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.
Still, she adds, the impact of the clouds on marine life is cause for concern, especially if they hit "some particular area that is the habitat of some endangered species."
Tracking the clouds is not easy. Despite weeks of effort, scientists are still uncertain of the full extent of undersea oil, which often seems to engage in a high-stakes game of peekaboo.
Scientists on one cruise would pick a spot where a previous ship had taken readings strongly suggesting oil was present in deep waters. Then, "67 hours later we'd go there, and there'd be nothing," says Larry Mayer, director of the University of New Hampshire's Center for Coastal and Ocean Mapping, just back from a plume-hunting cruise on the NOAA research ship Thomas Jefferson. "We'd go to a place where we had high readings 30 hours earlier, and there would be nothing."
"That's why we're backing off this idea of a plume," which suggests a continuous layer, he says. Instead, several researchers are thinking of the undersea oil as more cloudlike – patches of dilute oil in tiny particles, much like water or ice droplets in clouds.
Those droplets – many as fine as the mist from a can of hair spray – can form from the forces at play as oil leaves the ruptured well. But the chemical dispersants that BP is applying at the well are also designed to break up large blobs and streamers of oil into small droplets. They, too, are toxic.
Yet too little is known about the effect of dispersants at the depth in question – about 5,000 feet – to say how much of the "atomization" of oil is due to the oil's eruption or to the chemicals BP is applying to it.
Nor is it clear how the chemicals may or may not add to the concentration of potential toxins for marine life at those depths.
How scientists search for plumes
The hunt for answers is a slow, painstaking process, researchers acknowledge.
The hunt begins with scans by sonar "tuned" to detect organic material, explains Vernon Asper, a marine scientist at the University of Southern Mississippi. If the sonar picks up a signal that looks worth investigating, a research ship will begin "mowing the lawn" – traveling back and forth over the suspected plume, dropping and retrieving an assembly of specially designed water bottles, each triggered to gather a sample at different depths.
At the same time, instruments on the assembly – known as a CTD rosette – measure temperature, depth, and the water's electrical conductivity to gauge how salty or fresh the water is. The array also carries a small version of an ultraviolet "black light." It is tuned to bring out the unique fluorescent colors of different kinds of organic material, including oil.
In one area, "what we found was a very strong signal" at about 3,300 feet down to about 4,300 feet, he says. In many of the images coming from the fluorescence detector, the field was filled with tiny orange specs, strongly suggesting oil, along with tiny white specs – sometimes looking like clouds – which the team suspects are methane hydrate crystals that can form when methane accompanies oil billowing from an undersea blowout. Some estimates put the methane content of the blowout at about 40 percent of the oil-methane mix spewing from the wellhead – in contrast to roughly 5 percent methane in a typical well.
The images must be correlated with water samples, which themselves must be analyzed to confirm the presence of oil and to see if the oil's chemical fingerprints match those of samples from the BP blowout.
Day in and day out during a cruise, researchers drop and retrieve this rosette, or "dope on a rope," as Dr. Asper wryly calls it, "as often as we can with the time allotted" for the cruise. A single CTD "cast" can take up to two hours, then the ship moves to its next sampling point.
It's a frustrating pace, Dr. Mayer adds. "We're trying to measure something that varies in space and time over a large area with little dips" of a rosette at a target that's 4,000 feet away, he says.
8,332 animal and plant species in well area
One intriguing find: A cloud sampled by a research team including Asper contained lower concentrations of oxygen than did adjacent waters – in some spots, much lower. This suggests that microbes were feasting on the oil and methane in the water column.
"On the one hand, that's good," he says, because the process is removing oil. "On the other hand, you're removing oxygen from a layer of the ocean that isn't used to having oxygen removed from it."
The water at the depth the team sampled is highly enriched in oxygen to begin with, so researchers don't expect it to approximate the annual oxygen-starved "dead zone" that forms each year where the Mississippi River empties into the Gulf. Moreover, the Gulf region hosts a naturally occurring layer of low-oxygen water at depths of between 300 and 600 feet.
Still, "the question is: What effect is that going to have on the other organisms" living at the depth the team was sampling?, asks Asper.
A newly published catalog of Gulf marine life illustrates just how rich the area around the blowout is – home to 8,332 species of plants and animals, says Thomas Shirley, a marine ecologist with Texas A&M's Harte Research Institute for Gulf of Mexico Studies in Corpus Christi.
The depth range that includes the wellhead is home to more species than anywhere else in the Gulf at that same depth range. "I found that to be very surprising," he says.