Wastewater treatment and industrial plants would be asked to make costly upgrades to cut pollution while farmers would do so voluntarily under a long-awaited strategy released Monday to reduce harmful nutrients in Iowa waterways and the Gulf of Mexico.

Gov. Terry Branstad's administration released the 200-page document laying out plans to tackle one of Iowa's most pressing environmental issues, after two years of study and private meetings.

It comes in response to the U.S. Environmental Protection Agency's order in 2008 for 12 states along the Mississippi River to develop strategies to prevent nitrates and phosphorous from reaching the Gulf of Mexico, where excess levels of the substances make large areas unsuitable for marine and plant life.

Iowa is a leading contributor to the problem, which comes from runoff caused by fertilizers and manure used on farm fields and at wastewater and industrial plants.

For the first time, the 130 largest such plants would be required to take additional steps to remove phosphorous and nitrogen.

The Iowa Department of Natural Resources would require those plants over time to remove those nutrients as wastewater gets treated, which could cost taxpayers and businesses $1.5 billion over 20 years but reduce the amount of nitrogen discharged annually by more than 11,000 tons.

The state's 90,000 farmers, meanwhile, would be encouraged to reduce runoff through best land management practices and public and private conservation programs. The plans will start to be implemented across Iowa after a 45-day public comment period that ends Jan. 4.

"This strategy provides the most up-to-date scientific information available to farmers as they seek to use the best practices available to reduce nutrient delivery from their farm," Iowa Agriculture Secretary Bill Northey said in a statement. "This is not about rules or regulations. Instead, this strategy provides resources to farmers to help them improve water quality."

Environmentalists reacted leerily, saying they believed they were too friendly to the state's powerful agricultural industry, the leading source of the pollution. They accused the Republican administration of catering to the Iowa Farm Bureau, which has advocated for voluntary approaches to cut pollution.

"We need a strategy that has more teeth in it," said Democratic state Sen. Joe Bolkcom of Iowa City, who said voluntary approaches have not led to enough progress cleaning up Iowa's lakes and streams.

Ralph Rosenberg, director of the Iowa Environmental Council, said the strategy was developed in secret and appeared to include input from some interest groups and not others. He called on Branstad to extend the public comment period.

The biggest impact of the plan appears to fall on the 102 largest city-owned wastewater treatment plants and 28 large private facilities such as slaughterhouses and grain processing plants that discharge the most nutrients into Iowa's waterways.

As they seek to renew their operating permits from the DNR in the coming years, the plants will be required to to evaluate how many nutrients they discharge and find ways they could reduce them. The DNR will eventually require the plants to take steps such as installing new technology and changing their operations to achieve reductions.

Department of Natural Resources spokesman Kevin Baskins said the agency's staffers had concerns about an earlier version of the draft, but they were addressed in the final draft. He said the strategy marked the first time that all sources of runoff were addressed in a comprehensive way.

"The reason it took two years was to make sure that we could confidently go out to those affected and say, 'This is the science behind it that we're confident will make a difference,'" he said.