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

Creeks and rivers are critical conduits of a myriad of solutes and are a representation of the catchment processes happening on the ecosystem scale. Changes in hydrological and permafrost conditions within the Taiga Plains have had significant impacts on water quality. Permafrost thaw as well as floods and droughts threaten to alter the downstream delivery of dissolved organic carbon (DOC) and the bound/co-transported neurotoxin methylmercury (MeHg), as well as various other solutes. While several studies have researched the influence of permafrost condition on water quality, the impact of hydrological conditions on water quality and biogeochemical dynamics remains unknown in the Taiga Plains. In this study, we sampled twenty-seven streams in the Northwest Territories and Alberta over a five-year period from 2020 – 2024 approximately monthly across a variety of permafrost and hydrological conditions, as well as four smaller creeks more intensively from 2023 – 2024. The objectives were to: 1) determine the impact of permafrost and hydrological conditions on water biogeochemistry, including the export of Hg, MeHg and DOC, 2) assess the vulnerability of different streams during floods and droughts with regard to permafrost thaw, catchment size, and wildfires, and 3) assess the dominating diel environmental processes within ecosystems during drought conditions with respect to DOM generation, degradation, and transportation. MeHg and DOC exhibited a flushing response during high flows, however this was only in watersheds with little to no permafrost. On average, MeHg and DOC concentrations in southern catchments significantly increased by 80% and 50%, respectively, when comparing concentrations at the 10th and 90th flow percentile, whilst colder, more northern catchments experienced zero change to increased discharge and responded chemostatically. Peatland compositional differences driven by increasing permafrost extent in more northern catchments likely acted to limit the production of MeHg and DOC, but more importantly limited the transportation of these solutes by fragmenting the landscape via permafrost peat plateaus, which inhibited water and subsequent solute mobility. On a diel scale, photic processes were the primary driver of diel patterns within streams across a permafrost gradient, with consistent photodegradation occurring of fluorescent dissolved organic matter (fDOM) during extensive drought periods. With daytime fDOM values being 2% - 8% lower than nighttime values, this indicates that significant mobilization of fDOM, and other strongly associated solutes such as MeHg (R2 = 0.57), is occurring during dark hours , with limited differences in diel magnitude between the creeks that were along a permafrost gradient. The diel patterns suggests that we may be systematically underestimating carbon and mercury export in the Taiga Plains, given typical daytime sampling regimes. The findings of this study further recognize the complexities of hydrological and permafrost condition on water quality and catchment biogeochemistry. As northern regions continue to face more landscape disturbances associated with climate change, continued monitoring is needed to inform management and future research directions to ensure traditional land use can occur safely.

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