‘Dead zone’ cycle called persistent
Healing low-oxygen aquatic dead zones in the Chesapeake Bay and hundreds of other spots worldwide will be trickier than previously imagined, according to leading scientists on the issue.
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, Gulf of Mexico and other areas with dead zones. But these efforts have not yielded the expected results, scientists said Sunday at the annual meeting of the American Assn. 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 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 oxygen levels dipped even lower than would have been expected, given the amount of nutrients in the water. This 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, 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 are the cause. Less rainfall means less nutrient runoff from agricultural fields, overburdened sewage treatment systems and sidewalks.
This would logically mean a reduced dead zone. Perhaps more importantly, sea grasses which were once endemic to the Chesapeake but then disappeared have re-established themselves. Sea grasses pull nutrients out of 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 significant decline of oysters in the Chesapeake greatly exacerbated the dead zone, since these organisms both remove nutrients from the water. Low oxygen also kills the “nitrifying” bacteria that combine ammonia with nitrogen. The resulting nitrate, when mixed with other “denitrifying bacteria,” becomes nitrogen gas that bubbles to the surface and moves into the atmosphere.
Though Kemp attributes the recent improvements to dry weather the past few years, he hopes the gains can be maintained 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 including 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 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.
