Gulf oil spill plumes: what is known so far

Scientists have found evidence of at least two undersea plumes from the Gulf oil spill. The plumes appear to be diluted, but still a threat to the foundations of the Gulf food chain.

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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."

Peekaboo plumes

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.