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 NO(3)(-) threshold of 0.5 μmol L(-1), N deposition exceeds CL(Ndep) 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.