There are still nearly pristine coral reefs. They’re a blueprint for coral restoration
The coral reefs at Palmyra Atoll in the remote Pacific Ocean are some of the healthiest in the world, helping researchers understand the vital aspects of reef recovery and how it can apply to reef restoration worldwide.
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One of the most pristine coral reefs in the world is in the center of the Pacific Ocean, thousands of miles from any continent, in the jewel-like waters around a tiny atoll.
“I get a little lost for words on this reef,” Joe Pollock says, as he pops his head above the water. He’s senior reef strategy lead with The Nature Conservancy, a conservation group. “It’s just one of the most stunningly beautiful reefs that you could ever imagine.”
The corals of Palmyra Atoll are an example of what reefs are supposed to look like. Researchers have found its marine ecosystem is still largely preserved, from tiny fish to a thriving population of sharks. The reef is part of a U.S. national wildlife refuge and surrounds an atoll with no permanent residents, so it has remained largely free of human disturbances.
About 1,000 miles south of Hawaii, Palmyra Atoll is surrounded by a ring of coral reefs, formed around an ancient volcanic island that eroded under the sea.
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Still, climate change is reaching even the most far-flung places.
Palmyra Atoll’s corals are already showing signs of heat stress this year due to abnormally warm ocean temperatures. Corals are highly vulnerable to heat, which turns them a bone-white color, a process known as coral bleaching. Prolonged heat can kill corals. Worldwide, the current bleaching may become one of the worst on record due to the powerful El Niño climate pattern that’s raising temperatures globally.
Three years ago, another marine heat wave took a heavy toll on some of Palmyra Atoll’s reefs, but the corals are showing remarkable signs of bouncing back. Marine scientists are finding that healthy reefs like Palmyra’s have key ingredients that can help speed recovery.
Those lessons are being used at coral restoration projects around the world, where communities are hoping to give reefs a fighting chance to survive climate change. Coral reefs are reservoirs of biodiversity, but also support hundreds of millions of people who rely on them.
“Coral reefs are not doomed,” Pollock says. “Palmyra shows us that we’re not ready to write the obituary on coral reefs.”
Katie Franklin (left) and Joe Pollock of The Nature Conservancy visit one of Palmyra Atoll’s reefs. The islands themselves are made by the coral reefs, and their beaches are covered in large pieces of old coral.
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An underwater city
Palmyra Atoll’s reefs can feel like rush hour in a busy downtown. Schools of brightly-colored fish move among the towering corals. A peacock grouper rests on a piece of coral where a tiny cleaner wrasse darts out to nibble its dead skin and parasites, a visit to what’s essentially a fish spa. An octopus peeks from a hiding spot. Blacktip reef sharks slowly cruise the outskirts.
The corals themselves are the size of cars, sporting their own rainbow of colors. Some have delicate branching arms like deer antlers. Others are flat like undulating ribbons of fabric or round like massive boulders.
The marine ecosystem around Palmyra Atoll is teeming with life, having been impacted very little by pollution or human disturbance.
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“Almost every last square inch is covered in live coral, so much to the point that corals are trying to grow over each other,” Pollock says. “It’s coral on coral, which is exactly what you want to see.”
Palmyra Atoll’s remote location has largely shielded its reefs from human impact. More than 1,000 miles south of Hawaii, its tiny islands are formed by the coral reefs themselves, which surround an ancient volcanic island that has eroded under the sea.
Palmyra Atoll was once used by U.S. troops during World War II. Now, it’s home to a small research field station used by The Nature Conservancy and the U.S. Fish and Wildlife Service.
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The atoll was likely visited by Polynesians who crossed the Pacific Ocean and later became part of the U.S. after it colonized Hawaii. During World War II, the military constructed runways, roads and small buildings, but for the most part it’s had few human inhabitants. Since 2000, the atoll has been a hotspot of scientific research, where marine biologists can study a coral ecosystem that’s functioning as it should be.
Joe Pollock of The Nature Conservancy monitors Palmyra Atoll’s corals. “We can get reefs through this current bottleneck of climate change,” he says. “They may not look like the reefs looked in the past, but I believe if we do things right, we can have reefs into the future.”
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Bleaching comes to Palmyra Atoll
In 2023, a marine heat wave hit Palmyra Atoll, raising water temperatures for more than half a year. Pollock remembers seeing some of the atoll’s most vibrant corals turn completely white.
“It was devastating,” Pollock says. “You could feel in your body the suffering. Honestly, I didn’t know that it would bounce back.”
Heat disrupts a vital relationship for corals. They get most of their food from their roommates – tiny algae that live inside the corals’ tissues. When stressed, those algae get expelled, making coral lose their color. Bleached corals slowly starve and die if a marine heat wave lasts too long. But if temperatures drop, corals can recover when the algae move back in.
Some of Palmyra’s corals still show the lasting damage of the 2023 heat wave. But around them, Pollock finds a hopeful sign.
In 2023, a marine heat wave hit Palmyra Atoll, causing the corals to bleach. Some of Palmyra’s reefs have recovered strongly since, while others have been slower to bounce back.
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“It is just an absolute sea of baby corals,” Pollock says, pointing to what look like small cauliflowers. “The parents of these corals were the survivors of that bleaching event.”
Corals that survive bleaching often have a higher tolerance to heat, scientific research has shown. That ability can be passed on to the next generation, meaning these baby corals may have an advantage during future marine heat waves, thanks to their parents. Pollock says that kind of recovery is ideal after a bleaching event, but many reefs around the world are struggling to build that resilience.
On the seafloor, young corals are growing after a marine heat wave three years ago.
