SHAKOPEE — Oxygen levels are up in the Minnesota River, a key indicator that one of the state’s dirtiest waterways is getting healthier and that efforts to reduce pollution from wastewater treatment plants are working, the Minnesota Pollution Control Agency announced Monday.

MPCA staff monitored a 20-mile stretch of the lower river for three weeks in August to see if the hot, dry summer and low water flows would deplete dissolved oxygen levels. Such depletion had been a problem in previous droughts, but scientists were encouraged to find that oxygen levels remained high enough to support fish, bugs and other aquatic life despite the stressful conditions.

MPCA and Metropolitan Council officials credited the success to major public and private investments to reduce phosphorous discharges all along the river, which starts at Minnesota’s western border and cuts across the state to join with the Mississippi River just downstream from Minneapolis.

“It’s about a quarter-billion dollars collectively when you look at all the investment from all the municipal and industrial facilities,” Katrina Kessler, manager of the MPCA’s water assessment and environmental information section, said in an interview.

Phosphorous promotes the growth of algae, which depletes oxygen from the water when it dies.

The officials announced the findings at a news conference at the Blue Lake wastewater treatment plant in Shakopee, one of several in the Minnesota River basin that have undergone upgrades as part a 2004 phosphorous reduction plan.

“Everything that we do here not only benefits our local environment, our local economy, the fish and aquatic life here in the river here in the Minnesota, it has a positive impact on the Mississippi, on Lake Pepin, and all the way down to the gulf of Mexico,” said Glenn Skuta, the MPCA’s water monitoring supervisor.

Skuta said low oxygen levels when flows were low on both the Minnesota and Mississippi Rivers in the 1980s caused fish kills where the Mississippi widens into Lake Pepin. All the way downstream, the “dead zone” in the Gulf of Mexico is blamed in part on pollution from the Minnesota River, he noted.

“What it shows to us is a lot of the efforts that we initially designed are paying off, and the work of many small communities and wastewater systems across the Minnesota River watershed have added up to something pretty significant in terms of water quality improvement in the river,” MPCA Commissioner John Linc Stine told The Associated Press.

Stine said they’ve met phosphorous reduction goals ahead of the agency’s 2015 target date, and that’s good for the entire food chain in the river.

“We’re talking about all the critters, the bugs, the minnows, the fish — everything that relies upon dissolved oxygen to live,” Stine said.

When those creatures thrive, so do ducks, egrets, herons and even eagles that live along the river, he said.

Stine cautioned that the river is still struggling with other serious contaminants, primarily sediments from runoff and eroding river banks that keep it muddy. And other pollutants such as nitrogen from farm runoff still need to be cut, he said. Bacteria levels also are still too high.

Groups working to clean up the river applauded the findings.

“Twenty years of working to get people engaged and understand the worth that this river has to the state of Minnesota has paid off,” said Scott Sparlin, executive director of the Coalition for a Clean Minnesota River.

“Often we in the Minnesota River basin don’t share our success stories. We’re so busy trying to clean it up we don’t share the good news. … This is one of the areas where we can crow about our success,” said Lori Nelson, executive director of Friends of the Minnesota Valley.

The Minnesota has been treated as “an open sewer, a drainage ditch and a dumping ground for a hundred years,” said Patrick Moore, executive director of Clean Up the River Environment. People have stopped treating the river like a sewer and dump, he said, and now the state needs to help farmers not treat it like a drainage ditch.

“This step forward is one good step forward but it’s not the last step forward,” Stine said. “It’s going to take persistent efforts to address those contaminants and achieve water quality status that is no longer impaired.”