Most of the Bay health index indicators improved in 2015, with improvements in water quality contributing to a resurgence in aquatic grasses, an important habitat that provides a home for key species such as blue crab, bay anchovy and striped bass.

Sources of pollution to the Bay include agricultural manure and fertilizer; vehicle, industry and utility emissions; municipal, industrial and septic waste water; storm water; and chemical fertilizers applied to lawns, golf courses and other developed lands.

“Total nitrogen levels continued to improve throughout the Bay, corresponding to reductions in nitrogen entering the Bay being measured at monitoring stations. Dissolved oxygen—the amount of oxygen available in the water for the Bay’s creatures to survive—remained steady, while the benthic index of biotic integrity—what’s happening to life on the bottom of the Bay—showed improvement,” a press release for the study reported.

Chesapeake Bay Program executive director Nick DiPasquale told the Capital Gazette that the report shows progress toward the goal of increasing populations and development.

“You have to keep in mind we’ve had over two centuries of environmental assaults on this ecosystem,” he said. “I think it’s fantasy to think we can turn it around in two or three decades.”

The one indicator that worsened was total phosphorus, which had a lower score from the previous year. Excess phosphorus can lead to algae blooms, which reduce the amount of oxygen in the water available for Bay organisms.

“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.”