Critical Loads Map of Atmospheric Nitrogen in the Rocky Mountains, USA

Leora Nanus1*, David W. Clow2, Verlin C. Stephens3 and Jasmine Saros4

Critical loads are the amount of deposition of a given pollutant that an ecosystem can receive below which ecological effects are thought not to occur. In this study, maps are being created for highelevation areas in the Rocky Mountains showing (a) current atmospheric deposition rates of nitrogen (N), (b) critical loads of N, and (c) exceedances of critical loads of N. Deposition maps were developed at 400m resolution using gridded precipitation data and spatially interpolated chemical concentrations in snow and rain. Critical loads maps are being created based on chemical thresholds corresponding to observed ecological effects, and estimated ecosystem sensitivities calculated from basin characteristics.

Diatom species assemblages are being used as an indicator of ecosystem health to establish critical loads of N. Chemical thresholds (concentrations) will be identified for surface waters by using a combination of in-situ growth experiments and observed spatial patterns in surface-water chemistry and diatom species assemblages across a nitrogen deposition gradient.

Ecosystem sensitivity was estimated using a multiple-linear regression approach in which observed surface water nitrate concentrations at 530 sites were regressed against estimates of inorganic N deposition and basin characteristics (topography, soil type and amount, bedrock geology) to develop predictive models of surface water chemistry. Modeling results (r2 = 0.5, p < 0.01) indicated that the significant explanatory variables included percent slope, soil permeability, and vegetation type (including barren land, shrub, and grassland) and were used to predict high-elevation surface water nitrate concentrations across the Rocky Mountains.

Chemical threshold concentrations will be substituted into an inverted form of the model equations and applied to estimate critical loads for each stream reach within a basin, from which critical loads maps will be created. Deposition maps will be overlaid on the critical loads maps to identify areas where critical loads are being exceeded, or where they may do so in the future.

1* Corresponding author: Phone: 415-338-3849, Email: San Francisco State University, Department of Geosciences, 1600 Holloway Ave, San Francisco, CA 94132,
2 U.S. Geological Survey, MS 415 Federal Center, Denver, Colorado 80225; 303-236-4882x294;
3 U.S. Geological Survey, MS 415 Federal Center, Denver, Colorado 80225; 303-236 2101x226;
4 University of Maine, 137 Sawyer Research Center, Orono, Maine 04469; 207-581-2112;