Hydrologic Connectivity and Threshold Behavior Influences Nutrient Export from Arctic Hillslopes

Abstract:

Hydrologic intensification at high latitudes is leading to increased rates of precipitation, evapotranspiration and runoff. This hydrologic intensification is coupled with pronounced high-latitude warming and widespread degradation of permafrost, highlighting the need to understand coupled hydrologic and biogeochemical processes that control nutrient fluxes to downstream rivers, lakes, and the Arctic ocean. Here we focus on two related systems: collapsing upland permafrost known as thermokarst features and portions of Arctic hillslopes known as water tracks, which are zero-order curvilinear features that preferentially convey perennial subsurface flow downslope. These features are also sites in which carbon (C), nitrogen (N), and phosphorus (P) losses, transformation, and uptake are likely to occur. We observed significant increases in C, N, and P export during upland thermokarst formation, with the largest relative increases in nitrogen. These changes often returned to pre-disturbance levels after ground stabilization. Within water tracks, experiments revealed that net uptake of P was more common than net uptake of N, and increased with P availability. Furthermore, P uptake rates outpaced flow rates whereas N export potential was higher because of rapid net mineralization of N relative to flow rates. We also observed that runoff responses in hillslope water tracks exhibit varying thresholds to precipitation between sites, with storage thresholds moderated by differences in subsurface response between water tracks and non-water track hillslopes. These differences imply dynamic hydrologic connectivity that may also affect biogeochemical responses among both water track and thermokarst features. Because Arctic streams and lakes tend to be P limited, the preferential export of N by water tracks and thermokarst could exacerbate stoichiometric imbalances in freshwater ecosystems as the Arctic warms.