DENVER -- Airborne nitrogen pollution from vehicle exhaust and farm fertilizer is turning algae in the alpine lakes of Rocky Mountain National Park into junk food for fish, a study says.

A similar phenomenon is occurring in Sweden and Norway, according to the study of about 90 high-elevation lakes set to be published in the journal Science on Friday.

Arizona State University professor James Elser, the study's lead author, said the effect of airborne nitrogen on once-pristine lakes is greater than previously believed. The nitrogen's sources include vehicle exhaust, fertilizer used on farms and livestock feed lots and power plant emissions.

More nitrogen can reduce long-term lake biodiversity because algae become poor food for other microscopic organisms and, ultimately, fish. The algae are high in nitrogen, but low in phosphorous and less nutritious.

Previous studies have documented rising nitrogen levels in Rocky Mountain National Park, 70 miles northwest of Denver.

Elser likened the algae to junk food. "It's like eating marshmallows all day and expecting to grow. You can't do it," he said Thursday.

The fish in the park include the rare greenback cutthroat trout and other trout species.

The next step is to study how changes in algae populations are affecting the rest of the ecosystem, Elser said.

"This is filling in some of what we didn't know before," said Jill Baron, a scientist with the U.S. Geological Survey and adjunct professor at the Natural Resource Ecology Laboratory at Colorado State University in Fort Collins.

Baron has studied air pollution levels in Rocky Mountain National Park since 1981. Her research prompted Vaughn Baker, the park's superintendent, to push for efforts to cut the pollution.

Along with changes to the algae, park biologists blame nitrogen for an increase in sedges, compared to other grasses and flowering plants known as forbs.

"One of the main reasons this park was established was for the protection of tundra and alpine areas," park biologist Jim Cheatham said.

Rocky Mountain National Park has 60 peaks higher than 12,000 feet. It is home to elk, deer, moose, bighorn sheep, mountain lions, bears and eagles.

This year, the National Park Service reported increases in nitrogen-rich ammonium that could change ecosystems in 16 parks across the country.

A 2007 plan signed by state and federal officials and Rocky Mountain National Park aims to reduce airborne nitrogen levels over 25 years. Researchers said then that nitrogen levels were 20 times more than normal.

"We're admitting that we could never reach the natural levels" because of development elsewhere, Cheatham said.

The plan aims to cut nitrogen levels in half, said Mike Silverstein, manager of planning and policy for the Colorado air pollution control division. The nitrogen comes from nitrogen oxide, whose sources include vehicle and power plant emissions, and ammonium, whose sources include livestock feed lots, farms and water treatment plants.

Studies show the pollution is coming from the Denver area and northeast Colorado, one of the country's largest agricultural areas, as well as other states, Silverstein said.

Officials hope to cut nitrogen to 1.5 kilograms per hectare, or 2.47 acres. The current average is 3.1 kilograms.

Silverstein said state and federal efforts have reduced pollution from power plants and vehicles and plans are in the works to cut emissions by another 100,000 tons of nitrogen oxide per year in Colorado over the next 10 to 15 years.

Ammonium isn't regulated, Silverstein said. But state officials, Colorado State researchers and the agriculture industry are exploring ways to change farming practices to cut emissions.