ANNAPOLIS, MD — Less pollution runoff into Chesapeake Bay in 2015 earned the country's largest estuary a grade of C for its overall health, an improvement over recent years, scientists said.

The overall health of Chesapeake Bay improved last year, according to scientists at the University of Maryland Center for Environmental Science. The C grade in 2015 is one of the three highest scores since 1986. Only 1992 and 2002 scored as high or higher, both years of major sustained droughts.

"The high score for 2015 indicates that we're making progress reducing what's coming off the land," said Bill Dennison, vice president for Science Applications at the University of Maryland Center for Environmental Science.

Cleaner air, cleaner water and cover crop programs that protect against wind and water erosion are the keys to the steps forward. The Clean Air Act has cut nitrogen in the air by pushing the installation of smokestack scrubbers at power plants and catalytic converters on cars.

Money spent by the states of Maryland and Virginia to upgrade sewage treatment plants is also curbing pollutants, he told CNN.

»What do you think of the pace and efforts to clean up Chesapeake Bay? Tell us in comments below.

The overall score for the Chesapeake Bay Health Index for 2015 was 53 percent, compared with 50 percent in 2014 and 45 percent in 2013. Conditions improve in many regions throughout the Bay, including the Choptank River, Upper Eastern Shore, Lower Western Shore, and the Rappahannock River. There were no regions that had lower scores in 2015 compared to 2014.

The Lower Western Shore, which includes Anne Arundel waters like the Magothy, Severn, South, Rhode and West rivers, scored a D, reports The Capital-Gazette. The area has "poor ecosystem health," the report says, although there were significant improvements in chlorophyll a and ocean floor communities.

Factors in the Bay's overall health include several years of moderate weather, sewage treatment upgrades, use of winter cover crops by farmers, and reductions in atmospheric nitrogen deposition, the study says.

"We know why the Bay became degraded and what we need to do to restore it. This report card shows what's possible when we take action," said Donald Boesch, president of the University of Maryland Center for Environmental Science. "The positive results give us confidence that even greater improvements will be realized if pollutant loads are further reduced as committed."

While the scorecard shows progress, the bay is not on track to meet long-term goals to meet cleanup goals set by the Environmental Protection Agency to reduce the volume of contaminants and sediment flowing into the bay by 2025, said Chesapeake Bay Foundation Vice President Kim Coble.

"The bay jurisdictions, with EPA's leadership, need to do significantly more if we are to realize a restored bay by 2025," Coble told CNN.

Most of the indicators comprising the Chesapeake Bay health index improved in 2015. Chlorphyll a—a measure of phytoplankton concentration, which can lead to reduced water clarity and reduced dissolved oxygen levels — had improving scores throughout the Bay. Improvements in water quality contributed to the resurgence in aquatic grasses, one of the most important habitats in the Chesapeake Bay, providing a home for key species such as blue crab and striped bass.

Total nitrogen levels continued to improve throughout the Bay, while dissolved oxygen—the amount of oxygen available in the water for the Bay's creatures to survive—remained steady. The index of life on the Bay floor also showed improvement.

"We will make even more progress if we continue to innovate and partner with all sectors within the watershed, from agriculture to energy and transportation, and from the Conowingo Dam to the streams, wetlands, and aquifers that help define our national treasure," said Maryland Department of the Environment Secretary Ben Grumbles in a news release.

The amount of phosphorus in the water was down; a good sign since it can lead to algae blooms, which reduce the amount of oxygen in the water available for Bay organisms.

Important fish species in the Bay - striped bass, bay anchovy and blue crab -- earned moderately good scores. Striped bass held steady with a 98% score while both blue crab (47%) and bay anchovy (73%) improved.

"We should take the opportunity to celebrate these results, but we should also recognize that the long term success of our work to restore water quality and the health of this vitally important ecosystem will depend on stepping up and sustaining our efforts over the long-term to reduce nutrient and sediment pollution discharges to streams and rivers throughout the watershed," said Nick DiPasquale, director of the Chesapeake Bay Program Office.

For more information about the 2015 Chesapeake Bay Report Card including region-specific data and downloadable graphics, visit chesapeakebay.ecoreportcard.org.