- Experts documented the substantial recovery of coral reefs around the southern Line Islands in the central Pacific after the area was hit by a large-scale coral bleaching event in 2015 and 2016.
- Many factors may have contributed to the reef’s recovery, including the fact that the reef is seemingly untouched by human activity, which helped maintain a healthy and resilient ecosystem.
- But other experts question whether this reef would be able to recover after more frequent bleaching events, which are predicted to increase as global temperatures continue to rise.
In April 2009, ocean explorer Enric Sala and a team of marine biologists dove into the waters around the southern Line Islands, an uninhabited archipelago in the central Pacific that belongs to Kiribati, to find something increasingly rare in a world dominated by humans: a seemingly pristine coral reef.
The reefs surrounding the islands teemed with fish, including species like the endangered Napoleon wrasse (Cheilinus undulatus). The number of sharks and other predators even seemed to outnumber their prey, which was quite unusual to see on a coral reef, Sala noted.
“The southern Line Islands changed our understanding of coral reefs,” Sala wrote in an article published in in National Geographic. “Scientists like me had no idea what pristine reefs looked like.”
In 2015 and 2016, a strong El Niño event in the Pacific exposed the southern Line Islands to 15 weeks of warmer-than-usual temperatures that led to coral bleaching. When a research team went back to the area in 2017, they found that half of the corals had died from the heat stress— mainly the cauliflower corals in the Pocillopora genus and the stony corals in the Acropora genus.
But Sala was encouraged to see that the damaged corals had not been quickly overtaken by brown algae, which often happens after bleaching events. He hypothesized that an abundance of herbivorous fish, such as parrotfish and surgeonfish, is what helped keep the coral free of this algae. He also saw that the corals were covered by crustose coralline algae that can form a pink limestone crust, which is the preferred substrate for coral larvae.
In 2022, Sala and colleagues went back to the southern Line Islands to see if any of the reefs had recovered. While they noticed some changes, they saw that the reefs — including the corals and fish and all of the other life — had largely revived, almost as if nothing had happened to it. This led Sala to call them “super reefs.”
On the reef around the island of Vostok, Sala and his team saw Pocillopora corals slowly returning to life. But there were also blue, rose-like corals called Montipora aequituberculata that had sprouted on top of the Pocillopora.
“How could it have gone from dead cauliflowers to thriving roses in only five years?” Sala wrote. “Nobody was there watching — but we had a clue: the Montipora colonies were all about the same size. That suggests to me that corals elsewhere around Vostok had been reproducing sexually and releasing millions of eggs, which soon hatched and formed a massive cloud of larvae above the reef platform. A rain of Montipora larvae may have fallen and settled on the pink crust within a day — a single event that changed the seascape for years to come.”
Miriam Reverter, a marine biologist at the University of Plymouth in the U.K., said the remoteness and lack of chronic stressors like overfishing and pollution likely helped the reefs of the southern Line islands recover. However, she said scientists would need to conduct more research to understand how the reefs changed after the bleaching event, and if the reefs could withstand future pressures.
“This story is encouraging and brings much needed light in the dark forecast for coral reefs,” Reverter told Mongabay in an email. “However, we must be aware that [reef] community changes, although they might indicate apparent recovery, can be detrimental to the functionality (roles of the different ecosystem species) and resilience of the ecosystems. Line Islands may have recovered the coral cover, but we don’t yet know yet how stable and diverse these new reefs are, and therefore whether they will be able to withstand future stressors remain an open question.”
Maria Beger, a coral reef expert at the University of Leeds in the U.K., told Mongabay that if you leave a reef alone in any part of the world, some recovery will happen. However, she said that thermal stress can substantially reduce species diversity. To fully understand how a reef has fared after a bleaching event, Beger said researchers would need to conduct species diversity analysis.
“There’s been reports of the Great Barrier Reef recovering, and suddenly the coral cover’s high again, but there it’s very clear that the number of species has reduced,” Beger said. “So that’s a question here.”
Beger also questioned whether the reefs around the southern Line Islands would survive more frequent instances of thermal stress, which may not allow the reef to recover.
Rising global temperatures have led to warmer ocean temperatures, which have increased the frequency of extreme events like marine heat waves and coral bleaching across the oceans. As temperatures continue to rise, scientists say these events will become even more common.
A study co-authored by Beger found that coral refugia — places that have historically protected corals from thermal stress — would mostly disappear if the global temperatures rise by 2° Celsius (3.6° Fahrenheit) over pre-industrial levels. If policymakers don’t take drastic measures to reduce fossil fuel emissions, this level of warming could be reached in as little as 20 years, according to the Intergovernmental Panel on Climate Change (IPCC).
Besides thermal stress, coral reefs face other threats like ocean acidification, pollution, and even deep-sea mining. According to modeling produced by a coalition of scientists and mining opponents, pollution from seabed mining activities in the Pacific Ocean could spread to Kiribati waters in about three months. Deep-sea mining trials have already begun, and full-scale operations could begin in 2024.
Sala said the recovery of the reefs around the southern Line Islands shows that full protection of a reef “promotes resilience.”
“As the world tries to agree on how to reduce carbon pollution, we can buy time by protecting reefs and fostering their strength,” Sala wrote. “The difference between a dead reef and a super reef lies in how much we care.”
Banner image: At Millennium, a twin-spot snapper chases a school of convict tangs — another one of the herbivores that keep reefs in the southern Line Islands from becoming over-grown with algae. Protecting such fish, Sala says, helps make reefs resilient to global warming. Photo by EnricSala/National Geographic.
Citation:
Dixon, A. M., Forster, P. M., Heron, S. F., Stoner, A. M., & Beger, M. (2022). Future loss of local-scale thermal refugia in coral reef ecosystems. PLOS Climate, 1(2), e0000004. doi:10.1371/journal.pclm.0000004
Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.
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