Deep Reefs Might Not Be the Climate Refuge We Hoped

The descent into the reefs of Pohnpei, Micronesia.
Photo: Courtesy of Luiz Rocha (California Academy of Sciences)

By diving 500 feet down into the ocean’s twilight zone, a team from the California Academy of Sciences has learned that deeper-dwelling corals form entirely distinct ecosystems from those in shallow reefs—and they aren’t faring all that much better than their counterparts near the surface.

Many scientists had considered deep reefs as a sort of refuge for heat-stressed corals and the creatures that depend on them. Research last year showed reefs in the Ari Atoll in the Maldives, at least, were less likely to be bleached the deeper down they were. Bleaching, where corals turn lose their symbiotic algae, turn white, and start to starve, occurs when the water becomes too warm.

While that study concluded deeper reefs might offer some protection and a potential refuge from the effects of rising temperatures, a new study published in Science Thursday concluded differently. Deep reefs in five locations the team surveyed—Bermuda, the Bahamas, Curaçao, Philippines, and Pohnpei, Micronesia—all faced impacts from bleaching, pollution, and storms.


The team surveyed both shallow water reefs and deeper reefs in the mesophotic or twilight zone, which ranges from 100 to 500 feet. At each location, they found the fish in the reef systems differed: Only 27 percent of those in shallow waters reach the mesophotic zone. During these exhibitions, the team discovered about 25 new species in the deeper reefs, highlighting how much there’s left to learn about these ecosystems.

A taste of what one can find in the mesophotic zone.
Photo: Courtesy of Luiz Rocha (California Academy of Sciences)

“Shallow reefs are in even more trouble than we thought they were,” said Luiz Rocha, an associate ichthyology curator at the California Academy of Sciences to Earther. “Deep-sea corals are unique, and a lot of the threats we see in shallow reefs also extend to deep reefs.

To conduct this study, an average team of four, including Rocha, would descend into the twilight depths where they suspected reefs to be. The divers would take anywhere from seven to 10 minutes to reach the mesophotic coral reefs. Once there, the team would spent about 15 minutes conducting their research. Someone would be in charge of taking photos. A team of two would count and identify the fish to see how similar they were to those higher up. Others would take visual surveys of the reefs. After all this, the journey back up would begin. And that could take anywhere from four to five hours.


“Sometimes, we don’t find a reef, and we’ve wasted a whole day there,” Rocha told Earther.


Collectively though, the effort wasn’t a waste. This is some of the first research that’s directly documented deep coral reefs through observation. Down there, the impacts humans are having became clear. The team found old fishing lines, fishing gear, and plastic bags in the mesophotic zone. Then, there’s the impact from storms.

“Hurricanes, which a lot of people thought would affect only shallow reefs, also affect deep-sea reefs,” Rocha said.


In the Atlantic, the team visited reefs in the Bahamas just four days after Hurricane Matthew hit in 2016. Shallow reefs and deep reefs were both damaged. Entire deep reefs were covered in sand and sediment, which blocks the little light these corals receive from reaching their algae.

The Odontanthias borbonius in the deep reefs of the Philippines.
Photo: Courtesy of Luiz Rocha (California Academy of Sciences)

The team also saw the effects of sediment on deep reefs the Philippines. In Pohnpei, Micronesia, the crew documented fewer impacts and even spotted a blue Chromis circumarea, whose rarity hasn’t afforded it a common name. Reefs in Pohnpei and the Bahamas that are farther from humans tend to do better, the scientists note in the study.

This research stresses that deep reefs won’t be the saviors for corals around the world. Instead, they need the same kind of protections the rest of the reefs do. Rocha suggested humans get their shit together when it comes to fishing regulations and creating marine protected areas.


There’s hope for our reefs, but it’s going to take immediate action—and more research—to better understand and ultimately save them.

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About the author

Yessenia Funes

Senior staff writer, Earther. The one who "pulls the race card" in the name of environmental justice. You dig?

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