Deep-sea corals are still paying the price for the Deepwater Horizon ecological disaster, reveals a new study from Pennsylvania State University, and researchers warn that some reefs may never recover.

The Disaster

In April 2010, the Deepwater Horizon offshore drilling rig in the Northern Gulf of Mexico suffered a devastating blowout 1500 meters beneath the sea which released roughly 4.9 million barrels (780 million liters) of crude oil. For an excruciating 87 days, the oil pumped into the ocean depths, while seven million liters of chemical dispersant were applied at both the surface and at the wellhead on the sea floor in an attempt to combat the spill.

Thanks to both the chemical dispersant and the effects of ocean physics, most of the oil formed into a deep-water oil plume that lingered in the sea for months. The “perfect storm” combination of these many factors meant the Deepwater Horizon disaster had the potential for a devastating impact on marine life, and particularly on deep-water coral reefs.

Like the revered redwood trees of California, deep-sea corals are slow-growing and long-lived. Left in peace, they can reach truly staggering ages; one colony of the black coral Leiopathes was found to be over four thousand years old. And also like forests, coral reefs provide habitat for other species and support a thriving, biodiverse community of marine life which depends on them.

Yet the very traits which give these corals their ancient lifespan are what make them extremely vulnerable to destruction from reckless human activities – making the Deepwater Horizon disaster so much more than a temporary disturbance.


Fire engulfs the Deepwater Horizon oil rig as it sinks. Photo: Flickr/USEPA

Immediately after the disaster, anxious marine biologists began studying nearby deep-sea coral colonies to assess the impact of the oil spill. For months after the well was capped, deep-sea corals were still found covered in a strange brown film, a mixture of both oil and the applied chemical dispersant. By October 2011, the brown mixture appeared to have finally dispersed, but this revealed stark patches of exposed coral skeleton swiftly being smothered by opportunistic hydroids instead.

That’s when marine biologists Fanny Girard and Charles Fisher of Pennsylvania State University launched a long-term monitoring study of the Gulf’s deep-water coral colonies. Using cutting-edge imaging technology, they analysed the recovery of affected deep-sea corals from 2011 to 2017 – and their newly-published results are less then encouraging.

Girard and Fisher’s extensive study covered three impacted sites, as well as two reference sites which appeared to have escaped the initial spread of the oil.

“Over three hundred [deep-sea coral] colonies were imaged yearly between 2011 and 2017 at five sites,” the scientists explain in their new publication, “and the images were digitized to quantify health, hydroid overgrowth, identify branch loss, and track recovery patterns.”

This imaging technology allowed the researchers to monitor even small changes in the corals’ health over time. Over the years they tracked what proportion of the corals were healthy, unhealthy, or covered in hydroids, as well as noting when individual coral branches appeared to have snapped off.

The Consequences

Girard and Fisher’s monitoring revealed that reef recovery was slow, if it was even occurring at all. The proportion of damaged and dying corals at the impacted sites remained virtually unchanged after 2011. In other words, the corals have been suffering the effects of the 2010 oil spill just as keenly over the past seven years as they were in the year after the event.

“The effect of initial impact on recovery between consecutive years was still visible seven years after the spill, indicating a long-term, non-acute, impact on the colonies,” the researchers wrote. “Injured corals were also more likely to lose branches, and branch loss was still significantly higher at some of the impacted sites between 2016 and 2017, indicating an ongoing effect of the spill, which may eventually lead to delayed mortality.”

Paramuricea clavata (Risso, 1826)

Paramuricea clavata, like many corals, is the backbone of its marine community. Photo: Parent Gery.

The Deepwater Horizon oil spill may be fading into an environmental disaster of the past in the minds of many, but for these damaged habitats, the forecast for the future is grim. Girard and Fisher warn that based on their study, “many more years will be necessary for coral recovery and colonies with more extensive injury may never recover.”

For perspective, the researchers contrast Deepwater Horizon with the 1999 heat wave in the Mediterranean Sea that impacted colonies of Paramuricea clavata corals, a closely related species to those found in the Gulf. In that case, the Mediterranean corals showed signs of recovery over the first two years following the heat wave, and then their condition stabilised (although, the corals never did regain their pre-heat wave level of health).

In contrast, many of the coral reefs damaged by the Deepwater Horizon oil spill are not recovering and in fact have continued to slowly deteriorate for years. The corals damaged by the oil spill have remained very vulnerable to overgrowth by hydroids, which weaken the skeleton and ultimately cause struggling coral branches representing decades or centuries of growth to break off. Some broken coral branches recovered in this study have been analysed to be over six hundred years old.

It’s this alarming level of branch loss, combined with the corals’ slow growth rate, which leads Girard and Fisher to conclude that the coral reefs damaged by Deepwater Horizon may continue to shrink slowly over the coming years – and that the marine communities which rely on them could disappear as well.

The Future

Despite their seemingly isolated existence, deep-sea corals worldwide continue to face a variety of threats, including deep-sea trawling, offshore oil and gas extraction, and potentially even deep-sea cobalt mining.

On this note, Girard and Fisher encourage more widespread use of their incredible imaging technology in order to establish baseline data on coral biology, monitor coral reefs long-term, and assess the efficiency of Marine Protected Areas to protect coral.

The researchers also urge continued investment in science, and particularly in these types of long-term studies, stating: “With ever increasing energy extraction and the deepening impacts from fishing, ocean acidification and increasing water temperature due to climate change, the need for baseline data and monitoring is becoming acute.”

There are many lessons to be learned from the Deepwater Horizon disaster, and as this eye-opening new study shows, the consequences of humanity’s actions are long-term and cannot be simply swept under the rug – or under the sea.


Girard, F. and C.R. Fisher. (2018) “Long-term impact of the Deepwater Horizon oil spill on deep-sea corals detected after seven years of monitoring.” Biological Conservation 225: 117-127. DOI:


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