Long-term collaborative research helps Rocky Mountain National Park address nitrogen-based air pollution

Nitrogen is the most abundant element in the atmosphere. It is essential for life and is found in the amino acids, proteins, and genetic material of all living organisms. Nitrogen can also be a pollutant; it is a major cause of acid rain and dead zones in the world’s oceans and large lakes.

Nitrogen comes in a number of chemical forms including nitrate, nitrite, and ammonium. It is emitted naturally from soils after bacterial processing, as well as from human sources, such as automobiles, industrial operations, and agriculture (Figure 1). All these emissions are transported through the atmosphere and interact with vegetation, soil, and water.  Excessively high nitrogen levels in lakes and streams can change the composition and abundance of aquatic species and negatively affect ecosystem health and water quality.  High-elevation catchments are particularly sensitive to small changes in the flux of nitrogen and can act as harbingers of ecosystem change at lower elevations.

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Nitrogen deposition values and nitrate concentrations in a number of alpine and subalpine lakes within the Front Range of the Rocky Mountains are greater than those typically found in other remote, non-industrialized parts of the world. Atmospheric nitrogen deposition in the region not only exceeds pre-industrial deposition values, it has doubled since the mid-1900s. Current nitrogen concentrations exceed the requirements of many ecosystems, resulting in the export of excess nitrate into lakes and streams.  

Rocky Mountain National Park (Rocky Mountain NP), located within the Front Range in north-central Colorado, contains over 107,000 hectares of montane forests, grasslands, glaciers, snowfields, lakes, and streams (Figure 2).  More than half of Rocky Mountain NP is located at or above the tree line, which is the highest elevation at which trees can survive due to harsh weather conditions.  For many years, natural resource managers have discussed the potential impacts of elevated nitrogen deposition and the need to develop air quality standards to protect these fragile, high-elevation ecosystems.  In response to this concern, researchers from the U.S. Geological Survey Climate Research & Development Program (USGS) partnered with the U.S. National Park Service (NPS), Colorado State University (CSU), and the University of Colorado (CU) at Rocky Mountain NP to study watershed-scale ecosystem processes, including nutrient cycling and water quality, and their responses to climate variability and atmospheric deposition.

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The focal point of this cooperative research has been the Loch Vale watershed, located in Rocky Mountain NP (Figure 3). Loch Vale watershed is a high-elevation basin containing streams and four lakes within a narrow glaciated valley. Loch Vale drains 661 hectares of alpine and subalpine terrain, which are typical ecosystems in the Rocky Mountains.  Only 6% of the watershed is forested, and the dominant tree species are Engelmann spruce and subalpine fir (Figure 4).  Although the forests in Loch Vale have experienced natural disturbances including fire and avalanche, they have never been logged or settled. Researchers chose to focus on this small, pristine watershed because atmospheric and hydrologic inputs and outputs of nitrogen compounds and nutrients could be quantified.

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Using data on precipitation chemistry from the National Atmospheric Deposition Program (nadp.sws.uiuc.edu) and aquatic chemistry and stream flow data collected from tributaries and the outlet of Loch Vale, researchers developed a nitrogen budget for the watershed. Data collections extend as far back as 1982, providing insights on the variation in nitrogen concentration throughout the watershed and over time. Scientists also measured isotopic and chemical tracers of oxygen and nitrogen content from various water sources, including snowpack, snowmelt, rain, and surface and ground water to determine the origin of nitrogen exported by the streams (Figure 4).  Results showed that atmospheric deposition was the primary source of nitrate to the watershed. 

The research also shed light on how nitrogen from atmospheric deposition is cycled through ecosystems. Climate extremes, lake primary productivity, and microbial activity all play a role in the retention and release of nitrogen in the system. Additionally, USGS, CSU, and CU researchers documented changes in the species composition and concentration of algal primary producers (diatoms) in high elevation lakes in Loch Vale. Since AD circa 1950-1960, diatom species indicative of nitrogen enrichment increased significantly. Plant and soil communities in high-elevation Rocky Mountain ecosystems were also found to be affected by slight increases in nitrogen availability. Increased nitrogen deposition corresponded to changes in tundra plant community composition, increased soil microbial activity, and elevated nitrogen concentrations in the needles of old-growth spruce forests in the region. Together, these studies gave Rocky Mountain NP managers a large body of evidence on sources of nitrogen deposition to the park, the ecological effects of nitrogen on susceptible ecosystems, and pathways of nitrogen cycling.

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One of the most influential outcomes of this research program was its determination of critical nitrogen loads to the catchment. A critical nitrogen load is the amount of nitrogen necessary to cause harmful effects to sensitive organisms and the ecosystem.  Using these USGS-generated estimates on critical nitrogen loads as a foundation, the National Park Service, the Colorado Department of Public Health and Environment and the U.S. Environmental Protection Agency (EPA) worked collaboratively to develop and implement a plan that would focus on reducing nitrogen emissions and deposition in the park.  The Nitrogen Deposition Reduction Plan for Rocky Mountain National Park, released in 2007, described emissions, transport, sources, and trends of atmospheric deposition of nitrogen to Rocky Mountain NP. The plan also outlined a 25-year management strategy to reduce emissions so that deposition would fall below critical nitrogen loads. The plan targets reduced loads and achievable goals by 2032, with milestones for nitrogen reduction set every five years.  Local farmers, ranchers, dairy owners, the state of Colorado, the EPA, and the NPS have all come together to discuss best management practices outlined in the plan and determine strategies for voluntary nitrogen emissions reductions in the region. 

The issues described here are not exclusive to Rocky Mountain NP; increased atmospheric deposition of pollutants affects ecosystems at other national parks, especially in the eastern United States.  Data from these collaborations have implications for ongoing efforts to protect parks and natural areas from air pollution across the country. Other national parks and resource managers are watching to see what strategies can be developed, what goals are achieved, and what can be learned from these efforts. 

The unique aspect of this working group is the alliance among stakeholders. Rocky Mountain NP is one of the first parks in the country where such a large group of private business owners and government researchers have come together to reduce nutrient loads voluntarily for the health of the ecosystem, cleaner water, and improved air quality in the region.

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