Arctic Stream Networks are Sensitive Sentinels of Environmental Change in the Permafrost Zone.

Warming temperatures in the Arctic have led to unprecedented changes in permafrost conditions and subsurface flowpaths. Together, these changes could expose large pools of carbon and nutrients to transport and processing through Arctic surface waters. Increasing solute and nutrient concentrations in Arctic rivers suggest that lateral transport of solutes from the terrestrial landscape is increasing. However, the spatial and temporal drivers of these changing biogeochemical signals remain highly uncertain, preventing the inclusion of lateral transport in Earth system models. Here, we used a novel dataset with both high temporal and spatial resolution to characterize dissolved organic carbon (DOC) and nitrate (NO3) concentrations in three Arctic catchments. Across the 2016-2021 summer thaw seasons, we repeatedly sampled 35-50 locations in each catchment two to three times during each flow season. The catchments included a high-gradient alpine system, a low-gradient tundra system, and a low-gradient tundra system containing an abundance of lakes. With the spatially-extensive dataset, we found that the pattern of DOC concentrations in the alpine catchment remained relatively consistent during each sampling, but that the two low-gradient tundra catchments showed high spatiotemporal variability. Nitrate showed the opposite pattern, with greater variability in the alpine catchment than the tundra catchments. We found that the variance of DOC and NO3 was highest in the smallest subcatchments (<30 km2, <20 km2 and <10 km2 for tundra, lake, and alpine catchments respectively), indicating that relatively small catchments driving control lateral export of DOC and NO3. Finally, we characterized 18O and 2H stable water isotopes to assess the contributions of water from individual subcatchments, which showed that there is a positive correlation between volume of water contributed downstream and subcatchment area. We conclude that high-resolution hydrochemical data can improve estimates of current lateral carbon and nutrient flux while also serving as sensitive sentinels of Arctic change.