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.
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 Steve Murawski, the National Oceanic and Atmospheric Administration senior scientist who headed the federal team of researchers.
One reason that oxygen levels didn't drop too low was the natural mixing of water in the Gulf, which kept bringing in oxygen from other areas, Murawski said. Oxygen levels would have had to fall by three-quarters for the water to be classified as a dead zone, he 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, depleting the oxygen in a giant patch of the Gulf every summer.
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.
The use of dispersants has been a source of fierce debate because it involves an environmental trade-off: protecting the shoreline from oil at the risk of causing unknown problems in the deep. While dispersants make it easier for bacteria to degrade the oil, they tend to hide oil below the surface. There have also been concerns about the chemicals' toxicity and the long-term effects on marine life.
In May, the federal government convened about 50 scientists for advice on whether to continue using the dispersants. Though the researchers were divided before the meeting, they unanimously recommended continuing with the chemicals, said University of California Davis oil spill scientist Ron Tjeerdema.
"The best of two options — neither of which were great — was to continue dispersing," Tjeerdema said.
Louisiana State University researcher Ed Overton, who also was part of that meeting, said he feels vindicated. "Right now it looks like an incredibly good idea," he said. "It was a risky but necessary application. Damage was going to be done somewhere."
But Overton said it may be years before scientists know if there is long-term damage from the dispersants.
Last month, after federal officials said much of the oil had dissolved, dispersed or evaporated, outside researchers were skeptical. Two new studies called that into question, finding that invisible underwater plumes of oil remained deep underwater.
But Tuesday's report dovetails with another outside study, published last month, announcing the discovery of a new oil-consuming microbe in the Gulf that was flourishing on BP's spill.
The sagging oxygen levels also lend more weight to the government's claims last month that microbes are consuming oil, because there would be no dip in oxygen if the bacteria weren't feeding on the BP leftovers, Murawski said.
The new work is based on data collected from May through August at 419 locations by nine government and private research ships in the Gulf.
Larry McKinney, director of a Gulf of Mexico research center at Texas A&M University in Corpus Christi, said the new federal data showed that it was a "nearly perfect" outcome.
"They hit it on the head, which is good," said McKinney, who was not involved in the report.