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Globally, living coral has declined by about 10% since 1980, hit hard by repeated bleaching events, according to a report from the Global Coral Reef Monitoring Network. Marine heat waves are becoming more intense and frequent because the oceans have absorbed the majority of the extra heat in the atmosphere from burning fossil fuels.
Pollution, human development and overfishing also impact reefs around the world. A number of restoration projects are underway, from Hawaii to the Caribbean, where young corals are grown and planted in the ocean to build the reef back. Those projects are learning from Palmyra Atoll and other healthy reefs about what it takes to restore a vibrant, interconnected ecosystem. Here are a few of the lessons.
Parrotfish are essential herbivores of coral reefs, nibbling the algae that grows on coral and giving corals more space to grow.
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Lesson 1: Fish are friends
All over Palmyra’s reefs, throngs of brightly-colored parrotfish nibble at the corals. They’re eating algae, which grow quickly in the warm marine environment. The fish continually clean the surfaces with their grazing.
After a bleaching event, those janitorial services are crucial. Algae can quickly move in, covering the bleached and dead corals.
“They’ll take any chance they can get to take over that real estate,” Pollock says. “Once corals have given up that ground to the algae, it’s really hard to come back from that.”
Fish help keep the real estate open for corals, preventing them from getting overwhelmed by algae and making space for the next generation. When baby corals first form, they float in the current, searching for a place to settle. When a reef is largely clear of algae, there are more open landing spots available.
A stony, pink seaweed, known as crustose coralline algae, grows on rocks and dead coral. It’s a preferred landing spot for young corals looking for a place to live.
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Lesson 2: A bright pink crust
Coral larvae also get help from another organism: a bubblegum-pink crust that grows on the reef, known as crustose coralline algae.
This stony seaweed is a friend to corals, unlike many other algae. Its hard surface fills in gaps on the reef. Many baby corals prefer to settle on top of it when looking for a place to grow.
“What it does do is it saves space for those corals to come back,” Pollock says. “It’s basically a placeholder that keeps the algae out and it keeps that area open for when the environment is right for the baby corals to come back, to have a place to settle and to have a place to thrive.”
Palmyra Atoll has one of the highest densities of predators ever found by scientists, like this blacktip reef shark cruising the reef.
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Lesson 3: Sharks rule
Palmyra’s corals also benefit from species at the top of the food chain: sharks.
When biologists first started diving at Palmyra, they were astounded at the number of sharks. Studies show it has one of the highest densities of predators in the world. It’s a sign that the food chain is largely intact, which helps keep all species in balance.
The grazing fish that help clear algae are eaten by larger, predatory fish. But with plenty of sharks, those predatory fish that eat the grazing fish are kept in check, meaning populations of grazing fish don’t have too many predators and stay abundant.
“It’s really that interconnected ecosystem that we have here that unfortunately is fairly rare in more impacted areas around the world,” Pollock says. Due to overfishing, sharks have become sparse on many reefs around the world. Globally, sharks and rays have declined by 71% since 1970.
Research from Palmyra Atoll is helping inform coral restoration work around the world. A coalition of universities and non-profits is trying to identify “super reefs,” where conservation work could help protect the corals most likely to survive climate change.
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Putting the lessons into practice
As coral restoration projects ramp up around the world, researchers are trying to recreate some of the missing ingredients of healthy reefs. Efforts are underway to breed grazing animals that keep algae in check. In Florida, Caribbean king crabs are being released as part of coral restoration efforts, as well as sea urchins in Puerto Rico.
Scientists have also refined techniques at Palmyra Atoll to better target restoration work on reefs elsewhere. The Woods Hole Oceanographic Institution, Stanford University and the Nature Conservancy have developed a “Super Reefs” framework to identify reefs that stand the best chance at surviving a hotter future. They look at reef conditions, water currents, and genetically identify the most heat-tolerant corals. Working with local partners, they’re focusing on restoring those reefs, in places such as Maui and Belize.
Palmyra Atoll’s coral may still have other lessons to impart. While some corals bleached during the marine heat wave in 2023, others nearby were largely untouched. The reefs also recovered at different rates. Understanding these differences could help uncover more about what makes some corals more heat-tolerant.
Experts say it’s why studying the most pristine reefs is key for understanding how to save what’s left.
“We actually know so little about the biodiversity that exists on the reefs,” says Tali Vardi, executive director of the Coral Restoration Consortium, a non-profit focusing on reef restoration. “Most of it is unknown and there are still species being discovered.”
At most, only a few dozen people work at Palmyra Atoll National Wildlife Refuge at a time.
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The limits of restoration
Still, marine heat waves are becoming increasingly intense with climate change. At some point, most coral reefs won’t be able to keep up.
“Restoration is not the long-term solution,” Vardi says. “There’s no world in which restoration can keep coral reefs around without changing the trajectory of our climate.”
If the world gets more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than pre-industrial temperatures, between 70-90% of coral reefs worldwide won’t survive, according to a report by the United Nations’ Intergovernmental Panel on Climate Change. Currently, the world will hit 2.6 degrees Celsius (4.7 degrees Fahrenheit) of warming by the end of the century, if countries follow through on their pledges to cut heat-trapping emissions from burning fossil fuels.
Coral reefs are biodiversity hotspots, but are also vital for many human communities. They provide food and livelihoods for hundreds of millions of people, as well as coastal protection against waves from stronger storms.
“Given the value of coral reefs, given the fact that they support nearly a billion people’s lives and livelihoods around this Earth, it’s far too soon to give up,” Pollock says. “I honestly think we need to fight with everything we have to keep these reefs healthy.”
This story was supported by the Pulitzer Center.