Coral recovery during a prolonged heatwave offers new hope

Danielle Claar scuba diving in Kiritimati
Danielle Claar diving a reef off the coast of Kiritimati. Danielle Claar

Coral reefs serve important ecological functions, from providing habitat for countless species to protecting shorelines from erosion. Reef-dependent fisheries are also a vital source of food and income for hundreds of millions of people in tropical island nations where coral reefs are valued at $6.8 billion annually. 

The pressing concerns of climate change have placed the long-term health of the world’s coral reefs in jeopardy. However, new research inspires hope as some corals managed to survive a recent and globally unprecedented heatwave.

“Understanding how some corals can survive prolonged heatwaves could provide an opportunity to mitigate the impact of marine heatwaves on coral reefs, allowing us to buy time as we work to limit greenhouse gas emissions,” said lead author Danielle Claar, a postdoctoral researcher at the University of Washington who completed the work as doctoral student at the University of Victoria.

A coral reef site on Kiritimati before and after the 2015-16 marine heatwave
A coral reef site on Kiritimati before and after the 2015-16 marine heatwave, May 2015, 2018. Danielle Claar, Kevin Bruce.

Heat stress from the 2015-2016 El Niño event triggered mass coral bleaching and mortality on reefs around the world. The study published Dec. 8 in Nature Communications presents the discoveries made by an international research team as they tracked hundreds of coral colonies on reefs around Christmas Island (Kiritimati) in the Pacific Ocean, where the heatwave lasted an unprecedented 10 months.  

Climate change threatens the world’s coral reefs because corals are highly sensitive to the temperature of their surrounding waters. Warmer waters can trigger a coral bleaching where the coral turns white as it expels the symbiotic food-producing algae living in its tissues. Prolonged bleaching events often cause corals to die from starvation, but they can recover if they reclaim their food source within a few weeks.

To date, coral recovery from bleaching has only ever been observed after heat stress subsides. With global climate models predicting that heatwaves will continue to increase in both frequency and duration, a coral’s ability to recover its food source during a prolonged heatwave is essential to its survival. 

Researchers showed that corals only exhibited this capacity if they were not also exposed to other types of human-caused stressors, such as water pollution. This affirms that local reef management could help improve corals’ chances of surviving climate change.

“We’ve found a glimmer of hope that protection from local stressors can help corals,” said Julia Baum, a University of Victoria marine biologist and the study’s senior author. 

“Although this pathway to survival may not be open to all corals or in all conditions, it demonstrates an innovative strategy for survival that could be leveraged by conservationists to support coral survival,” said Claar. 

The research was supported by the Natural Sciences and Engineering Research Council of Canada, the U.S. National Science Foundation, the David and Lucile Packard Foundation, The Pew Charitable Trusts’ Pew Fellows Program in Marine Conservation, the Rufford Foundation, the Canadian Foundation for Innovation and the Shedd Aquarium.

For more information, contact Claar at danielle.claar@gmail.com 

Adapted from a University of Victoria news release.

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