Long-term (2003 - 2016) dissolved inorganic nitrogen fluxes from paired High Arctic catchments

Abstract

Climate change is expected to have strong impacts on permafrost and the frequency of landscape disturbances are expected to increase in response to warming temperatures and overall permafrost change. These disturbances can increase the export of dissolved inorganic nitrogen (DIN) from High Arctic catchments, having important effects on downstream aquatic ecosystems, thus motivating the need to better understand the impact of permafrost change on nitrogen exports. This study examined fluxes of DIN (ammonium (NH4+) and nitrate (NO3-)) from paired catchments at the Cape Bounty Arctic Watershed Observatory, Nunavut. Water samples were collected from the outlet of the rivers during the melt season from 2003 to 2016. A range of surface water draining different landscape types were also sampled during the 2016 season to identify how specific locations in the catchments control downstream DIN concentrations. Both physical disturbances, including active layer detachments (ALDs), and thermal perturbations related to widespread deepening of the active layer in response to warm summers, occurred during this record. Widespread physical disturbances occurred at the end of the 2007 field season in response to exceptionally warm temperatures and a large rainfall event. Additionally, 2012 was the warmest sampling year, and is one of the warmest years on record (1970 – 2016).

River NO3- fluxes increased in 2008, responding to the 2007 disturbance, as the nitrification and mobilization of nitrogen is inferred to increase as soils were brought to the surface and exposed to flushing. The DIN fluxes recovered rapidly at the catchment scale, reaching pre-disturbance values by 2010. DIN fluxes increased strongly in 2012 as warm temperatures are assumed to result in deep active layer thaw and increased subsurface flow, resulting in high DIN export, especially during the baseflow and stormflow hydrologic periods. Recovery from widespread permafrost thaw was not observed after four years.

Fluxes are similar between the rivers, with minor deviations associated with variations in catchment morphology. Spatial analysis of the DIN concentrations from upstream locations along the rivers does not show a distinct pattern throughout the season, but rather indicates a complex interaction of inputs, dilutions, and biogeochemical transformations.