Gov. Mark Dayton became a clean water convert when he read a report last year showing that half the lakes and streams in southern Minnesota are too polluted for safe swimming or fishing.

"I wasn't aware of the severity of the problem," Dayton said in an interview at the governor's residence this week. "It really stunned me."

Dayton has been preparing for a water summit that is expected to draw hundreds of interested Minnesotans to St. Paul on Saturday for a discussion on how to tackle the state's water problems.

But even as Dayton has pledged to take action, political realities have threatened to overwhelm his hoped-for clean water legacy. His water agenda almost by necessity puts him at odds with a powerful and cherished Minnesota interest group: farmers.

Cleaning up Minnesota waterways cannot be accomplished without solving agricultural pollution that comes from the fertilizers and pesticides that — no matter how carefully applied — still wind up contaminating the state's vast system of interconnected waterways.

At the Legislature, the influence of agriculture is far greater than the sheer number of farms, which is now just 75,000. Even after the Legislature passed a Dayton measure aimed at securing buffers around waterways to protect them from farm pollutants, agricultural interests pushed back hard and got Dayton to cave on a key regulatory piece of the new law.

Dayton — whose grandfather owned a farm where the young man came to appreciate rural byways — likes to emphasize the farmer part of the Democratic Farmer Labor Party, often appearing at the State Fair and Farmfest, touting Minnesota's $19 billion farm economy and bucolic way of life.

The governor must carefully navigate the issue in an election year when Democrats believe they have a good chance to win control of the House, which now has a Republican majority. To do that, they will need to win a few seats in battleground rural areas after suffering a drubbing outstate in 2014.

On the other side, environmental groups hope Dayton stands strong on water.

Steve Morse, executive director of the Minnesota Environmental Partnership, said he wished Saturday's summit would focus more on "the elephant in the room" — agricultural water pollution.

"They are bending over backward not to call out ag," Morse said, noting that 72 percent of the nitrogen pollution in the state's waters comes from farming practices, according to the Minnesota Pollution Control Agency.

Taxpayers spent nearly $125 million last year to clean up Minnesota lakes, streams and groundwater contaminated by farming, according to a Star Tribune analysis of state and federal budget data published last year.

Dayton has swung back and fourth on the issue of farm pollution in the last year.

Last August at the annual meeting of the Minnesota Environmental Partnership, Dayton was blunt: "I refuse to believe we have to accept this kind of contamination because it's farm country. We don't accept it in mining country. We don't accept it in the metropolitan area," he said.

"We are not just going to turn our backs and say we are going to provide free rein to people even if they are doing really important work. If that makes me an enemy of agriculture, I regret that, but there is too much at stake here," Dayton said.

Since then, he has softened his tone.

In the interview this week, he emphasized the importance of agriculture: "Agribusiness is the mainstay of our economy. The agricultural economy is vitally important to the state of Minnesota and especially Greater Minnesota," he said.

He said that many farmers are taking responsibility for water quality. Their best practices merely need to be embraced more widely, he said.

When he was asked about ethanol — which is responsible for about one-third of corn production and faces growing skepticism from environmentalists and economists as an inefficient alternative to fossil fuels — Dayton defended it, approvingly citing a report from the U.S. Department of Agriculture on ethanol's increased efficiency.

He dismissed the idea of fundamentally rethinking American agriculture and its heavy reliance on corn and soybeans that go to animal feed, saying farmers are reacting to the American consumer's demand for meat.

Whitney Clark, executive director of Friends of the Mississippi River, who praised Dayton for bringing the state together for the water summit, nevertheless said agriculture must change.

"Corn at the scale that we plant it is a disaster for water, wildlife habitat and pollinators," he said. "We need to be farming less corn acres."

Dayton proposed a record $220 million in bonding for clean-water projects earlier this year, three-fourths of which would go to help cities, including many in farm country, clean up polluted water.

He said Republican lawmakers threatened to deny his request if he didn't back down on a key provision of the water-quality bill they passed last year, which had legislators of both parties and agriculture interests fighting Dayton's regulators about which waterways would be protected.

Dayton relented, in a retreat that left environmentalists deeply disappointed but had the DFL-aligned Farmers Union thanking him in a recent letter.

For their part, Republican legislators and farm interests, which had been battling with Dayton for the better part of a year over water issues, emphasized collaboration more than conflict.

"I think there's more work to do on a bipartisan basis," said Rep. Denny McNamara, R-Hastings, who chairs the House Environment and Natural Resources Committee.

Doug Busselman, director of public policy for the Minnesota Farm Bureau, a lobbying group on agriculture issues, said he would be at the water summit, cautiously optimistic there will not be a target on the backs of farmers: "We were told it isn't intended to be an attack on any one sector, but is intended to be a conversation that presents the water issue as something that everyone has a responsibility for," he said.

But Busselman also subtly signaled that actions could have consequences in November of an election year.

"In this particular cycle both the Senate and the House are up for election," he said. "We've been working with rural senators from both parties. I would imagine [Dayton's] interested in it from that standpoint."