Scientists Warn of Persistent 'Dead Zones' in Bay, Elsewhere

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.

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