Storm hydro-biogeochemistry, of boreal catchments disturbed by fire and permafrost thaw

Abstract

High-latitude ecosystems are vulnerable to a rapidly warming climate, which has led to thawing permafrost and increased frequency and intensity of wildfire. Such disturbances change pools and fluxes of carbon and nutrients within boreal catchments generating biogeochemical signals apparent in streams. However, a warming climate also results in more variable precipitation and larger storms, potentially obscuring biogeochemical signals of disturbance. I used a space-for-time approach to assess how disturbances influence biogeochemistry of high-latitude catchments by monitoring catchments occupying orthogonal contrasts in permafrost extent and fire history. We analyzed concentration-discharge (C-Q) dynamics (fDOM, NO₃-, specific conductance, and turbidity) during storms, when catchments are most hydrologically connected to streams. Sourcelimited C-Q dynamics and dilution of nitrate and specific conductivity in all catchments suggested storm runoff comprises overland flow and shallow subsurface flowpaths that bypass mineral-rich soils regardless of catchment context. A positive correlation between burn extent and the variability of C-Q dynamics for inorganic solutes suggested wildfire contributes to the patchiness in hydrologic connectivity within catchments. Greater source limitation of nitrate in catchments with greater burn extent suggest smaller pools of nitrate that are accessible to shallow flowpaths. Mobilization of fDOM during storms varied among catchments, with dilution of nearstream sources of fDOM in low relief catchments and flushing following delayed mobilization in steeper catchments. A positive correlation between deciduous cover and the variability of flushing dynamics for fDOM suggest more heterogenous flowpaths in permafrost-free catchments compared to catchments underlain with permafrost. Antecedent moisture conditions and seasonality had weak effects on storm C-Q dynamics and the strong imprint of catchment attributes on hydrology and biogeochemistry during storms persisted despite storm- to seasonal-scale variation in moisture conditions. Results of this study show 1) stream biogeochemistry measurements can be used as a proxy for upstream soil, thermal, and post fire processes and 2) long-term monitoring of catchment biogeochemistry during storms could therefore provide indicators of change in the state of terrestrial ecosystems resulting from permafrost thaw or changing fire regime.