Earlier in August, top federal officials declared the oil in the spill was mostly "gone," and it is gone in the sense you can't see it. But the chemical ingredients of the oil persist more than a half-mile beneath the surface, researchers found.
The oil is degrading at one-tenth the pace at which it breaks down at the surface. That means "the plumes could stick around for quite a while," said study co-author Ben Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts, which led the research published online in the journal Science.
Monty Graham, a scientist at the Dauphin Island Sea Lab in Alabama who was not involved in the study, said: "We absolutely should be concerned that this material is drifting around for who knows how long. They say months in the (research) paper, but more likely we'll be able to track this stuff for years."
The underwater oil was measured close to BP's blown-out well, which is about 40 miles off the Louisiana coast. The plume started three miles from the well and extended more than 20 miles to the southwest. The oil droplets are odorless and too small to be seen by the human eye. If you swam through the plume, you wouldn't notice it.
The scientists used complex instruments - including a special underwater mass spectrometer - to detect the chemical signature of the oil that spewed from the BP well after it ruptured April 20. The equipment was carried into the deep by submersible devices.
With more than 57,000 of these measurements, the scientists mapped a huge plume in late June when the well was still leaking. The components of oil were detected in a flow that measured more than a mile wide and more than 650 feet from top to bottom.
Federal officials said there are signs that the plume has started to break into smaller ones since the Woods Hole research cruise ended. But scientists said that wouldn't lessen the overall harm from the oil.
The oil is at depths of 3,000 to 4,000 feet, far below the environment of the most popular Gulf fish like red snapper, tuna and mackerel. But it is not harmless. These depths are where small fish and crustaceans live. And one of the biggest migrations on Earth involves small fish that go from deep water to more shallow areas, taking nutrients from the ocean depths up to the large fish and mammals.
Those smaller creatures could be harmed by going through the oil, said Larry McKinney, director of Texas A&M University's Gulf of Mexico research center in Corpus Christi.
Some aspects of that region are so little known that "we might lose species that we don't know now exist," said Graham of the Dauphin Island lab.
"This is a highly sensitive ecosystem," agreed Steve Murawski, chief fisheries scientist for the National Oceanic and Atmospheric Administration. "The animals down at 3,300 to 3,400 feet grow slowly." The oil not only has toxic components but could cause genetic problems even at low concentrations, he said.
For much of the summer, the mere existence of underwater plumes of oil was the subject of a debate that at times pitted outside scientists against federal officials who downplayed the idea of plumes of trapped oil. Now federal officials say as much as 42 million gallons of oil may be lurking below the surface in amounts that are much smaller than the width of a human hair.
The Associated Press contributed to this report.