Emerging role of summer rainfall events in driving High Arctic terrestrial-aquatic connectivity

Abstract:

Climate warming is changing precipitation patterns and causing permafrost thaw and disturbance in the High Arctic, which in turn is fundamentally modifying material mobilization, composition, and transport pathways along the terrestrial-aquatic continuum. We used data from the hydrological and biogeochemical research program at the Cape Bounty Arctic Watershed Observatory (2003-2019) to integrate changes in the timing and magnitude of fluvial energy (stream power) with organic and inorganic matter transfer and qualities along the terrestrial-aquatic continuum. We show that the timing of fluvial energy is shifting from a skewed (snowmelt-dominated) to a multi-modal (snowmelt- and rainfall-dominated) distribution across the circum-Arctic. At CBAWO, this shift enhanced connectivity for dissolved material fluxes (DOC, TDN, major ions). To couple watershed-scale particulate material fluxes (POC, suspended sediment), summer rainfall events must produce enough fluvial energy to overcome the watersheds’ buffering capacity. Physical permafrost disturbance (2007; active layer disturbances; ALDs) provided a unique opportunity to compare watershed-scale effects of disturbance versus concurrent greening (+0.19 to +1.3% yr^-1 NDVI) and increases in summer rainfall. Disturbance (1.2-2.7 % of the watersheds’ areal extent) reduced DOC export enough to offset increased DOC export observed in a paired undisturbed watershed -- but did not cause a multiyear increase in watershed-scale POC flux. Further, disturbance played a weaker role than increased magnitude and frequency of rainfall events in altering C export along the watershed terrestrial-aquatic continuum, but the ALDs have primed the landscape for accelerated geomorphic change when future rainfall magnitudes and consequent pluvial responses exceed the watersheds’ energy buffering capacity. Our identified energy threshold, which was necessary to connect the terrestrial-aquatic continuum at the watershed-scale, may explain disparities in the literature where C cycling in some Arctic aquatic systems (e.g. headwaters and large Arctic rivers) are heavily influenced by allochthonous input and terrestrial connectivity while lower-energy systems (e.g. small lowland ponds) primarily internally cycle autochthonous C.