Spatial variation of atmospheric nitrogen deposition and critical loads for aquatic ecosystems in the Greater Yellowstone Area

Current and historic atmospheric nitrogen (N) deposition has impacted aquatic ecosystems in the Greater Yellowstone Area (GYA). Understanding the spatial variation in total atmospheric deposition (wet + dry) of N is needed to estimate air pollution deposition critical loads for sensitive aquatic ecosystems. This is particularly important for areas that have an increasing contribution of ammonia dry deposition to total N (TN), such as the GYA. High resolution geostatistical models and maps of TN deposition (wet + dry) were developed using a variety of techniques including ordinary kriging in a geographic information system, to evaluate spatial variability and identify areas of elevated loading of pollutants for the GYA. TN deposition estimates in the GYA range from <1.4 to 7.5 kg N ha⁻¹ yr⁻¹ and show greater variability than wet inorganic N deposition. Critical loads of TN deposition (CL_TNdep) for nutrient enrichment in aquatic ecosystems range from less than 1.5 ± 1.0 kg N ha⁻¹ yr⁻¹ to over 4.0 ± 1.0 kg N ha⁻¹ yr⁻¹ and variability is controlled by differences in basin characteristics. The lowest CL_TNdep estimates occurred in high elevation basins within GYA Wilderness boundaries. TN deposition maps were used to identify critical load exceedances for aquatic ecosystems. Estimated CL_TNdep exceedances for the GYA range from 17% to 48% depending on the surface water nitrate (NO₃⁻) threshold. Based on a NO₃⁻ threshold of 1.0 μmol L⁻¹, TN deposition exceeds CL_TNdep in approximately 30% of the GYA. These predictive models and maps can be used to help identify and protect sensitive ecosystems that may be impacted by excess atmospheric N deposition. - Highlights: • Atmospheric deposition maps of TN identify hot spots and vary spatially. • TN critical loads are sensitive to the nitrate threshold value for ecological effects. • Lowest TN critical loads (<1.5 kg N ha⁻¹ yr⁻¹) are in high elevation basins. • Over 30% of aquatic ecosystems in the GYA may be impacted by excess TN deposition. - Total Nitrogen critical loads for nutrient enrichment of aquatic ecosystems show that over 30% of the Greater Yellowstone Area may be impacted by excess deposition.