The massive oil spill that inundated the Gulf of Mexico in the spring and summer of 2010 severely damaged deep-sea corals more than 11 kilometers from the well site, a sea-floor survey conducted within weeks of the spill reveals. Although 10 more distant sites examined during the survey did not show any ill effects, future studies will be needed to confirm that they did not suffer long-term detriment from any exposure to oil, scientists say.
Starting with an explosion onboard the Deepwater Horizon oil drilling rig on 20 April and continuing for 85 days, the worst oil spill in U.S. history released an estimated 4.9 million barrels of oil -- about 20 times the amount spilled by the Exxon Valdez in Alaska in 1989. Between 3 and 4 months after the well was capped, researchers used the deep submersible vehicle Alvin and the remotely-operated vehicle Jason II to revisit several sites along the continental shelf known to host corals, says Charles Fisher, a team member and deep-sea biologist at Pennsylvania State University, University Park.
The researchers also used previously collected sonar data to identify a possibly rocky patch of sea floor where corals could thrive about 11 kilometers southwest of the well site. At that 1370-meter-deep site, which hadn't been visited before but had been right in the path of a submerged 100-meter-thick oil plume from the spill, the researchers found a variety of corals—most of them belonging to a type of colonial coral commonly known as sea fans—on a 10-meter-by-12-meter outcrop of rock. Nearby, boulders poking up through the sediment hosted isolated colonies of coral. Many of the corals were partially or completely covered with a brown, fluffy substance that Fisher variously calls "frothy gunk," "goop," and "snot."
Samples of the material contained mucus secreted by the corals—a sign the colonies had recently been under stress—as well as fragments of dead coral polyps, saturated and unsaturated fatty acids commonly found in biological tissues such as cell membranes, and a mélange of petroleum residues. Although the chemicals related to petroleum—including long-chain hydrocarbons, polycyclic aromatic hydrocarbons, and a group of compounds known as hopanoids—could have originated from other oil wells or natural sea floor seeps in the area, measurements of the ratios of specific hopanoids identify the Deepwater Horizon spill as the source of the oil, the researchers report online today in the Proceedings of the National Academy of Sciences. "It's like a fingerprint," says Helen White, a geochemist at Haverford College in Pennsylvania and a co-author of the new research.
In almost half of the 43 corals studied at the site, the majority of animals had died or were showing signs of stress, the researchers say. And in more than one-quarter of the corals, more than 90% of the animals showed such damage. Also, more than half of the brittle stars, a relative of starfish, found clinging to the sea fans were partially or completely bleached white, another certain sign of stress, says Fisher.
The new findings "show clearly the very negative effects in deep-water communities from this spill," says Samantha Joye, a biogeochemist at the University of Georgia in Athens who wasn't involved in the research. The true extent of damage from the spill is, for now, tough to determine because so much of the sea floor hasn't been examined, she notes. "The deeper you look, the more you're going to find."
Also, Joye notes, areas that weren't immediately damaged by oil plumes in the wake of the spill may be later exposed to oily material lofted from the ocean floor by strong currents or by human activities such as trawling. "This stuff is like the foam on a latte," she says. "It's very fluffy."