Six years ago, Lake Rebecca was so polluted with algae that fish died and the public swimming area had to be curtained off to keep the slime at bay. Now the 256-acre lake in western Hennepin County has made a remarkable comeback, thanks to several actions and an infusion of money.

"Absolutely, it's a success story and an example for other lakes," said Jerry Wise, commissioner of the Pioneer-Lake Sarah Watershed Commission.

Lake Rebecca, the geographical heart of a popular Three Rivers Park District reserve, is like dozens of others in the metro area and statewide that contain too much phosphorus, which comes from crop fertilizer and manure runoff, leaking septic systems, and urban pollution such as lawn fertilizers.

Too many nutrients washing into the water will supercharge the growth of algae and other plants, including invasives such as curly-leaf pondweed and Eurasian watermilfoil. The stew of vegetation causes some lakes to look like pea soup by midsummer, with slimy, stinky water that's no fun for boaters, swimmers and anglers.

In Lake Rebecca's case, decaying algae robbed the water of oxygen and killed more than 30 muskies and six bass in 2007. By 2008, pollution officials listed it as "impaired," with excessive phosphorus and poor water clarity.

But in only a few years, the amount of phosphorus has been cut in half, and Lake Rebecca is meeting water quality standards for the first time in more than 20 years.

"There's been a huge difference," said John Barten, natural resources director for Three Rivers Park District.

Visitors have noticed, Barten said. Massive algae blooms no longer muck up the lake's surface in July and August, and there's no need for a curtained-off swimming beach. Water that was murky by midsummer now remains clear at least 6 feet down, he said.

A horse farm cleans up its act

What led to the turnaround was a combination of changes, including one unusual source: the stomping grounds of the Zuhrah Shriners horse patrol.

The organization's local unit owns 93 acres just over a hill from Lake Rebecca. The complex includes offices, horse stables, an indoor arena, paddocks and pasture land.

Larry Messerli, board chairman of Horsemen Inc., the Zuhrah Shriners local unit, said state officials told the farm 10 years ago that its horses were a big part of Lake Rebecca's problems. "We didn't believe them, but we asked what we could do," Messerli said.

Over the next few years, the Shriners did plenty. They fenced off a stream that runs through the property to keep horses out of it and planted a buffer strip of grass on both sides to filter runoff.

They built a concrete manure-stacking slab so barn waste stored over winter could be trucked off-site in spring by farmers eager for crop fertilizer. They built a system to capture clean rainwater and snowmelt from a neighbor's land and divert it through pipes, preventing it from running across a paddock and washing manure into a stream that feeds the lake. And the Shriners built a couple of storage ponds to intercept any other runoff that might pollute the lake.

Messerli showed off the changes recently at the farm as Arabians, American quarter horses and other breeds, some sporting pinto coats, roamed the hilly land. About 66 horses are boarded at the farm, he said, including 24 owned by members, 30 by nonmembers, and a dozen owned by the Minneapolis Police Department.

Standing next to the manure pad, Messerli smiled. "You've got to be a good neighbor, and everybody seems happy with what we did," he said.

Lessons for other lakes

Barten said the Shriners were a huge help, but much more needed to be done.

"There's no one silver bullet to improve the water quality of a lake," he said. "It takes a concerted effort to address all the sources of phosphorus."

Three Rivers crews have treated the lake with an herbicide each spring since 2009 to kill invasive curly-leaf pondweed. They also spread liquid aluminum sulfate across the lake in fall 2010 and spring 2011, thanks to a $450,000 grant from the state's Legacy Outdoor Heritage Fund. The compound mixes with phosphorus, creating heavy particles that drop to the bottom of the lake and remain inactive there. And Three Rivers also reduced erosion into the lake by stabilizing part of its shoreline.

Barten said Lake Rebecca has several advantages that made the turnaround happen faster than might be possible at other lakes. Because it has no lakeshore development, it has no lawns, streets, houses and driveways to contribute runoff. And because large motors aren't allowed, bottom sediment loaded with phosphorus does not get churned up and re-suspended in the water.

But other sick lakes can also reverse directions if there's enough time, money and commitment, Barten said.

At Lake Rebecca, the Park District did not install a protective curtain at the swimming beach in 2011 and 2012, the first time that's happened in more than a quarter century. Earlier this month, crews were spreading clean sand and sprucing up the area for this summer.

"In the past, we'd basically pull in algae-free water and blow out surface water just to maintain a decent swimming area, and even that was marginal," Barten said.

"Now we don't need the beach curtain any more. The water quality is really excellent."