Ohio's attempt at slowing down the toxic algae turning Lake Erie green hinges on a plan that some environmental groups say relies too much on voluntary programs and lacks the sense of urgency needed to thwart the growing threat to the region's drinking water.

The state's strategy outlined recently by Ohio's Environmental Protection Agency is centered on new regulations and programs put in place within the past year.

Now the question is will it be enough to thwart the .

How well those changes—stopping farmers from spreading manure on frozen fields and encouraging them to use the right amount of fertilizer—work won't be known for a couple more years.

The moves are part of the state blueprint for a 40 percent reduction in the amount of that fuels the in the lake's western end. Ohio, along with Michigan and the Canadian province of Ontario, all have pledged to reach that goal within the next 10 years.

"It's possible but it's going to be difficult," said Mike Shriberg, regional executive director of the National Wildlife Federation's Great Lakes office.

Last year's algae bloom was the biggest on record, and another in 2014 left more than 400,000 people in Toledo and southeastern Michigan unable to drink tap water for two days.

Ohio's plan deserves credit, Shriberg said, for including a strong focus on coordination and monitoring efforts to fight the algae. But he doesn't see the sense of urgency that's needed.

"This plan makes the assumption that doing more of the same will get us there," Shriberg said.

More than anything, the state should be requiring and verifying that all farmers are taking steps to reduce field runoff, he said.

Scientists say fertilizers and livestock manure from the region's farms are the main sources of the phosphorus feeding the . Sewage from treatment plants and leaking septic tanks play a role, too.

Farm organizations say they are doing their part by promoting voluntary efforts aimed at cutting runoff and by contributing to ongoing research projects.

A report released earlier this year by the University of Michigan Water Center said not enough farmers in Ohio and Michigan are taking part in the voluntary practices and say sweeping changes are needed to cut enough phosphorus to stop the algae blooms. Agriculture groups criticized the study for focusing only on farms.

Karl Gebhardt, deputy director for water resources at the Ohio EPA, said the state's plan will be evolving over the next decade and if what's being done now isn't working, changes will be made.

He said what's most important is "are we seeing an improvement in water quality?"