Terrestrial and in‐stream influences on the spatial variability of nitrate in a forested headwater catchment

Abstract

A vast majority of monitoring programs designed to assess nutrient fluxes from headwater systems rely upon temporally intensive sampling at a single position within the stream network, essentially measuring the integrated response of the catchment. Missing from such an approach is spatial information related to how nutrient availability varies throughout the network, where freshwater biota live and where biogeochemical processes ultimately shape the downstream water chemistry. Here, we examine the spatial distribution of nitrate (NO3−) concentrations within the Paine Run catchment, a forested headwater catchment in Shenandoah National Park, Virginia. Nitrate concentrations throughout the stream network were measured as part of synoptic surveys conducted in 1992–1994, in the aftermath of region‐wide gypsy moth defoliation that caused dramatic increases in stream water NO3− concentrations. A follow‐up synoptic survey was conducted in 2007, when the stream water NO3− concentrations had returned to predefoliation levels. Common to each of the eight synoptic surveys were observations of multiple‐fold declines in NO3− concentration along the main stem of the stream network from the headwaters to the catchment outlet. A portion of this decline was caused by dilution, as water input by tributaries at the lower elevations of the catchment tended to have lower NO3− concentrations. A stream network model was applied to determine the relative contributions of terrestrial versus in‐stream processes to the spatial variability of the NO3− concentrations. Model results suggest that even though nitrate removal within the stream network can be substantial, terrestrial factors that determine the NO3− inputs to streams account for the vast majority of the spatial variability in stream water NO3− concentrations.