Aided by favorable weather, the Chesapeake Bay’s ecological health made modest but widespread gains last year, according to a new assessment.

The Bay’s condition earned a middling “C” grade for the fourth straight year in the 2015 report card issued Tuesday by the University of Maryland Center for Environmental Science. But the overall score given for water quality, habitat and fish abundance ticked upward to 53 percent of what scientists consider a healthy ecosystem -- the third highest rating since restoration efforts began in earnest.

“That’s really good news,” said William C. Dennison, UMCES vice president for science applications, who oversees the annual assessment. “We’re laying the groundwork. Some good things may be coming.”

Only twice before has the Chesapeake’s overall health been rated as high or higher — in 1992 and 2002. Both of those were drought years, when a severe lack of rainfall meant less nutrient and sediment pollution washed into the Bay to feed algae blooms and the annual “dead zone” of oxygen-starved water. Though not a drought last year, precipitation and stream flow were below normal, so there was less runoff than usual.

The Bay also benefited from a relatively mild summer, which tends to help underwater grasses and improve levels of dissolved oxygen in the water that fish and crabs need to breathe. Heat waves in 2005 and 2010 caused a dieback in aquatic vegetation, particularly eel grass. Even though 2015 registered as the hottest year on record for air temperatures, the UMCES report noted, water temperatures were “relatively mild.”

“This is one of those cases where if you get the right conditions … and the right [conservation] activities, you can have both good corn crops and crabs,” Dennison said. “I’m liking that scenario, because it isn’t pitting the farmer against the waterman.”

Indeed, the report card tallied gains in key fish populations. The Bay’s stock of striped bass or rockfish remained relatively abundant, while blue crabs and bay anchovies (small forage fish) both improved.

Water clarity, which has been a chronic problem in recent years, increased dramatically last year, and algae concentrations improved as well, the report card noted. Those likely aided the expansion of Bay grasses, which depend on sunlight penetrating the water for their growth. Scientists recently reported that “submerged aquatic vegetation” last year reached its greatest extent in more than 30 years, about halfway to its restoration goal.

Levels of nitrogen in the water declined, the report noted, meaning there was less of that plant nutrient to feed algae blooms and the oxygen-starved “dead zone” that forms every spring in the Bay. Concentrations of dissolved oxygen earned a “very good” rating and remained stable.

The one trouble spot in the otherwise glowing report was an increase in phosphorus levels, the other problem nutrient.  Water-quality monitoring shows that phosphorus concentrations have come down  from where they were 30 years ago, but have leveled off and even increased in places in the last decade.

“We should, like the nitrogen, be seeing reduced phosphorus, and we’re not,” Dennison said. “It is a bit of an enigma.”

Phosphorus levels could be on the rise because of runoff from phosphorus-saturated farm fields on Maryland’s Eastern Shore, the UMCES scientist said. Or, he suggested, the increase may stem from the Conowingo Dam losing its ability to trap phosphorus-laden sediment that’s being carried down the Susquehanna River from Pennsylvania and New York.  Both issues are under study.

Conditions in the lower mainstem of the Bay in Virginia rated a “B,” while the Mid  and Upper Chesapeake earned “C” grades, as did most of the tributaries. Conditions improved in most of the rivers, even the Baltimore area’s Patapsco and Back rivers, which improved from failing scores in previous years to a D-minus. The lower Western Shore and Patuxent River in Maryland garnered ‘D’ grades, while Virginia’s York and Elizabeth rivers upped their ratings to D-plus.

Dennison said that he’s cautiously optimistic that the latest report card shows the Bay is building resilience to the vagaries of weather that can cause water quality conditions to yo-yo from year to year.

“We’ve got these nutrient reductions starting to kick in,” he said, referring to the cutbacks in nitrogen, phosphorus and sediment discharges and runoff mandated as part of the Chesapeake Bay Total Maximum Daily Load. There have been “massive” plantings of cover crops to prevent the runoff of nitrogen from farm fields in winter, he noted, and there’s less nitrogen raining down on the Bay watershed because of improved air emission controls on power plants and motor vehicles.

“I’m not a total Pollyanna about this, but I do think there’s a couple factors working in our favor,” he said.

Even so, the UMCES scientist cautioned that the Bay’s recovery is still relatively fragile, and could suffer setbacks from tropical storms or other extreme weather.

“When we have had particularly bad years … you can backslide a long way, and it takes a long time to creep up again,” Dennison said. “The gains are incremental and difficult; the backsliding, unfortunately, can be pretty dramatic. So, we’re not out of the woods. But we are on the right track.”

Kim Coble, vice president of the Chesapeake Bay Foundation, said the improved score in the UMCES report card indicates progress is being made in reducing Bay pollution, but said it's not good enough.

“The region is not on track to meet its long-term goals," Coble said, "and therefore, the Bay jurisdictions, with (the Environmental Protection Agency's) leadership need to do significantly more if we are to realize a restored Bay by 2025, as the states and EPA committed to achieving.”