The Deepwater Horizon explosion in the Mexican Gulf is still the largest marine oil spill in U.S. history, and its effects will be felt for decades to come.

Oil burns during a controlled fire on May 6th, 2010, in the Gulf of Mexico. (Photo: Justin Stumberg/U.S. Navy via Getty Images)

In the years since the 2010 Deepwater Horizon disaster—the largest marine oil spill in United States history—scientists have been monitoring the effects of the so-called "dirty blizzard" that showered the seafloor. Oil particles and chemical dispersants released in the spill hijacked a ride on phytoplankton and flecks of biological debris that naturally fall to the bottom, forming a toxic storm over deep-sea ecosystems.

Today, as President Donald Trump and Congress push to expand offshore drilling, scientists are still coming to terms with the spill's effects on deep-sea corals in the Gulf of Mexico, an important but poorly understood part of this ecosystem. Even as oil spill research funding moves on to new priorities, scientists say the story of how these slow-growing corals will fare could take decades or more to conclude.

"It's going to take a minimum of 50 years, and it could be as long as hundreds of years, to get back to the size of the corals that we lost," said Erik Cordes, a deep-sea ecologist at Temple University who has been studying the Gulf of Mexico since five million barrels of oil gushed into its waters more than seven years ago. Scientists estimate 15 to 30 percent of the spill eventually settled on the bottom.

Cordes is part of ECOGIG (Ecosystem Impacts of Oil and Gas Inputs to the Gulf), a research consortium that has been monitoring several coral sites since 2010 through annual visits in a remotely operated underwater vehicle. The research funding granted due to the spill, he said, has offered a rare opportunity to study the same corals year after year, allowing for new insights into both the effects of oil contamination and the role of deep-sea corals in the wider Gulf ecosystem.

Deep-sea corals are found all over the world and, because of their depth, are mysterious ecosystems that are difficult to study. In the Gulf, they live as deep as 9,000 feet on rocky surfaces that dot the seafloor, providing a habitat for a diverse array of animals like lobsters, fish, and some sharks. They are considered predators, capturing zooplankton that drift by with their polyps.

After the BP spill occurred, this toxic "marine snow" left a scattershot layer of oil and chemicals covering an area the size of Manhattan. Some corals lost branches or developed tissue damage that left bare spots, allowing them to be colonized by tiny, jellyfish-like animals called hydroids.

Researchers are still analyzing images and samples from the last expedition in June, but so far the group has found mixed results on coral recovery at sites spanning a 15-mile radius from the doomed BP well. Some coral communities showed heavy damages, Cordes said, but none have died yet. Some, to his surprise, are starting to recover.

"Some of the corals are more resilient than I thought when we first saw the impact," he said.

ECOGIG researchers roughly estimate that, if less than 20 percent of a coral's body was impacted by the spill, it has done well so far. Those that were significantly harder hit are showing fewer signs of recovery or, in some cases, are getting worse. Currently, Cordes says ECOGIG researchers are working to build models that will help predict which corals will survive over the long term.

"These animals live so long and respond to impacts so slowly that even seven years after the spill, the fate of impacted corals is still unclear," said Fanny Girard, a Ph.D. student working on the ECOGIG expeditions.

The spill's longer-term effects on wider deep coral populations are even less certain. Because these animals only reproduce once every few decades, no data exist on their reproduction rates since the spill. Girard worries that, if they are similar to shallow-water coral species, however, experiments suggest ingested oil could slow reproduction with uncertain ripple effects across the Gulf population and the broader ecosystem.

Cordes echoed this uncertainty. "We're just starting to learn how they interact with the rest of the deep sea," he said. "It could be that you see a collapse in a different part of the ecosystem, and it could be because we lost these coral habitats. And we won't even know that."

While scientists say more research is needed, funding isn't likely to flow quite as freely nearly a decade out from the spill. Since the $20.8 billion BP settlement was finalized last year, attention is shifting from assessment to restoration and recovery. About $270 million has been allocated for "open ocean" restoration activities, a quarter of which will be directed toward deep-sea and coral habitats. The Gulf state trustees who manage the funds are expected to release a plan on deep-sea restoration early next year. Scientists and engineers will be working in uncharted territory—oil spill restoration at that depth and scale has never been attempted before.

Some research funding will live on as part of the BP settlement and fines. The National Academy of Sciences and National Oceanic and Atmospheric Administration Science Program will disburse hundreds of millions of dollars over the coming decades. But one of the biggest sources for spill research money—a $500 million, 10-year grant from BP in the wake of the spill—is about to end. The Gulf of Mexico Research Initiative, which funded the ECOGIG work, awarded its final round of two-year grants in September.

Harte Research Institute for the Gulf of Mexico Studies researcher Paul Montagna, who worked on the federal Deepwater Horizon Natural Resource Damage Assessment and has studied marine habitats for more than 30 years, says that the post-BP influx of money was an aberration—the federal government used to be the only source for research funding. According to one analyst's estimate, federal agencies managing offshore oil rights have spent $1.3 billion on environmental studies since 1973. That's roughly the same amount spent by BP on research and damage assessments in the wake of the spill.

Federal spending on environmental studies seems unlikely to increase under the Trump administration. But at least one federally funded research project is looking at sensitive seafloor ecosystems, this time on the Atlantic coast. Cordes is leading DEEP SEARCH, a multiyear project to study and map deep-sea coral, canyon and gas seep ecosystems. The U.S.Bureau of Ocean Energy Management says it will use the research findings to inform future offshore drilling decisions, potentially including Atlantic sites. The project just completed its first expedition in September.

The research comes amid an aggressive push by Trump for offshore drilling in the Arctic and Atlantic oceans, the latter of which has not seen drilling since the 1980s. The Department of Commerce has also drafted a report, which has not been made public, with recommendations on whether to change the boundaries of 11 existing marine sanctuaries to open up those areas for offshore drilling.

This all may be bad news for deep-sea corals, Cordes says. "We're trying to provide the best advice we can, which is to drill as far away from these areas as you possibly can," he said. "So not if, but when, the next spill happens, it happens far away from these sensitive communities."

This article originally appeared on Oceans Deeply. You can find the original here. For more in-depth coverage of ocean health, you can sign up to the Oceans Deeply email list.