Published online 19 August 2010 | Nature | doi:10.1038/news.2010.420

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Extensive chemical analysis confirms that undegraded oil remains at ocean depths.

Oil from the Deepwater Horizon spill persists underwater. CHRISTOPHER BERKEY/ EPA/Photoshot

The swathe of oil still stretching from the Deepwater Horizon spill is over 35-kilometres long, according to a new report. The study, published in Science1, is the first major peer-reviewed analysis of the underwater oil plume. It also indicates that the plume has persisted for several months, with oxygen measurements showing little sign of the oil being degraded quickly by microbes in the water.

A team led by researchers at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts found that the main plume is suspended at a depth of 1,000–1,200 metres below the surface and in some places is more than 2 kilometres wide and 200 metres thick. Other oil plumes are present in the Gulf of Mexico, but this was the first to be identified and is the most thoroughly sampled.

"Up to this point, people had identified hydrocarbons in subsurface waters, but they weren't able to say just how wide the plume was, how tall it was, or how long it was, or that it was continuous," says lead author Richard Camilli, an oceanographer at the WHOI.

Beginning at the site of the blown-out oil pipe, Camilli and his colleagues studied the plume's properties by zigzagging an autonomous underwater vehicle (AUV) through the plume. They suspect that the plume was longer than 35 kilometres, but their measurements were stopped short in late June by the approach of Hurricane Alex. But between 19 June and 28 June, the team took more than 3,500 real-time measurements of hydrocarbon concentrations and tracked the presence of 10 chemicals in the water column by using a mass-spectrometer attached to the AUV. They made another 2,300 chemical measurements while sampling oxygen concentrations in the water using a device lowered down on a cable.

The team has a "technological capability that is second to none on this planet," says John Kessler, a chemical oceanographer at Texas A&M University in College Station. "They can basically swim the AUV like a fish through this plume, measuring all the different oil and gas hydrocarbons and do a much more efficient job of mapping the area of this plume than anyone else can."

Slow degradation

In contrast to the findings of other groups, the new report concludes that ocean oxygen levels have not dipped substantially in the region of the plume since the spill began and that little of the oil had degraded in the regions they sampled. Oil- and gas-consuming microbes are naturally present in the ocean, and consume oxygen as they break down hydrocarbons, making oxygen levels a good indicator of how quickly oil is being degraded.

Click for a larger version of this image. Science/AAAS

"Our findings suggest that the microbial rate of degradation was relatively slow," says Benjamin Van Mooy, an oceanographer at the WHOI and a co-author of the study. "What that would mean is it would take quite a long time before we got to the low oxygen levels that might be harmful to animals." But that would also mean that the plume will not disappear any time soon.

In some cases, the group found lower-than-normal oxygen levels, but they attributed this to flawed instrumentation. Oxygen sensors can become fouled by oil; when the team used a backup method known as Winkler titration, which allows oxygen concentrations to be measured precisely, they could not reproduce the low oxygen values in some cases.

But Kessler's group, which sampled the same plume as the WHOI team earlier in June, reported oxygen depletions of up to 30%, indicating that biodegradation was occurring quickly. Another team found that oxygen concentrations were 30–50% lower than normal in some places along the plume's path. And both of these teams reproduced their own findings by using independent methods (see 'Muddying the waters on Gulf oxygen data').

Both situations could be occurring within the plume, says Kessler. Camilli's team "could be sampling in areas that just haven't witnessed those sorts of oxygen reductions, whereas we have sampled in areas that had a slightly different hydrocarbon soup mixture".

Oil buffet

David Valentine, a geomicrobiologist at the University of California, Santa Barbara, who was on the same vessel as Kessler, says that microbial respiration rates are likely to vary with location and maturity of the plume. Valentine compares this to a buffet: "You put out something people actually like, such as a filet mignon, and that's going to go quickly. But your stale nachos at your taco bar, they're still going to be there."

But Valentine disagrees with the conclusion that the respiration rates reported in the study indicate slow metabolism. "That's a very fast rate for something happening at those kinds of depths," says Valentine.

The initial reports of oil and gases were lingering deep in the water rather than rising to the surface (see 'Oil cruise finds deep-sea plume') came as a shock to many scientists. Oil on the surface is relatively easy to clean up, whereas hydrocarbons that lurk deep underwater could expose marine organisms to their toxic effects for much longer.

Among the chemicals that Camilli's team measured were benzene, toluene, ethylbenzene and xylenes, collectively called BTEX. These light-weight volatile organic compounds (VOCs) are typically the first molecules to evaporate from oil at the ocean surface. It is unclear how long they might linger in the water column. "VOCs are considered to be acutely toxic but short-lived in most oil spill situations," says Judy McDowell, a physiological ecologist at the WHOI. "In this instance, for BTEX to persist at depth would suggest that acute toxic effects would occur."

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Another group of researchers, from the University of South Florida in St Petersburg, has preliminary evidence that phytoplankton and bacteria are being exposed to toxic concentrations of chemicals northeast of the ruptured oil pipe.

But researchers cannot say exactly which ingredients remain in the toxic soup of the plume or how they might affect ocean creatures. "It's very hard right now to get any handle as to the total inventory in the plume," says Christopher Reddy, a co-author of the study who is also based at the WHOI. Oil has thousands of components which behave differently from each other in water. Some of the breakdown products of oil and gases might have risen to the surface, whereas others are still trapped in the plume, Reddy says. "We'll probably know in a month or so when we get a much better, fuller data set."

The WHOI researchers note that their sampling was a "snapshot" in time, and that the plume has probably changed since late June. In fact, researchers are not currently sure of the whereabouts of the plume, according to Valentine.

"I think that we're going to see that many of the hydrocarbons in these oil plumes are going to be spinning around the Gulf, probably staying around the same depth range for some time to come, certainly months, if not years," says Valentine.