Scientists Warn of Persistent 'Dead Zones' in Bay, Elsewhere
Tuesday, February 17, 2009
CHICAGO -- Healing low-oxygen aquatic "dead zones" in the Chesapeake Bay and hundreds of other spots worldwide will be trickier than previously imagined, leading scientists on the issue said Sunday.
That's because the low oxygen levels that make it impossible for most organisms to survive also kill bacteria crucial to removing nitrogen from the water.
Dead zones are caused primarily by excess nutrients -- nitrogen and phosphorus -- that feed massive algae blooms. Those, in turn, soak up most of the water's oxygen and leave little for other life forms -- a condition known as hypoxia. In recent years there have been extensive efforts to reduce nitrogen and phosphorus loads in the Chesapeake Bay, the Gulf of Mexico and other areas with dead zones. But those efforts have not yielded the expected results, scientists said at the annual meeting of the American Association for the Advancement of Science.
"We've been working for 20 years to breathe life into these dead zones, but we've found it much harder than we thought," said Donald F. Boesch, president of the University of Maryland Center for Environmental Science. "Even when the nutrient loads are reduced, the hypoxia is generally not recovering with the rapidity we assumed it might."
In the Chesapeake Bay, what scientists call a "regime shift" happened in the early 1980s, when bottom-water oxygen levels dipped even lower than would have been expected, given the amount of nutrients in the water. That trend continued for two decades.
"Managers advocating spending money to restore the system have been losing faith because it seems like nothing is happening," said W. Michael Kemp, a professor at the Maryland center.
There have been hopeful signs of late. Since 2006, Kemp said, it appears that the Chesapeake has swung back to its pre-1980s pattern, with a greater oxygen content, relative to nutrient load. He said scientists have no proof of what is causing the new trend, but he theorizes that a few years of relatively little rainfall may have played a key role. Less rainfall means less nutrient runoff from agricultural fields, overburdened sewage treatment systems and sidewalks in the bay watershed.
That would logically tend to cause a reduced dead zone. Perhaps more important, sea grasses that were once endemic to the Chesapeake but then virtually disappeared have reestablished themselves. Sea grasses pull nutrients from the water and sequester them in the sediment.
"We have cautious optimism that the system is returning to its old regime," Kemp said. "We don't know exactly what's going on here, but it is giving us renewed hope for restoration."
Kemp said the previous disappearance of sea grasses and the significant decline of oysters in the Chesapeake greatly exacerbated the dead zone, since both organisms remove nutrients from the water. Low oxygen also kills the nitrifying bacteria that combine ammonia with oxygen. The resulting nitrate fuels denitrifying bacteria, which transform the nitrate into nitrogen gas. When nitrogen is transformed into this gaseous form, it becomes essentially unavailable to support the excessive growth of algae.
Though Kemp attributes the recent improvements to dry weather the past few years, he hopes the gains can be maintained even if rainfall increases.
"When conditions are bad, they get worse, but when conditions start getting better, there is a reinforcing effect and they get even better," he said. "Now that the sea grasses are there again, they have an ability to modify their natural environment" -- through removing nutrients and keeping the water clear -- "so that it will be hard to get rid of them."
Multiple dead zones line the U.S. Atlantic coast. In the Gulf of Mexico, fertilizer runoff from Midwestern agriculture into the Mississippi River is responsible for a dead zone the size of New Jersey. In Lake Erie, a dead zone is putting a $1 billion fishing industry at risk.
There are also significant dead zones off countries such as India, Japan, Australia, Brazil and Mexico. In the Baltic Sea, a dead zone the size of Denmark has been so persistent that scientists are considering major engineering solutions to inject oxygen directly into the water. The Swedish government has granted $2.5 million and promised $40 million for such experiments, and a private group called Baltic Sea 2020 has donated $60 million.
The Baltic Sea has had periodic dead zones over the course of its 8,000-year life, scientists have determined from geologic records, including one thought to have been caused by Viking agricultural practices.
Now the sea suffers massive nutrient contamination from agriculture and livestock operations, cruise ships dumping waste, and urban sewage runoff.
Daniel Conley, a professor at Sweden's Lund University, noted that it would take up to 60,000 rail cars of liquid oxygen annually to directly re-oxygenate the sea. Lime or aluminum could also be dumped to provoke chemical reactions that would reduce nutrients, but that would be expensive and could have unintended ecological effects.
"There's no silver bullet," Conley said. "And all these things are just temporary solutions. We still need to greatly reduce nutrient runoff."