Deep-water corals in the Gulf of Mexico are still struggling to recover from the devastating Deepwater Horizon oil spill in 2010, scientists report at the Ocean Sciences Meeting in New Orleans. Comparing images of more than 300 corals over 13 years—the longest time series of deep-sea corals to date—reveals that in some areas, coral health continues to decline to this day.

The spill slathered hundreds of miles of shoreline in oil, and a slick the size of Virginia coated the ocean surface. Over 87 days, 134 million gallons of oil spilled directly from the wellhead at a depth of 1520 meters (nearly 5000 feet) into the Gulf. While the spill was most visible at the surface, negative ecological impacts extended hundreds of meters into the ocean.

In a presentation on Tuesday, 20 February, scientists will show that deep-water corals remain damaged long after the spill. Over 13 years, these coral communities have had limited recovery—some even continuing to decline.

“We always knew that deep-sea organisms take a long time to recover, but this study really shows it,” said Fanny Girard, a marine biologist and conservationist at the University of Hawai’i at Mānoa who led the work. “Although in some cases coral health appeared to have improved, it was shocking to see that the most heavily impacted individuals are still struggling, and even deteriorating, a decade later.”

The findings can help guide deep-water restoration efforts following oil spills.

A few months after the Deepwater Horizon well was capped, an interdisciplinary team of researchers surveyed the ocean floor 6 to 22 kilometers (3.7 to 13.7 miles) from the wellhead to record the damage. About 7 miles away and at 1,370 meters (4,495 feet) depth, they found a dense forest of tree-like Paramuricea corals that looked sickly.

“These corals were covered in a brown material,” Girard said. Testing showed the sludge contained traces of a combination of oil and chemical dispersants. A few months later, the researchers found two additional coral sites at 1,580 meters and 1,875 meters (4921 and 6233 feet, respectively) deep that were similarly damaged.

Deep-sea corals are suspension feeders and may have ingested contaminated particles, leading to the observed health impacts, the researchers said. Direct exposure to toxic chemicals contained in the mixture of oil and chemicals may have also damaged coral tissue. However, to date, scientists still do not exactly know how the oil and dispersants affected these vulnerable organisms.