DENVER (AP) -- A study says increasing nitrogen pollution is turning algae in Rocky Mountain National Park's alpine lakes into junk food for fish.

Arizona State University professor James Elser, the study's lead author, says the effect of airborne nitrogen on once-pristine lakes in the park is greater than previously believed.  The nitrogen comes from vehicle exhaust, fertilizer used on farms and livestock feed lots.

The study appears Friday in the journal Science. It says more nitrogen can reduce the long-term biodiversity of the lakes because algae become poor food for other microscopic organisms and, ultimately, fish.

Previous studies have documented rising nitrogen levels in the national park 70 miles northwest of Denver.

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