Nitrogen and Sulfur Deposition Effects

Tidal Marsh

Sulfur and nitrogen compounds are emitted as pollutants into the atmosphere by a variety of sources, including industry, power plants, and automobiles. After transport and transformation in the atmosphere they may be deposited by either wet deposition or dry deposition. Once deposited into ecosystems they may cause acidification, fertilization, or eutrophication. Effects include changes in water chemistry that affect aquatic vegetation, invertebrate communities, amphibians and fish. Deposition can also cause chemical changes in soils that affect soil microorganisms, plants, and trees. Plant species composition and abundance may change where nitrogen overstimulates growth, favoring some types of plant species and inhibiting growth of others. The deposition of nitrogen also contributes to nutrient enrichment in coastal and estuarine ecosystems, which can cause toxic algal blooms, fish kills, and loss of plant and animal diversity.

High Mountain Wilderness

High elevation ecosystems in the Rocky Mountains, Cascades, Sierra Nevada, southern California, and the upland areas of the eastern U.S. are generally the most sensitive to atmospheric deposition due to their limited ability to neutralize acid deposition or to absorb excess nitrogen. Streams in both Shenandoah and Great Smoky Mountains National Parks are experiencing chronic and episodic acidification, affecting brook trout fisheries. Nutrient poor ecosystems in parks, including some high elevation lakes, alpine tundra, coastal sage ecosystems, serpentine grasslands, and lichen communities are experiencing changes in plant species and soil nutrient cycling due to excess nitrogen deposition. For example, aquatic and terrestrial ecosystems in Rocky Mountain National Park are currently undergoing subtle changes attributable to atmospheric deposition of nitrogen.

"Critical load" is a term used to describe the amount of pollution that initiates harmful changes in sensitive ecosystems. Scientists have developed a critical load for nitrogen deposition to high-elevation lakes in Rocky Mountain National Park. Research is underway in other parks to develop critical loads for communicating resource condition and evaluating management strategies.

Protecting Resources on Federal Lands: Implications of Critical Loads for Atmospheric Deposition of Nitrogen and Sulfur (July 2005, pdf 249kb)

Translating science into policy: Using ecosystem thresholds to protect resources in Rocky Mountain National Park (October 2007, pdf 574kb)