The Chesapeake Bay's ecological health has actually dipped slightly in recent years, even though pollution levels improved in 2013, according to federal scientists.
The amounts of nutrient and sediment pollution flowing into the bay fell in 2013 below the long-term average for the past 25 years, the U.S. Geological Survey reported Friday.
But most of the bay and the tidal portions of its rivers remain seriously impaired, the agency said, meeting just 29 percent of water-quality standards from 2011 to 2013. That's down 2 percent from the previous three years.
"We're fluctuating on kind of a flat path for the moment," said Peter Tango, who oversees bay region water-quality monitoring for the survey.
Maryland and the other five bay states have been striving to meet a pollution "diet" imposed four years ago by the Environmental Protection Agency. While progress has been made in upgrading sewage treatment plants and taking steps to reduce polluted runoff, changing weather makes it hard to detect water quality trends from year to year.
Excess nutrients and sediment are among the leading causes of the bay's poor health. Nitrogen and phosphorus from sewage, farm and suburban runoff and air pollution can fuel the growth of algae blooms and formation of fish-stressing "dead zones" in the bay.
According to Scott Phillips, the survey's bay coordinator, pollution levels dropped in 2013 because there was less rainfall than normal to wash nutrients and sediment off the land into the bay's rivers. But he suggested cleanup efforts by Maryland and other bay region states also contributed to the reduction.
The bay's lack of response, though, likely reflects the lingering effects of near-record pollution flushed into the bay two years earlier, Tango and Phillilps said. That year, 2011, was marked by unusually heavy spring rains and two late-summer tropical storms.
Despite year-to-year variations, water quality has improved modestly overall in the bay and in most of its rivers since cleanup efforts by the states and federal government began in the mid-1980s, Phillips noted.
"We do see a long-term signal that some of the efforts to reduce nitrogen and sediment are having a positive impact," he said.
Tango also pointed out there have been localized improvements in some rivers just downstream of sewage plants once they're upgraded, including Baltimore's Back River.
But in the past decade, levels of pollution that were declining have either leveled off or actually begun to go back up in some rivers, the scientists note.
They say water quality may be slow to improve in part because nitrogen from past overapplications of fertilizer on farm fields has soaked down into ground water, where it is taking years and even decades to ooze out. Phosphorus pollution is also worsening in places, they noted.
Scientists have laid that trend in part to runoff from farm fields that have been repeatedly overdosed with the bay-fouling nutrient by farmers' practice of using animal manure as a crop fertilizer.
In its final months, the O'Malley administration acted to impose limits on how much phosphorus farmers could put on their fields in Maryland. The regulations were opposed as too costly by Eastern Shore farmers, many of whom rely on chicken manure as fertilizer.
Gov. Larry Hogan blocked the rules this week upon taking office, saying he wanted further review and public input. His move drew criticism from environmentalists, who say the science shows limits are warranted.