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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 the proposed study, maps will be created for high-elevation areas in the Rocky Mountains showing (a) atmospheric deposition rates of nitrogen and acidifying compounds, (b) Critical Loads of N (nitrogen) and S (sulfur), and (c) Critical Loads exceedances. The deposition maps will be created using gridded precipitation data and spatially interpolated chemical concentrations from snowpack and wet deposition sampling. Critical Loads maps will be created based on chemical thresholds corresponding to observed ecological effects, and estimated ecosystem sensitivities calculated from basin characteristics. Chemical thresholds (concentrations) will be identified for surface waters by comparing diatom species composition to surface-water chemistry at a suite of lakes in the Rockies, and identifying the concentration of inorganic N or alkalinity at which diatom species shifts occur. 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.