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CATHY WURZER: All this month, MPR News has been taking a close look at Minnesota's threatened water quality. The green scum that grows on contaminated lakes in the summer is when the most recognizable signs of the state's water problems, and cleaning up those lakes is a big job. Mark Steil reports.

MARK STEIL: Every time rain falls on the city of Worthington, it means trouble for the community's most important natural resource.

[THUNDER RUMBLING]

The city of 13,000 people in Southwest Minnesota surrounds Okabena Lake. The lake is a national resource. With low banks and abundant Prairie Winds blowing across the water, Okabena provides great sailing, and it's hosted three national windsurfing competitions. But the lake is also heavily polluted. When it rains, soil particles and fertilizer wash into the lake from the nearby city and from the thousands of acres of farmland that surround the community. Dan Livdahl, head of the local watershed district, says the impact on the lake is immediate.

DAN LIVDAHL: We can have the sediment coming into the lake, turning the lake a brown color and not have it clear up for the rest of the season after one big rain event. Our water clarity will go from pretty good to terrible.

MARK STEIL: It's an outcome Livdahl has seen many times. Right now, he's in a boat floating on Okabena Lake about 100 feet from shore homes in Parkland lined the banks of this mile and a half-long lake. The average depth of the brown, green water is about 7 feet. Livdahl is doing a quick check of the lake's health. He leans over the side of the boat and plops an 8-inch black and white disk into the water.

DAN LIVDAHL: This instrument is called a Secchi disk, and it is used all around the world as a way to measure water clarity.

MARK STEIL: The disk descends about two feet before it disappears into the murk. For Okabena, 2 feet is not bad. But in a clear Minnesota lake, you might be able to see the disk or even the lake bottom at 10, 15, or even 20 feet. Jason Vogt is President of the Lake Improvement Association. Sitting on his Lakeshore dock, he says water clarity gets worse in late summer. That's when months of warm temperatures can create algal blooms that produce what's been described as an overwhelming sewage like smell.

JASON VOGT: When the wind is in the wrong direction depending upon where you're standing, you will certainly know it's there. It'll stop walkers and runners around the lake because it gets really, really bad. The problems in Okabena afflict many of Minnesota's 4,000 or so shallow lakes. But they weren't always the cloudy algae-ridden bodies of water many have become.

Detailed analysis of core samples from the bottoms of several state lakes show that 150 years ago or so, they were fairly clear. Changes began after farmers started draining soggy prairie land to plant crops in the late 1800s, and urban areas began developing. The resulting runoff carried more nutrients from fertilizer and manure into lakes. University of Saint Thomas Biologist Kyle Zimmer says, the extra phosphorus and nitrogen in the runoff starts a cycle of decline.

KYLE ZIMMER: The more of those two nutrients in a lake the faster the algae can grow, the more the clear water state is destabilized.

MARK STEIL: In a Clearwater state, a lake has plentiful vegetation growing on the bottom. As those plants grow, they can sop-up the extra nutrients pouring in, but only up to a point.

KYLE ZIMMER: If nutrient levels get so high that the growth rates of submerged aquatic plants can't keep up, that's when the system gets basically overloaded, and all the nutrients start shifting over to algae up in the water column, and you flip over to a turbid water state.

MARK STEIL: At that stage, the unabsorbed fertilizer and manure feed, the algae, which flourish, the more they grow, the more they cloud the water fatally cutting off sunlight to the bottom vegetation, snuffing it out. That leaves even more nutrients to fuel algae growth. Eventually, they can grow so thick that dense clusters form mats on a lake surface, releasing the sewage stench. Algal blooms can even become toxic and have killed dogs and sickened children.

Fish populations in high nutrient waters change too from clear water species to rough fish like carp that thrive in poor quality water. These bottom feeders disturb the lake releasing nutrients trapped in sediment, feeding algae even more. The stirred-up sediment also adds to the clouding. Okabena lake in Worthington has seen all of this happen. Despite disagreements over the best course of action, there is a consensus the lake needs help.

BILL GORDON: Okabena is a great attribute to Worthington. It's one of our attractions. It's great for recreation.

MARK STEIL: Bill Gordon in his family farm within a couple of miles of Okabena Lake. They and other farmers upstream of the lake have contributed to its problems. The Minnesota Pollution Control Agency estimates drainage from rural areas accounts for about 2/3 of the phosphorus entering Okabena. But Gordon and his family have done more than most to reduce nutrient runoff. They've sacrificed economically productive land to protect Okabena Lake.

Gordon says they've converted roughly 10% of their 2000 acres to grass and wetlands, which remove soil and nutrients from runoff before it enters the lake. Gordon estimates the change has reduced his farm revenue by as much as $60,000 in some years. The family converted another 35 acres into water holding areas.

The system of ponds allows pollution shin to settle out of the water before it's released through the spillway to the lake. Despite the sacrifices of Gordon and other farmers in the area, more are needed. The MPCA estimates farmers need to cut the pollution they send to Okabena Lake by another 2/3. But even Bill Gordon is balking at that. He says farmers couldn't give up enough cropland or fertilizer to meet that goal and also survive financially.

BILL GORDON: We want healthy safe water, that's the most important thing, but use common sense, cost effective answers. We can get this done.

MARK STEIL: But for now, getting it done is far from a certainty. State water managers have demonstrated they can restore a lake from an algae [? clotted ?] milk-chocolate color back to clear water. But oftentimes, that restoration brings new problems. Bill Doyscher walks to the end of his dock on Lake Shaokatan, about 150 miles southwest of the Twin Cities. He likes what he sees.

BILL DOYSCHER: I think it's been a lot better shape than he was when we first came out here. The water quality is better.

MARK STEIL: Doyscher says 25 years ago, the lake surface was often covered with algal blooms in the summer. Residents, though, banded together to improve water quality.

BILL DOYSCHER: The lake had the inherent capability to clean itself up, if we could help it and try and control some of the things that were going on that maybe were bringing nutrients to the lake.

MARK STEIL: A concerted effort among state and federal agencies and area farmers sharply reduced nutrients flowing into the lake. Livestock feedlots took steps to keep manure from reaching the water residents fixed leaky septic systems. Property owners restored runoff cleansing wetlands. The lake cleared. Aquatic plants returned to the lake's bottom. But then, the vegetation exploded because the water still had too much phosphorus and other nutrients. Last year, large clumps of vegetation massed on the lake's surface.

BILL DOYSCHER: By the end of the summer, it was pretty well covered. Travel one end of the lake to the other, and sometimes it didn't work.

MARK STEIL: Okabena Lake has a long way to go to reach even that stage. But Worthington resident Terry Morrison says, decisions affecting the lake's future will test the region's moral and ethical backbone.

TERRY MORRISON: Water to me really means life, and I believe that it represents the overall health of our existence. And without water, we simply die.

MARK STEIL: Saving Okabena Lake will take a lot of work. There have been small improvements, but no breakthroughs. Meanwhile, the alarm keeps sounding.

[THUNDER RUMBLING]

Mark Steil, Minnesota Public Radio News, Worthington.