Spatial Variation of Atmospheric Nitrogen Deposition and Estimated Critical Loads for Aquatic Ecosystems in the Greater Yellowstone Area

Abstract:

Aquatic ecosystems at high-elevations in the Greater Yellowstone Area (GYA) are sensitive to the effects of atmospheric nitrogen (N) deposition. Current and historic N deposition has impacted aquatic ecosystems in the GYA and N deposition is increasing in many areas. Anticipated changes in atmospheric emissions may further affect these sensitive ecosystems. Understanding the spatial variation in atmospheric N deposition is needed to develop estimates of air pollution critical loads for aquatic ecosystems in complex terrain. For the GYA, high resolution (400 meter) maps were developed for 1993-2014 to identify areas of high loading of mean annual Total N deposition (wet + dry) and wet deposition of inorganic N (nitrate and ammonium). Total N deposition estimates in the GYA range from ≤ 1.4 to 7.5 kg N ha^-1 yr^-1 and show greater variability than inorganic N deposition. Spatially explicit estimates of critical loads of N deposition (CL_Ndep) for nutrient enrichment in aquatic ecosystems were developed using a geostatistical approach. CL_Ndep in the GYA ranges from less than 1.5 kg N ha^-1 yr^-1 to over 10 kg N ha^-1 yr^-1 and variability is controlled by differences in basin characteristics. The lowest CL_Ndep estimates occurred in high-elevation basins with steep slopes, sparse vegetation, and exposed bedrock, including areas within GYA Wilderness boundaries. These areas often have high inorganic N deposition (>3 kg N ha^-1 yr^-1), resulting in CL_Ndep exceedances greater than 1.5 kg N ha^-1 yr^-1. The N deposition maps were used to identify CL_Ndep exceedances for aquatic ecosystems, and to explore scale dependence and boundary issues related to estimating CLN_dep. Based on a NO₃^- threshold of 1.0 μmol L^-1, inorganic N deposition exceeds CL_Ndep in 12% of the GYA, and Total N deposition is in exceedance for 23% of the GYA. These maps can be used to help identify and protect sensitive ecosystems that may be impacted by excess N deposition in the GYA.