The role of catchment characteristics, discharge, and active layer thaw on seasonal stream chemistry across ten permafrost catchments

Abstract. High latitude catchments are rapidly warming, leading to altered precipitation regimes, widespread permafrost degradation and observed shifts in stream chemistry for major arctic rivers. At headwater scales, stream discharge and chemistry are seasonally variable, and the relative influence of catchment characteristics, climate and active layer thaw on this seasonality has been poorly addressed. To provide new insight into mechanisms driving changes in streamflow chemistry within permafrost watersheds, we measured discharge and sampled major ion and dissolved organic carbon (DOC) concentrations across ten permafrost catchments in Yukon Territory, Canada. We incorporated concentration-discharge relationships within generalized additive models to resolve the distinct influence of discharge and seasonal active layer thaw on stream chemistry and identify the role of watershed characteristics on the magnitude and seasonality of solute concentrations. After accounting for seasonal variations in discharge, results indicate both major ions and DOC were highly seasonal across all catchments, with DOC declining and major ion concentration increasing post freshet. Seasonal variability in major ion concentrations were primarily driven by active layer thaw, whereas DOC seasonality was strongly controlled by flushing of soil organic carbon during freshet. While major ion concentrations were geologically mediated, greater permafrost extent led to enhanced seasonality in major ion concentrations. Catchments with strong topographical gradients and thinner organic soils had higher specific discharge, lower DOC concentrations but greater relative seasonality. Our results highlight the important role catchment characteristics play on shaping both the seasonal variations and magnitude of solute concentrations in permafrost underlain watersheds.