Permafrost conditions in peatlands govern riverine flushing of dissolved organic carbon, methylmercury, and nutrients

Abstract. Permafrost thaw and intensified droughts and floods threaten to alter the mobilization of dissolved organic carbon (DOC), nutrients and methylmercury (MeHg) from boreal peatlands, with cascading impacts on aquatic ecosystem functions and traditional food sources. Here we monitored 27 peatland-dominated (>30 %) catchments in western Canada (150 to 52,000 km²) over a five-year period (2020 – 2024) which captured extreme hydroclimatic conditions. These catchments spanned a climatic and permafrost gradient (mean annual temperature -0.2 to -2.8 °C), which provided a space-for-time framework to assess impacts of continued thaw and warming. Our results demonstrated that catchment climate, and thus permafrost conditions, strongly controlled the hydrological response of solute concentrations. Warmer catchments showed a pronounced flushing response where DOC and MeHg concentrations increased by 50 % and 80 %, respectively, as discharge increased from the 10th to 90th percentile. In contrast, colder catchments maintained a chemostatic response, where concentrations remained stable and low despite hydrological variability. Similar climate-hydrology interactions were found for total nitrogen, total phosphorous, and inorganic phosphorous, but not for inorganic nitrogen. It is likely that permafrost conditions in peatlands affect both the production of solutes and their hydrological connectivity to the stream network. The presence of permafrost in peatlands may act to both ensure connectivity during droughts but also preclude full connectivity during floods, yielding the observed patterns. Our findings suggest that ongoing peatland permafrost thaw will increase mobilization MeHg, DOC, and nutrients during high flow periods. This shift necessitates further monitoring to understand the long-term consequences for aquatic ecosystems and northern communities.