The fish-smothering "dead zone" now forming in the middle of the Chesapeake Bay is likely to be one of the smallest in the past 25 summers, scientists predicted Tuesday, a brighter outlook they credited to favorable weather as well as to long-running efforts to clean up the estuary.
Researchers with the University of Maryland Center for Environmental Science forecast that this summer will produce the fifth-smallest stretch of water in the bay's depths deprived of the oxygen that fish, crabs and oysters need to breathe.
Whether that means the bay is on the road to recovery depends on which scientist you ask.
"We're cautiously optimistic," said William C. Dennison, the UM center's vice president, who oversees the annual forecasts of the bay's dead zone. "If we can just get through this summer with a good year environmentally, we'll do our fisheries a world of good."
But Denise Breitburg, a senior scientist with the Smithsonian Environmental Research Center, cautions that shrinking the dead zone is no panacea for the bay's woes, particularly for restoring the Chesapeake's historic bounty of oysters, crabs and finfish.
"Just solving the water-quality problem alone is not going to get us back to the production of fish we saw earlier," she said in a briefing for reporters aboard a Smithsonian research vessel in the Rhode River. "What worries me is if we promise benefits we can't see, we may lose public support" for cleanup efforts.
Dissolved oxygen levels on the bay bottom dip to zero in June from the Bay Bridge south to the Potomac River, forcing fish and crabs into a thinner and thinner layer of water just below the surface as the zone expands through summer. Shrinking that dead zone and raising oxygen levels to where fish can breathe more easily has been a central goal of the multi-state Chesapeake Bay restoration effort for decades, so annual fluctuations are closely watched by scientists, government officials and environmentalists.
Oxygen levels get depressed because the water has become overenriched with nutrients from sewage, from fertilizer and animal manure washing off farmland and lawns, and from vehicle and power plant emissions that rain down from the sky. Those nutrients — the same nitrogen and phosphorus used to fertilize gardens — feed massive growths of algae in the water, which consume the available oxygen as they die and decay on the bottom.
But there haven't been massive algae blooms so far this year, said Dennison, despite heavy snow and rain early in the year. That precipitation washed above-normal amounts of nitrogen and phosphorus off Pennsylvania farmland into the Susquehanna River, but the pulses of pollution hit before algae started growing. Rains tapered off in late spring and favorable prevailing winds have helped circulate the bay's waters, limiting the extent of the oxygen declines.
"We haven't had major plankton blooms out there," Dennison said. "The water clarity remained pretty good late into the spring. So we're not getting those conditions that sort of set the scene for the bad oxygen [in the water]."
The dead-zone outlook could change if weather conditions shift, the UM scientist cautioned. An updated forecast for late summer will be issued later.
"Doing everything we can to improve water quality is very important," said Breitburg, who likened the creation of the dead zone in the bay to bulldozing a national park and converting it into a poorly maintained farm.
But dead zone in the middle of the bay is not the only place where fish and crabs suffer from the effects of nutrient pollution, she said.
Research has shown that oxygen levels can drop precipitously in shallower waters on the bay's edges, or in rivers and coves. Breitburg's Shady Side lab south of Annapolis is studying the effects on oysters of nightly drops in oxygen levels seen in some impaired waters. She said preliminary findings suggest the short-term fluctuations may make the shellfish more susceptible to the diseases that have devastated bay oysters since the late 1980s.
But while watermen and environmentalists frequently blame the bay's poor water quality for meager harvests of fish, crabs and oysters, Breitburg said studies here and abroad have found no firm relationship between commercial seafood landings and the levels of pollution that can cause dead zones.
Fish in the Chesapeake and elsewhere are affected by how much water habitat they have in which to feed, reproduce and hide from predators, she said. They're also heavily influenced by fishing pressure, both commercial and recreational.
So improving water quality without addressing such issues may yield disappointing results, she warned.
"One of the things I worry about is overpromising what we'll gain if water quality improves,'' she said.
Dennison acknowledged that eliminating the dead zone by itself will not restore the bay to its prior vitality.
"Yes, there are other important things" affecting the bay's fish communities, he said, such as the hardening of shorelines by waterfront homeowners, the loss of most of the bay's oysters and the complex interactions among species.
"But the dead zone is a good indicator, a good barometer of what's going on," he said. "If the dead zone went away, the Chesapeake Bay would clearly be in better position for restoration, so we're going to track it."
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