A nonprofit that tracks pollution in America's largest estuary said Wednesday that the health of the Chesapeake Bay is improving, but huge challenges remain as manure and storm water continue to flow into the watershed.

The Chesapeake Bay Foundation released a report analyzing efforts to improve water quality. Last year marked a halfway point toward the implementation of a federally mandated "pollution diet" for the bay by 2025.

"Unless the states and their federal partners expand their playbooks and push harder, the bay and its rivers and streams may never be saved," foundation president William C. Baker warned in a statement.

The EPA has required states to cut phosphorous, nitrogen and sediment pollution, which has come from sewage treatment plants and runoff from farms and cities.

The good news in Wednesday's report is that the bay's notorious oxygen dead zones are shrinking. Underwater grasses and oysters are rebounding. The water is clearer.

But the foundation warned that some states, particularly Pennsylvania, are not doing enough to stop manure from leaking off farms.

The foundation also took aim at President Donald Trump's administration for rolling back regulations on power plant emissions, which Baker said pollute the bay.

Baker also warned that the Chesapeake Bay Foundation would sue against any efforts to push back the clean-up timeline. And he said the federal Environmental Protection Agency should hold states more accountable if they fail to meet their goals.

"The blueprint is working," Baker said during a conference call with reporters. "The scientific data show that clearly."

The foundation analyzed EPA data for each state in the watershed, which runs from New York to Virginia.

Sewage treatment plants are responsible for the biggest reductions in pollution. They're filtering more contaminants out of treated wastewater that's released into rivers.

Overall, the foundation said states are reducing enough phosphorous and sediment pollution. But they're behind in curbing nitrogen.

The foundation said Pennsylvania in particular is having trouble reducing agricultural runoff. The Susquehanna River flows from Pennsylvania into the bay in Maryland. The report also said Maryland and Virginia could do more to limit runoff from farms and cities.

Solutions would include better storm water management systems, the foundation said. Farmers could also do a better job of storing and spreading manure and protecting streams from livestock.

Deborah Klenotic, a spokeswoman for Pennsylvania's Department of Environmental Protection, said the state "faces an enormous challenge to reduce runoff" on 33,000 farms in the bay's watershed.

But she said major efforts are underway. They include increased farm inspections and finding more money to help farmers improve manure management practices. The state is also using data to pinpoint the counties where efforts will be most impactful.

Jay Apperson, a Maryland Department of Environment spokesman, said the state is well-positioned to meet its goals. The office of Virginia Gov. Ralph Northam said in a statement that his administration is fully committed to meeting the 2025 deadline.

David Sternberg, an EPA spokesman, said the EPA will be releasing its own midpoint assessment on efforts to clean up the bay in the coming weeks.

The states in the bay's watershed are Delaware, Maryland, New York, Pennsylvania, Virginia and West Virginia as well as the District of Columbia.