Extent of Lingering Gulf Oil Plume Revealed

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

Amanda Mascarelli
19 August 2010 |
Nature | doi:10.1038/news.2010.420
http://www.nature.com/news/2010/100819/full/news.2010.420.html

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.

"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."

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.

References
Camilli, R. et al. Science advance online publication
doi:10.1126/science.1195223 (2010).

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