Between 20 April and 15 July, BP released some 4.9 million barrels of oil into the Gulf of Mexico. Within weeks of the leak being plugged, researchers reported on the oil's rapid disappearance. Others are now challenging those early claims.
So is the oil gone or not?
At the surface, the oil does appear to be almost gone. But the big question is whether oil droplets are still around below the surface, and if so how long they will linger. Researchers are divided on this.
For months, the government and BP burned and skimmed oil off the surface. What's more, hot temperatures boosted evaporation and microbial communities that consume surface oil. Estimating what's going on further down the water column and in sediments along the sea floor – is much more challenging.
Of particular interest is the fate of enormous plumes of oil droplets that were seen near the broken wellhead when it was still gushing oil. Richard Camilli and colleagues at the Woods Hole Oceanographic Institute in Massachusetts show in a study out this week that at one point the plume was 2 kilometres wide and 200 metres high. But their measurements were made from 19 to 22 June, before the leak was plugged early this month (Science, DOI: 10.1126/science.1195223).
In another study published this week, Robert Hallberg of the National Oceanographic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, used models to estimate how long it would take the Gulf's prevailing currents and oil-eating microbes to disperse and degrade the oil. He found that oil near the surface can abate within weeks, whereas oil trapped in the colder waters below about 1100 metres can take up to two months to disappear (Geophysical Research Letters, DOI: 10.1029/2010gl044689, in press).
Contrary to other reports, Camilli also found evidence that oil-munching bacteria were only slowly working through the suspended oil. Together, his and Hallberg's studies suggest that oil will probably remain deep in the water column for at least another month.
But Terry Hazen, a microbial ecologist at Lawrence Berkeley National Laboratory in California, says that he has studied the same plume as the Woods Hole group. His results, which have yet to be published, show that microbes are rapidly eating up the plumes – so much so, he says, that the oil should already have vanished. Hazen is adamant: "The plume is no longer there. It's gone."
Why are the results so different?
For starters, different groups are measuring different things, all of them toxic. Oil is an assortment of hydrocarbons, and microbes consume each component differently. The Woods Hole group is looking at the degradation of monochromatic hydrocarbons known as BTEX, which stands for benzene, toluene, ethylbenzene and xylenes. Hazen, on the other hand, is studying long-chain hydrocarbons such as alkanes.
But the discrepancy still puzzles Steven Lohrenz, an oceanographer at the University of Southern Mississippi, Stennis Space centre campus. He is surprised by the Woods Hole group's findings. "I wouldn't expect [the BTEX]to persist for a very long time in seawater," he says.
The difference in the rates at which the researchers believe microbes are breaking down the oil is another point of difference. Of the three, Hazen is the only one to have measured what microbes in the Gulf are actually doing. What's more, other microbial biologists, including Gary King of Louisiana State University in Baton Rouge and Jay Grimes of the University of Southern Mississippi in Ocean Springs agree with his numbers.
Didn't a federal report also find most of the oil has gone?
Yes. The federal government's National Incident Command (NIC) looked at all of the oil that had been released since 20 April. It factored out the oil that had been captured directly at the wellhead, oil that had been burned or skimmed, oil that had evaporated, oil that had been dispersed (both naturally and by chemical dispersants) and oil that microbes had broken down. Combined, that added up to 74 per cent of all the oil that escaped the well. In other words, they say, only 26 per cent of what NIC calls "residual" oil remains in a form that we should be worried about.
But earlier this week researchers at the University of Georgia and the Georgia Sea Grant challenged that interpretation. Almost 80 per cent of the oil has not been recovered, they say. They took particular issue with the NIC's dismissal of dispersed oil hidden below the surface. "One major misconception is that oil that has dissolved into water is gone and, therefore, harmless," says Charles Hopkinson at the University of Georgia in Athens, director of Georgia Sea Grant.
At stake here is tha toxicity of dissolved oil in water. According to Hallberg, the Environmental Protection Agency claims that a billion droplets of water contaminated with a droplet of oil is safe to drink. So if, as the NIC suggests, the oil is reaching that point of dilution in the Gulf, we're in the clear. Not so fast, others retort. Even if we can handle some oil in our water, deep-sea animals may not be able to. Unfortunately, it's too early to know how these organisms are faring.
Are researchers still looking for deep oil?
Yes. Just because some researchers can't find plumes doesn't mean that we should stop looking, says David Valentine, a microbiologist at the University of California, Santa Barbara. "It's easy to miss a plume," he says. "There's no way you can go out there and know where this thing is going to be."
Several teams have plans to travel to the Gulf and look for oil trapped in deeper waters. A group headed by Joseph Montoya, a biologist at the Georgia Institute of Technology in Atlanta, is scheduled to begin an extended deep-sea cruise within a week.
Other groups are looking for regions of abnormally low oxygen, which would indicated the oil has been broken down: whenever microbes consume oil, they also deplete the oxygen around them. NOAA observations are already showing extensive regions of depleted oxygen, says Lohrenz.
If these teams find that underwater oil is gone, do we still have to worry about it?
Yes. It's likely that the oil spill has changed the fabric of the ecosystem: generations of fish may already have been wiped out, erosion in marshes may accelerate and the microbial population of the Gulf is already changing. For instance, should large swaths of deeper waters become low in oxygen, microbes capable of living in such hypoxic conditions thrive while others die.