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 feedlots 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.