Gulf oil spill plumes: what is known so far

Scientists are groping in the deep, lightless waters of the Gulf of Mexico, trying to take the full measure of oil that has been detected several thousand feet below the surface.

The undersea clouds are tenuous. Oil concentrations measured so far have been very small – akin to diluting a half-ounce of oil in every 1 million ounces of seawater.

Still, the oil and its companion, methane, represent an unseen but worrisome presence from the Gulf oil spill, now in its ninth week, researchers say.

Some of the creatures most vulnerable to diluted oil are among the ocean's smallest and least able to avoid plumes – various forms of plankton, as well as larvae of fish and corals. Early research suggests that, while microbes are eating the oil, they are also consuming oxygen vital to undersea ecosystems.

Scientists are wary of drawing sweeping conclusions from what so far are limited data. Too much is still unknown about the situation – from the spread of the plumes to the effect of oil and chemical dispersants in deep water. But scientists are gathering as much information as possible on the plumes, knowing that new data are crucial to projecting the plumes' potential effect on the Gulf's undersea ecosystems and the fisheries they sustain.

Which animals are at risk?

To date, scientists say they have uncovered two clouds. One is west of the Deepwater Horizon blowout site that spans some three miles across. The first evidence of the plume appeared at a depth of 3,600 feet, and in places it is up to 1,500 feet thick.

In addition, others have detected evidence of a cloud northeast of the blowout in a kind of undersea layer cake. One cloud appeared at 1,200 feet, another some 1,800 feet deeper.

Even at the low concentrations found so far, the oil could be toxic to many marine creatures, says Marion Nipper, a marine environmental toxicologist at Texas A&M University's Center for Coastal Studies in Corpus Christi.

At the top of the list are bottom-of-the-food-chain life-forms that are literally drifters, moving with the currents they encounter.

Many of these creatures move up and down the water column each day to feed or to escape becoming another animal's meal. Beyond that, they have little or no ability to swim out of an oil cloud they unexpectedly come to inhabit. Even organisms that aren't in the path of an approaching cloud may still have to migrate through it during these daily excursions.

As a result, "the exposure time seems to be very long," Dr. Nipper says, raising the likelihood that creatures caught up in the clouds or that pass through them repeatedly could succumb to the oil.

Nipper is careful not to overexaggerate the potential impact of the plumes.

Given the size of the Gulf, "you will hardly wipe out a whole population" of a given species "because the plumes are still small relative to the size of the Gulf," she says.

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