WASHINGTON — Government scientists studying the BP disaster are reporting the best possible outcome: Microbes are consuming the oil in the gulf without depleting the oxygen in the water and creating "dead zones" where fish cannot survive.
Outside scientists said this so far vindicates the difficult and much-debated decision by BP and the government to use massive amounts of chemical dispersants deep underwater to break up the oil before it reached the surface.
Oxygen levels in some places where the BP oil spilled are down by 20 percent, but that is not nearly low enough to create dead zones, according to the 95-page report released Tuesday.
Dissolved oxygen levels would have to drop by an additional 70 percent to create dead zones, and that's unlikely, said Steve Murawski, the National Oceanic and Atmospheric Administration's chief scientist for fisheries and the head of the group of federal scientists who analyzed data and produced the report.
In an unusual move, BP released 771,000 gallons of chemical dispersant about a mile deep, right at the spewing wellhead instead of on the surface, to break down the oil into tiny droplets.
The idea was to make it easier for oil-eating microbes to do their job. But the risk was that the microbes would use up the oxygen in the water. So BP had to perform a delicate balancing act.
"Has it hit the sweet spot? Yes. Was it by design? Partly," said Murawski.
The lower oxygen levels were detected within 60 miles of BP's blown-out wellhead at water depths of 3,300 to 4,300 feet — a layer that's relatively rich in oxygen. The natural mixing of water from surrounding areas with the depleted layer is helping counter the decline, Murawski said.
The Gulf of Mexico already has a yearly major problem with a natural dead zone — this year, it is the size of Massachusetts — because of farm runoff coming down the Mississippi River. Fertilizer in the runoff stimulates the runaway growth of algae.
Federal officials had been tracking oxygen levels and use of dispersants since the spill, which spewed more than 200 million gallons of oil into the gulf between April and July. Had the oxygen plummeted near dangerous levels, the dispersant use would have been stopped, said Greg Wilson, science adviser at the Environmental Protection Agency's emergency management office.