.

Understanding the interactive impacts of permafrost conditions, wildfire and inter-annual climate variability on water quality is essential for managing and protecting the health of northern communities, ecosystems, and food webs. This research provides vital data to inform decision-making, support the resilience of local First Nations, and guide effective environmental stewardship in the face of ongoing climate change. We believe our findings are likely representative of northern peatland-rich regions broadly and will thus be of great interest to understand the global impacts of permafrost thaw and wildfire on carbon and mercury cycling.