Mapping critical loads of nitrogen deposition for aquatic ecosystems in the Rocky Mountains, USA

Spatially explicit estimates of critical loads of nitrogen (N) deposition (CL_Ndep) for nutrient enrichment in aquatic ecosystems were developed for the Rocky Mountains, USA, using a geostatistical approach. The lowest CL_Ndep estimates (<1.5 ± 1 kg N ha⁻¹ yr⁻¹) occurred in high-elevation basins with steep slopes, sparse vegetation, and abundance of exposed bedrock and talus. These areas often correspond with areas of high N deposition (>3 kg N ha⁻¹ yr⁻¹), resulting in CL_Ndep exceedances ≥1.5 ± 1 kg N ha⁻¹ yr⁻¹. CL_Ndep and CL_Ndep exceedances exhibit substantial spatial variability related to basin characteristics and are highly sensitive to the NO_{3− threshold at which ecological effects are thought to occur. Based on an NO3− threshold of 0.5 μmol L⁻¹, N deposition exceeds CLNdep in 21 ± 8% of the study area; thus, broad areas of the Rocky Mountains may be impacted by excess N deposition, with greatest impacts at high elevations.}