Critical loads for atmospheric deposition of nitrogen and sulfur compounds in national parks of the USA

Background/Question/Methods

Critical loads are being increasingly developed and used in the USA to evaluate ecosystem condition, develop management goals, and assess the efficacy of air quality improvement strategies.  A critical load, simply defined, is the amount of nitrogen or sulfur atmospheric deposition below which harmful effects to ecosystems, either from acidification or nitrogen nutrient enrichment, are not expected.   The EPA has used critical loads to develop a framework for National Ambient Air Quality Standards to protect streams and lakes from acidifying effects of nitrogen and sulfur deposition, and the National Acid Precipitation Assessment Program (NAPAP) has used critical loads to evaluate progress towards reducing air pollution under the Acid Rain Program of the Clean Air Act.  Managers of federal lands, including national parks and forests, are also using critical loads to identify areas most vulnerable to deposition and to guide planning efforts.  A database is now available for many ecoregions of the USA that includes both critical loads for acidification of streams, estimated with a steady-state model, and critical loads for nutrient enrichment, estimated from thousands of empirical observations of nitrogen effects.  For this project, we used a GIS approach to map critical loads from the database for national parks with significant natural resources, including large national parks like Mount Rainier National Park (WA) and Shenandoah National Park (VA), and smaller units like Valley Forge National Historic Park (PA). 

Results/Conclusions

The resulting maps show critical loads for a variety of ecosystem types and ecosystem receptors in over 200 national parks.  In addition, we show maps of areas where current deposition of nitrogen or sulfur exceeds the critical load.  The maps suggest that many parks are in ecoregions where the critical loads are exceeded for nutrient nitrogen effects to mycorrhizal fungi, lichens, forest ecosystems, and herbaceous plants and shrubs.  A number of parks are in areas where critical loads for acidification are exceeded.   These maps are powerful management tools for the National Park Service and are being used to assess resource conditions in parks, identify park ecosystems most at risk, and assist park planning efforts.