After the 2016 El Niño, coral reefs in Moorea are thriving -- why?

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At a time when corals around the world are under siege, marine biologists have found hope for reefs near the island of Moorea in French Polynesia.

Scientists were concerned that 2016's El Niño would devastate reefs in the Pacific Ocean and Caribbean Sea as a result of higher water temperatures.

While that proved true for many reefs, including the Great Barrier Reef off Australia, Peter Edmunds and other ecologists at California State University, Northridge (CSUN) found that not only did Moorea's reefs survive, but El Niño coincided with the growth, or "recruitment," of new corals.

"In the case of Moorea, coral recruitment in 2016 was the highest we've recorded in the past decade," Edmunds said. "Part of the take-home message is that there are multiple voices in the scientific community, and many of those are describing the devastating effects of El Niño -- but the key is that we can't paint the picture with a single color. While El Niño was bad in many locations, Moorea tells a slightly different story."

Edmunds and colleagues received a National Science Foundation (NSF) Rapid Response Research grant to find out whether rising water temperatures would affect the growth rate and arrival of coral larvae at Moorea's reefs.

The results of the study are published this week in the journal PLoS ONE.

"We directed a lot of our focus into why the coral reefs of Moorea appear to be resilient," Edmunds said. "We went into the El Niño year thinking, 'Moorea has been pretty resilient when devastated by cyclones and coral-eating sea stars, but El Niño is going to be particularly bad.' Coming so soon after a major cyclone in 2010, we feared the effects would be bad indeed."

But the study "presents a hopeful possibility for future coral reefs in a warming ocean," said Dan Thornhill, a program director in NSF's Division of Ocean Sciences, which funded the research at NSF's Moorea Coral Reef Long-Term Ecological Research (LTER) site. "Resilient corals that don't bleach or die when stressed, and numerous young corals, keep reefs healthy and vibrant."

Long-term studies of coral reefs are important, according to Edmunds. "The only way we know that these disturbances have these effects, and can evaluate their magnitude, is because we know the reefs' history and can assess changes."

Edmunds said there are two likely reasons why Moorea's reef colony survived El Niño as well as it did.

"Despite the forecast for very warm water in Moorea, the warming of the seawater was much more moderate than people had expected," he said. "El Niño created a very clear signal. If you looked at 2016, it was different from other years, but in Moorea, it wasn't sufficiently different to create the high temperatures that were catastrophic for corals."

Another factor lies in the area's history, he said, pointing to the reef's recovery from an infestation of crown-of-thorns sea stars and a 2010 cyclone. Covered in long, poisonous spines, the sea stars are a well-known and voracious coral predator.

"It's like the reef was riding a wave of coral recovery, and it had reached the peak of the wave when El Niño struck," Edmunds said. "That wave of success allowed the corals to survive through El Niño."

He compared it to a healthy person who comes down with a severe illness. That person is often able to survive because his or her body is equipped to deal with the assault. The same applies to coral reefs, Edmunds said.

"In the reverse, in St. John in the U.S. Virgin Islands, the corals have taken a nose dive," he said. "They were getting toward the bottom of the trough, then they were hit by two Category Five hurricanes. The responses to these disturbances are going to be very different.

"Whether coral reefs are on the crest or in the trough -- whether they are healthy or are debilitated -- has a huge effect on how the reefs respond to disturbances."

The next step, he said, is understanding why Moorea and the reefs that surround it were doing well when El Niño arrived.

One answer may lie in the fact that few people live in the middle of the South Pacific in French Polynesia. But, Edmunds said, that's a hypothesis, one that he invites other scientists to explore.