SPECIAL SERIES | DEEP IMPACT
Deciphering the unseen, underwater effects of the Gulf oil spill.
Nearly five months after the Deepwater Horizon oil rig exploded in the Gulf of Mexico (map), causing the worst oil spill in U.S. waters, BP is set to permanently cap the damaged well as soon as this week.
But the discovery of widespread oil on the seafloor and studies of remnant undersea oil plumes suggests that the debate over the ecological impact and ultimate fate of the Gulf oil spill—which released an estimated 4.9 million barrels of crude—is just warming up. (One barrel equals 42 gallons, or 159 liters.)
In early August, a high-level U.S. government official asserted that more than three-quarters of the oil from the Gulf spill was "gone"—based on preliminary National Oceanic and Atmospheric Administration (NOAA) estimates. Since then a fiery backlash has erupted from independent scientists who have been tracking and studying the spill.
"The oil budget NOAA came out with was just a joke, a fairy tale scenario," said Samantha Joye, a marine biogeochemist from the University of Georgia and one of the first researchers to detect and measure the deep plumes of oil.
"I understand why people want it to disappear, but who in their right mind would believe that? It makes absolutely no sense."
Gulf Oil Buried in Seafloor
Joye, who has been taking sediment cores in the Gulf aboard the research vessel Oceanus for the past month. So far she's discovered layers of oil in ten sediment cores taken around a mile (1.6 kilometers) deep and up to 80 miles (129 kilometers) north of the well.
In some places, the oil layer was up to two inches (five centimeters) thick, she found.
Though further analysis would confirm if the oil is from the Gulf spill, Joye said the cores are unlike those taken near natural oil seeps, in which the oil is distributed throughout the core.
(Read more about the Gulf of Mexico's natural seeps.)
In one core sample, the oil layer covered dead organisms such as shrimp and marine worms.
In addition, Joye notes, the plumes have also changed since they were first detected. They're much more diffuse, with lower methane gas concentrations and very active microbial communities.
"The oil is not gone," Joye said in a recent email from Oceanus. "You only find it, however, if you look in the right place. The sediment signal is robust. Water column is patchy, but that is not surprising."
Methane Gas Ignored in Oil Estimates?
The contribution of methane gas to the oil estimates is another bone of contention for Joye and her colleague Ian MacDonald, a marine microbiologist at Florida State University.
"All the reports of the pollutant load discharged from the well have been issued in barrels—a unit of liquid volume—and have ignored the gas," MacDonald testified before the U.S. Congress in August.
"In fact, if calculated in equivalent units of weight [mass] or energy [barrels of oil equivalents], the magnitude of the oil plus the gas is equal to 1.5 times the oil alone," MacDonald said.
In other words, if 4.1 million barrels of oil escaped into the Gulf, the total discharge of liquid oil plus gas, would be equivalent to more than 6 million barrels of oil.
While methane is less toxic than oil and breaks down faster, bacteria still need oxygen to degrade it—creating another oxygen sink in the system, Joye said.
"It would take four times the [oxygen] volume of the plume to consume the gas," she said.
"That gets you to low, low oxygen levels in a large chunk of water."
And when oxygen levels are low, it takes much longer for microbes to break down the oil. (Read more about how nature is fighting the oil spill.)
For instance, Woods Hole Oceanographic Institution researchers are still finding layers of oil in low-oxygen sediments from a 1969 barge spill in Massachusetts' Buzzards Bay (map). The team has also documented lingering impacts of that spill on marine life.
Low-oxygen levels are a serious concern for Gulf biologists, especially since fertilizers and sewage flowing down the Mississippi River contribute to an annual, low-oxygen "dead zone" in the Gulf that this summer was as large as Massachusetts.
But so far NOAA researchers sampling the water column outside the annual dead zone have not found evidence that the oil is having a severe impact on dissolved oxygen levels in the Gulf: There are reduced oxygen levels, but none too low to support fish.
Toxic Oil Harming Marine Life
A recent study from the Department of Energy's Lawrence Berkeley National Laboratory suggests that most of the undersea plume has already dispersed or degraded, though that claim is contested by other researchers.
One of those is chemical oceanographer David Hollander of the University of South Florida. In mid-August, he and fellow researchers found oil in sediments and the water column up to 50 miles (80 kilometers) northeast of the well, including the DeSoto Canyon, a critical spawning area for commercial fish species on the West Florida Shelf.
Though Hollander is still two weeks away from chemically fingerprinting the oil, he said he's "100 percent sure" it's from the Gulf oil spill.
"Everywhere we went east of the wellhead we found it," Hollander said. "It's not like a blanket, but a settling of fine microdroplets so small you can't see them. Under UV light it looks like a constellation of stars in the southern sky. They are pinpricks, but there is plenty of it there."
The lighter gases and short-chain hydrocarbons such as methane and benzene are mostly gone, leaving more complex, longer chain hydrocarbons that are harder for microbes to break down, Hollander noted.
"After about 24 carbon molecules, bacteria just give up," Hollander says. "And there they sit at 200 to 500 parts per billion. It's below acute toxicity levels, but it's still there."
Perhaps more concerning is that Hollander's colleague John Paul, also of USF, found the remaining oil is still toxic to marine organisms, such as tiny microscopic plants called phytoplankton and bacteria.
In microbial assays using Gulf water sampled from the surface, at 820 feet (250 meters), and at 902 feet (275 meters) deep, Paul found the oil may be causing bacteria to mutate.
Paul suspects the mutations may be due to residual polynuclear aromatic hydrocarbons (PAHs) from the oil, which are known carcinogens.
"The impact on commercially important larvae that are bathed in this stuff is hard to say," Paul said.
"We might see groupers with tumors three years from now. It's a long process."
Bright Spots for Gulf Oil Spill Recovery
There's some encouraging news from the Gulf, however. For instance, the impact on wildlife appears to be far lower than during the 1989 Exxon Valdez oil spill, when 3,100 marine mammals and more than a hundred thousand birds succumbed in Alaska's Prince William Sound.
By contrast, more than a thousand turtles, 70 marine mammals, and 4,000 birds have died so far from the Gulf oil spill, according to NOAA. (See "Gulf Oil Spill Pictures: Birds, Fish, Crabs Coated.")
Winds and currents kept most of the oil away from critical wetlands, and the dense marsh grasses themselves prevented much of the oil from penetrating far inland. (See pictures of oil seeping into Louisiana marshes in May.)
And so far, Gulf Coast seafood has been deemed safe to eat: U.S. Food and Drug Administration tests of more than a thousand fish samples taken from areas closed to fishing during the spill have found only a small fraction contaminated with PAHs. The levels detected have been a hundred to a thousand times lower than levels of known risk.
(Watch a related video: "Oil Inside Gulf Crabs May Be Shed.")
Long-Term Effects Still a Concern
But it's the long-term effect on the food web of the Gulf that still remains a worrisome unknown, experts say.
"My fear is that [the public will] say, Hallelujah, the oil is gone!" the University of Georgia's Joye said.
"People will forget about it and walk away and we'll never learn what is happening," she said.
"Clearly we didn't learn anything from Exxon Valdez. ... We can do much better than this. If you can do better and choose not to, it's inexcusable."