DENVER - With high levels of nitrogen that scientists fear could lead to fish die-offs and other permanent environmental changes, the superintendent of Rocky Mountain National Park is recommending cutting air pollution levels in half at what is often called the "crown jewel" of Colorado.

Environmentalists hailed the recommendation by park superintendent Vaughn Baker as "a truly historic step." Baker said levels of nitrogen detected at the park are 15 to 20 times the amount found in nature.

"Two decades of scientific research have shown that the world-class alpine lakes, forests and Colorado's renowned greenback cutthroat trout are hard hit by rising air pollution at Rocky Mountain National Park," said Dan Grossman, director of the regional office of Environmental Defense.

Vickie Patton, staff attorney for Environmental Defense, said Baker's recommendation is based on more than 20 years of research and is the first time a national park in this country has used that kind of science-driven management tool. She said "critical loads" for pollutants, the threshold above where adverse effects occur, have been established in Canada and Europe.

"The time for meaningful policy action is here if we're going to protect Colorado's crown jewel for future generations," Patton said.

The Colorado Department of Public Health and Environment is charged with enforcing federal clean-air rules in the state. The agency has signed an agreement with the National Park Service and the Environmental Protection Agency to cut the pollution in the park that attracts nearly 3 million visitors each year.

"We've all agreed by the end of June that we will come out together to establish a goal," said Mike Silverstein, manager of planning and policy for the state air pollution control division.

Silverstein said the parties will then explore how to reach the goal, possibly establishing benchmarks to the ultimate target. Officials were scheduled to meet Thursday.

In a May 9 letter to state health officials, Baker said research shows the critical load for Rocky Mountain National park is 1.5 kilograms of nitrogen per roughly 2.5 acres. A monitoring station that has collected data the last 24 years in a central point of the park shows levels twice that amount.

Park Service biologist Karl Cordova said research also shows significant changes in the park's ecosystem since the 1950s. Certain species of algae have become more dominant, which has upset the balance and eventually could lead to damage similar to that caused by acid rain.

Baker, citing research by U.S. Geological Survey scientist Jill Baron, said higher-than-normal levels of nitrogen have been found in the park's water, trees and soil. He said nitrogen levels have increased by about 2.5 percent yearly the past two decades.

The sources of pollution include fertilizer used on farms and vehicle and power plant emissions. Failure to reduce the pollution, which also contributes to haze hovering over the park, could lead to such effects as "episodic and chronic acidification and the associated lethal effects" on fish and forests, Baker said in his letter to the state health department.

In 2004, Environmental Defense and Colorado Trout Unlimited petitioned the federal government, seeking a declaration that air pollution is endangering Rocky Mountain National Park. They invoked provisions of the federal Clean Air Act and the law that created the National Park Service, which requires the agency to protect park resources.

Rocky Mountain National Park, about 70 miles northwest of Denver, has 60 peaks higher than 12,000 feet, and is home to elk, deer, moose, bighorn sheep, mountain lions, bears and eagles.