In the Pacific, Some Coral Survived the Last El Nino, Thanks to Ocean Currents

El Nino has officially arrived. The trade winds that blow across the Earth weaken during El Niño, causing changes in global weather, including extreme heat in the Pacific. With El Nino’s recent arrival, there are already concerns about corals in the eastern Pacific experiencing heat stress in the high temperatures.  

In the 2015-16 El Nino, the heat of the cyclical climate pattern combined with the extreme heat caused by global warming  drove the longest global coral die-off event ever recorded up to that time. 

New research published on Wednesday in Science Advances found that, while a lot of coral in the Pacific died during the bleaching events, El Nino caused ocean conditions to change near the central pacific atoll of Palmyra, a national wildlife refuge about halfway between Hawaii and American Samoa, and brought much needed nutrients to heat stressed coral in the area. 

While this study only focused on Palmyra, it’s possible there are other areas where geography and different conditions in ocean currents can help coral survive during ocean heat waves, which are becoming increasingly common. Coral reef managers who monitor restoration projects can use this information to better make more informed decisions about which areas to prioritize. 

“What this tells us is that there are processes in the ocean that are driven by much larger climate and ocean patterns that have really important consequences for small-scale things like reefs and individual corals,” said Michael Fox, assistant professor at King Abdullah University of Science and Technology and a lead author on the study. 

Surviving Bleaching

During extreme heat, coral undergo a process called bleaching, in which they push out algae living in their tissues, turning their color white. This algae normally provides food for the coral, which lose an important source of nutrition during bleaching events. 

But algae isn’t the coral’s only food source. “The most underappreciated aspect of coral biology is the fact that they’re very good at eating things,” including plankton, Fox said. Often during extreme heat, the plankton the coral eat also suffer. But in Palmyra, changes in currents and other oceanic processes brought plankton and nutrients to the reefs and allowed them to survive during a bleaching event. 

Because Palmyra is isolated, Fox said, researchers can understand how reefs work without dealing with pollution caused by humans.

“Palmyra Atoll provides an opportunity to study the impacts of global threats—like warming oceans—in an ecosystem largely unaffected by local stressors such as pollution or overfishing,” Joseph Pollock, a senior coral scientist at The Nature Conservancy, said.  

During the El Nino in 2015-16, a current that affects Palmyra called the North Equatorial Counter Current intensified. This current, when combined with a shallower surface layer around Palmyra, caused the upward movement of cooler, nutrient rich water, known as an upwell.

This process brought plankton to Palmyra’s coral reefs during the bleaching event, giving them access to a larger food source than other reefs in the area. 

The upwell likely helped Palmyra, but other Pacific islands lost the upwelling they normally have during El Nino. When combined with the warming of the water by climate change, the 2015-16 El Nino had a devastating effect on many other reefs in the pacific. 

For instance, Kiritimati and Jarvis, islands approximately 435 miles south of Palmyra, experienced some of the most extreme heat scientists had seen in that area. This heat, along with the lack of upwelling normally seen in those islands, led to much of the coral dying. 

It’s difficult to make exact comparisons between the impacts on Palmyra and these other islands, especially since the reefs surrounding Kiritimati and Jarvis experienced some of the most extreme temperatures that area has ever seen and were hotter than Palmyra. So while the coral didn’t have access to plankton to supplement their food, “it was also so hot that some of them may have just been cooked,” Fox said. 

Knowledge about how larger systems like El Nino impact coral reefs that are relatively close to each other can provide important insights for coral reef managers, he said. Because these managers are trying to monitor so many other problems, such as pollution and overfishing, knowing more basic information about how coral reefs function can help inform management decisions. 

Verena Schoepf, an assistant professor at the University of Amsterdam who was not involved in the study, said certain methods used in the study were “elegant and powerful.” 

While Palmyra reefs experienced benefits from the extra nutrients that upwelling provided in this case, a different study showed that upwelling of nutrients and extreme heat actually increases coral bleaching in the Red Sea, Schoepf said. 

“What I find particularly interesting about this specific study is that it shows that these impacts can vary depending on regions,” Schoepf said. 

“A Broader Puzzle” 

If researchers and managers can use these findings to determine which reefs might be more naturally protected from climate change, their decisions on where to focus resources will improve, Fox said. 

The data suggests that the ocean processes that benefited Palmyra in the 2015-16 El Nino heat wave also occurred during the three previous El Nino heatwaves. But it’s possible small changes in current and even hotter heat waves could mean this process can’t sustain Palmyra’s reefs in the future.

As El Nino continues to affect the state of the Pacific during the coming months, it’s important to understand how those conditions continue to impact reefs, Pollock, at the Nature Conservancy, said. While these circumstances do have broadly negative effects on reefs, this study shows that those negative effects don’t necessarily apply everywhere. 

“In a sense, this study gives us another clue to a broader puzzle of how reefs are going to function under climate change and what changing ocean conditions mean for them,” Fox said. 

Sometimes natural conditions can help places like Palmyra recover quickly from bleaching events. But, ultimately, if coral reefs experience too many instances of temperature stress in a row, they don’t have enough time to regenerate. 

“This is not a silver bullet for reefs to survive climate change,” Fox said. “What we discovered here is a reason for hope in one particular spot, but we have a lot of work to do to figure out where else this is beneficial.”