A landmark scientific study of deteriorating Lake St. Croix water quality calls for public activism up and down the river to reverse decades of phosphorous contamination.

Fertilizers, animal waste, storm-water runoff and other pollutants threaten recreational pursuits such as fishing on the popular St. Croix River unless all of the people who depend on the river rally to repair it, the first-ever comprehensive report concludes.

"The scientific knowledge base here is almost unequaled," said John Hensel, a scientist at the Minnesota Pollution Control Agency and one of the study's authors.

Explaining to residents along the river how they can act on the study's recommendations will present a challenge because of its technical nature, said several people familiar with it.

"I'd like to see it boiled down to a level that an average person could understand," said Brian Zeller, mayor of Lakeland, a river city.

Lake St. Croix, the portion of the river most popular with boaters, fell onto the impaired waters list in 2008 after monitoring of water quality showed excess phosphorous had created large oxygen-sucking algae blooms. The lake extends from Stillwater south to Prescott, Wis.

The good news is that as watershed districts, local governments and citizen advocacy groups step up the tempo, more people than ever want to help restore the river, Hensel said.

"In my view, this is really where the action is going to happen," he said. "Everyone's more directly motivated in their backyard."

The report, known as a Total Maximum Daily Load study, seeks to reduce phosphorous contamination in Lake St. Croix to 360 metric tons a year -- the same as 1940s standards. Without corrective action, the study concludes, phosphorous contamination could reach 540 metric tons a year by 2020.

Hensel said that "with proper management and planning," the river will heal.

Ignoring the study would bring consequences for the entire St. Croix region, because the economy is built around the river's scenic and recreational value, said Deb Ryun, executive director of the St. Croix River Association.

Left unchecked, the degrading of the river could kill off native fish and disrupt boating and swimming, causing the river to lose its luster as a recreational destination, she said.

"It's a world-class resource that very few other people have. It's a national treasure," Ryun said.

People up and down the river have already reported that the water looks different because excess phosphorous has turned it blue and green in places, she said.

Most of the excess phosphorous in the St. Croix basin comes from agriculture, because that industry covers the largest area, Hensel said. A goal of the study is to provide information to farmers to help reduce fertilizer runoff without hurting their livelihoods, Hensel said.

Ryun acknowledges that farming represents the largest source of phosphorous, with development second, but she said challenges facing the St. Croix are more complex than that.

"It's everybody's problem, and everybody will be part of the solution," she said. "I hope it doesn't come to pointing fingers at one source."

The study, which the MPCA and the Wisconsin Department of Natural Resources completed in December, has gone to the U.S. Environmental Protection Agency for review. Once that's completed this spring, planners will decide how to involve all the people who have a stake in the river.

Hensel acknowledges that the study has its skeptics, who will contest findings and potential costs, although "we're really not taxing people per se," he said.

"I don't think everybody has an interest in wild and scenic rivers," he said. "The heartening thing is that even in this era we live in, a lot of people who move to the basin are motivated by this very beautiful river."

Kevin Giles • 651-735-3342