A new survey of deep-sea corals in the Gulf of Mexico has found that the Deepwater Horizon oil spill four years ago has affected more of the ocean than previously thought.

The work is a follow-up to another survey that found a large area of oil-coated corals shortly after the nearly five million barrel spill, said co-author Erik Cordes, marine ecologist and associate professor of biology at Temple University.

"About 50 percent of the colony was covered in this black substance. There was mucus being produced by the coral — that's their first stress response," he said. "And then where it was really bad, there was tissue falling off the bone, basically, there was bare skeleton, and literally, tissue melting off of the bone."

The oil is now gone, but indications of its earlier presence abound for miles beyond the initial spill site.

The findings

Publishing this week in the journal PNAS, the team reported additional sites where many corals — stripped of their tissue by the oil— have been colonized by other species, causing them further damage.

Cordes is one of many researchers who have tracked coral health in the region by taking photos, which can reveal whether the marine invertebrates have dead tissue or are being taken over by hydoids, a type of predatory animal related to the jellyfish.

In some — but not all — locations, they found corals in similar condition to those known to be affected by the Deepwater Horizon spill. In one case, damage was evident on coral located more than 14 miles from the oil rig, and 500 meters deeper than previously documented.

"They may have been impacted by oil that traveled all the way up to the surface, got broken down, mixed with some of the plankton in the surface waters, and then rained back down to the deep sea in what they're calling the dirty blizzard," said Cordes.

Unable to escape, corals receive the brunt of any environmental disturbance to the ocean. But because of their role in the ecosystem, the poor health of the organisms could hurt other species as well.

"Corals provide habitat for a lot of fishes and sharks to actually lay their eggs as nursery grounds that can affect these wider populations all across the Gulf of Mexico," said Cordes.

Two sites that were not damaged from oil nevertheless did have signs of human interference: a fishing line was tangled in coral and trash had found its way to the sea floor.