ANNAPOLIS — (CNN) Conditions may be “poor to moderate” in Chesapeake Bay but scientists still found a reason to celebrate.

America’s largest estuary got a C (53%) on its health report card for 2015. That’s its highest score in a non-drought year since 1992 and it represents the third year of consecutive growth, according to researchers at the University of Maryland Center for Environmental Science.

It’s also one of the three highest scores since 1986. Only 1992 and 2002 scored as high or higher, but those were years of sustained droughts, meaning there was little runoff water to wash pollutants into the bay.

The continued improvement in the absence of a major drought suggests efforts to reduce pollution are paying off, said Bill Dennison, vice president for science applications at the center.

The annual survey of bay conditions compares seven indicators to scientific thresholds. Those indicators are combined into an overall health index represented as a percentage toward a broad set of ecosystem goals.

Water clarity increased from 2014 to 2015 along with prevalence of aquatic grasses, an important habitat for key species including blue crab and striped bass. Meanwhile, nitrogen and phosphorous pollution fell, researchers said.

Blue crab, bay anchovy and striped bass populations also grew from year to year, though they are not included in the score.

Grades varied throughout the region. Ecosystems in the Lower Bay showed B-level improvement while conditions in the Patapsco and Back rivers in and around Baltimore drew Ds and a D-, the survey found.

Chesapeake Bay Foundation Vice President Kim Coble welcomed the signs of progress but said the score is only “part of the story.” There’s still a long way to go to meet designated cleanup goals set by the Environmental Protection Agency to reduce the volume of contaminants and sediment flowing into the bay by 2025.

“The region is not on track to meet its long-term goals, and therefore, the bay jurisdictions, with EPA’s leadership, need to do significantly more if we are to realize a restored bay by 2025.”

The University of Maryland researchers see the results in a sunnier light.

Dennison attributed the progress to three factors: Cleaner air, cleaner water and cover crop programs that protect against wind and water erosion.

Thanks to the Clean Air Act, the atmospheric fallout of nitrogen that has been occurring for decades is starting to measurably diminish, he said.

For one, the act required catalytic converters on cars, eliminating nitrous oxide pollution from cars, Dennison said. At the same time, the act introduced scrubbers that clean the gases passing through power plant smokestacks, reducing the formation of acid rain.

“As a result we’ve reduced the amount of nitrogen that falls out in rainwater,” he said.

Meanwhile, dedicated funding in Virginia and Maryland to upgrading sewage treatment plants is paying off, he said.

“We’re reaping the benefits of improved water quality from sewage treatment plants.”

Finally, in recent years an incentive program for farmers has encouraged the planting of cover crops after the summer harvest to alleviate runoff into the bay. Cover crops recycle unused plant nutrients remaining in the soil from the previous summer crop and protect fields against wind and water erosion.

“We think it’s a combination of these three factors that’s starting to turn the tide,” Dennison said.

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