Researchers at the Johns Hopkins University and the University of Maryland Center for Environmental Science teased from 60 years' worth of water-quality measurements what they described as one of the first clear signs of progress in the costly 27-year-old campaign to clean up the bay.
The study, published in the current issue of the scientific journal Estuaries and Coasts, appears to explain away recent research finding no real improvement in the "dead zone," where oxygen levels in the bay drop so low each summer that fish and shellfish struggle to survive. The oxygen gets sucked out of the water by the breakdown of massive algae blooms that grow every spring, fed by sewage, farm and urban runoff and air pollution.
In fact, the dead zone that formed early this past summer was the largest ever, scientists say, stretching from above the Bay Bridge to the Potomac River.
Weather plays a big role in how big the dead zone gets, scientists found. A bigger slug of oxygen-starved water forms when heavy spring rains wash more algae-feeding nutrient pollution into the bay from the sprawling six-state watershed.
The size of the dead zone in early summer has actually been getting worse, on average, in recent years, recent research had found. That was puzzling and troubling to scientists, who'd wondered why the zone wasn't shrinking in response to pollution reduction efforts. Water sampling has tracked long-term declines in algae-feeding nitrogen flowing into the bay from the Susquehanna River, the Chesapeake's largest tributary.
"If there's something else controlling [the dead zone] other than nutrients, then maybe this is a waste of time," said Michael Kemp, co-author of the study and an ecologist at UM's Horn Point environmental laboratory near Cambridge. "I didn't believe that. So this was a big surprise, and we set out to explain it."
Earlier research had focused on measurements of the dead zone in early summer, when it first forms. The Hopkins-UM team expanded their analysis, though, to look at the dead zone throughout the summer.
They found that the size of the early-summer dead zone was influenced by changing physical conditions in the bay, possibly long-term shifts in prevailing winds and a slight rise in sea level.
But looking at the dead zone in late summer, the researchers found that since the mid- to late 1980s, it has gotten smaller overall and broken up a little sooner each year.
"On average, the late summer [mid-July through August] dead zone volume in the last decade was about 80% of the volume in the mid-1980s," Rebecca Murphy, a Hopkins doctoral student who is the study's lead author, said in an email. The improvements roughly parallel declines in nitrogen measured in the Susquehanna, she said.
Murphy called those "really encouraging results," indicating that the multibillion-dollar effort to reduce nutrient and sediment pollution is on the right track.
On a scientific level, Kemp, who has been studying the bay for 30-some years, said he was glad the new study resolved doubts about whether the Chesapeake is responding as other water bodies have to reductions in nutrient pollution. "After all this scratching of heads and beating our heads against the wall, we just realized it was right there, had been there all along," he said. Kemp called that twist "embarrassing" but also reassuring.
William C. Baker, president of the Chesapeake Bay Foundation, also took heart from the study. But he contended that "this is a fragile improvement" that could stall or even reverse if Maryland and other bay states let up. Farmers, developers and some local and state officials have complained that the "pollution diet" the Environmental Protection Agency has prescribed for the bay is unwarranted and too costly, and the American Farm Bureau has sued to block EPA from enforcing it.
"We're not out of the woods yet by a long shot," said Baker.