Coral reefs in some parts of the world actually benefit from warming El Niño events

As marine heatwaves increasingly become a worrisome byproduct of climate change, our vibrant coral reefs are under threat. These complex ecosystems, the backbone of our ocean’s biodiversity, are facing significant challenges.

But recently, scientists have discovered a phenomenon that offers a glimmer of hope. It could potentially enable us to manage and safeguard our precious coral reefs better.

Our story begins with El Niño, a recurring climate pattern that significantly affects the tropical Pacific. Winds, weather, and ocean temperatures alter drastically during this period. One such potent El Niño event unfolded between April 2015 and May 2016, placing a spotlight on the Central Pacific region.

Unexpected impact of El Niño on coral reefs

This El Niño episode led to an increased ocean temperature. The thermal stress triggered a catastrophic mass bleaching episode amongst the coral reefs.

The stressed corals expelled their symbiotic algae and turned white in a desperate struggle for survival. It was during this period that an international team of scientists, including researchers from KAUST, discovered a hopeful anomaly.

As the ocean’s temperature rose, the coral reefs on the Central Pacific island of Palmyra not only survived but thrived. The researchers uncovered local ocean processes that provided much-needed support to these reefs during the intense El Niño. Their discovery helps us understand why coral reefs react differently to similar stressors.

“El Niño was not something we expected to benefit the corals,” says Michael Fox, a coral reef ecologist at KAUST. “What caused the death of coral reefs on equatorial islands in the Central Pacific resulted in beneficial conditions a bit further north. The surprising part is that something good for corals could happen during such an intense El Niño.”

Research team uncovers fascinating chain of events

The beneficial outcome Fox mentions refers to a fascinating chain of events during 2015-16. Coral reefs on Palmyra survived, while those only 700km south on Kiritimati and Jarvis islands suffered severe damage.

El Niño weakens the ocean current at the equator, depleting nutrient-rich waters typically delivered to the surface. However, slightly further north, Fox and his colleagues noted an increase in the strength of the North Equatorial Counter Current, especially as it approached Palmyra’s western shores.

In concert with a shallower sea surface layer around Palmyra, these currents ushered in cooler, plankton-rich waters to the island’s coral reefs. This vital influx allowed the reefs to combat the heat stress triggered by the ocean surface temperature increase.

The team found that the processes that saved the Palmyra reefs during the 2015-16 El Niño were also present during the other two major El Niño events in the past fifty years. They discovered a promising pattern — these processes might help these corals survive even the most extreme marine heatwaves in the future.

“The biggest threat to coral reef ecosystems worldwide comes from heatwaves associated with El Niño,” Fox explains. “These events can cause mass coral mortality across vast areas. Identifying coral reefs that have a higher survival chance during these extreme events is crucial for understanding the future of coral reef ecosystems.”

Does this phenomenon occur in other parts of the world?

This new understanding of coral survival during El Niño raises an intriguing question: Are there other locations where ocean conditions during El Niño help reefs fare better than expected?

Fox believes this study offers a guide to search for such resilient reefs. “Our findings provide a roadmap for finding reefs that resist the global trend of decline. This information is vital for Pacific islanders deciding where to establish marine protected areas or planning for the future.”

The researchers chose Palmyra, a U.S. federally protected, uninhabited island home to a small research station, for this study. This isolated ecosystem allowed the scientists to make precise connections between ocean processes and coral survival.

Joseph Pollock, senior coral reef resilience scientist of The Nature Conservancy’s Hawaii & Palmyra Programs, asserts, “Palmyra Atoll serves as an ideal living laboratory to study climate change impacts on coral reefs and, importantly, to identify keys to reef resilience.”

“Palmyra’s remoteness and protected status have helped maintain its reefs’ ecological integrity, whereas many global reefs have suffered from local stressors like pollution or overfishing,” Pollock explains. “By comparing Palmyra’s reefs with those in more degraded areas, researchers can isolate and better understand climate change’s specific impacts.”

What’s next for this coral reef research team?

This surprising study is a collaboration involving KAUST, Bangor University in the U.K., Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration in the U.S., and The Nature Conservancy. Looking ahead, Fox’s Ecological Oceanography Lab at KAUST aims to continue its research in Palmyra and extend its focus to the Red Sea.

“Our group combines organismal physiology, community ecology, and oceanography to identify patterns and processes to help reefs survive climate change. We are exploring similar questions in the Red Sea and working to identify physical processes that can help these unique reefs persist in such an extreme environment,” Fox concludes.

Unearthing such discoveries about how and why coral reefs respond differently to the same stressors could be pivotal. It may allow conservationists to identify naturally protected reefs that can serve as the foundation for repopulating more climate-exposed corals.

The fight to save our coral reefs may have a tough road ahead, but the invaluable lessons learned from Palmyra Atoll have brought new hope to the table.

More about coral reefs

Coral reefs are underwater ecosystems characterized by reef-building corals. These remarkable structures house a myriad of species, making them one of the most diverse habitats in the world. Occupying less than 0.1% of the ocean’s surface, they provide a home for approximately 25% of all marine species.

Coral reefs are not only critical for biodiversity, but they also hold enormous economic value. They provide billions of dollars in services such as tourism, fisheries, and coastal protection. Furthermore, reefs are a significant source of new pharmaceuticals, and they are invaluable in terms of cultural heritage for many coastal and island communities around the world.

However, despite their importance, coral reefs are among the most threatened ecosystems on Earth due to climate change and other anthropogenic disturbances. Here are some of the ways climate change impacts coral reefs:

Coral Bleaching

This phenomenon is one of the most visual and damaging effects of climate change on coral reefs. Coral bleaching occurs when corals undergo stress due to changes in conditions such as temperature, light, or nutrients. These stressful conditions lead the corals to expel the symbiotic algae living in their tissues, causing them to turn completely white (hence the term “bleaching”). This doesn’t kill the coral immediately, but it puts them at a higher risk of mortality. The rise in ocean temperatures due to global warming has led to more frequent and severe bleaching events.

Ocean Acidification

As levels of atmospheric CO2 increase due to human activities, more of it is absorbed by the oceans, leading to their acidification. Corals, much like other marine organisms with calcium carbonate structures, struggle to maintain their skeletal system in an increasingly acidic environment. This can hamper coral growth and make them more susceptible to damage.

Increased Storm Intensity and Frequency

Climate change is expected to bring more frequent and severe storms. These can physically damage or destroy coral reefs.

Sea Level Rise

Rising sea levels can affect coral reef growth. If sea level rises too quickly, deeper waters can limit the amount of sunlight reaching the corals, affecting their growth and survival. Conversely, if sea levels drop too quickly, corals may find themselves exposed above the water line.

Changes in Ocean Currents and Weather Patterns

Climate change can lead to alterations in ocean currents and weather patterns, which can affect the distribution of nutrients and food availability for corals and reef creatures. Changes in these oceanic and atmospheric circulations can also affect the recruitment and dispersal of coral larvae.

In addition to climate change, other human activities, such as overfishing, destructive fishing practices, pollution, and coastal development, are also contributing to the rapid decline in coral reefs. These threats, combined with the impacts of climate change, pose a significant challenge to the survival of coral reefs.

Despite the bleak outlook, there is ongoing work to protect and restore coral reefs. Strategies include reducing greenhouse gas emissions, managing local stressors like overfishing and pollution, and various restoration techniques such as coral gardening and reef rehabilitation projects.

In some instances, researchers are working on breeding “super corals” that can withstand warmer and more acidic waters. Understanding the resilience mechanisms of corals, like the recent findings related to El Niño events, can also contribute to these conservation efforts.