Some coral reefs are keeping pace with ocean warming

Some coral communities are becoming more heat tolerant as ocean temperatures rise, offering hope for corals in a changing climate.

After a series of marine heatwaves hit the Phoenix Islands Protected Area, or PIPA, in the central Pacific Ocean, a U.S. National Science Foundation-funded study finds the impact of heat stress on the coral communities lessened over time.

"The protected area gives us a rare opportunity to study pristine and isolated coral reef ecosystems," said co-author Anne Cohen, a marine scientist at the Woods Hole Oceanographic Institution.

While a 2002-03 heatwave devastated coral communities in PIPA, the reefs recovered and experienced minimal losses during a similar event in 2009 and 2010. Then, in 2015 and 2016, a massive heatwave put twice as much heat stress on the corals, yet the die-off was much less severe than expected, according to the new research published in Geophysical Research Letters.

The authors of the study suspect that heat-tolerant offspring from the surviving corals are repopulating the reefs, allowing the community to keep pace with warming seas, at least for the time being.

The study could help coral reef managers identify coral communities most likely to survive in a warming ocean, improving conservation and restoration outcomes.

"It's easy to lose faith in coral reefs," said first author Michael Fox, a coral reef ecologist at WHOI. "But in PIPA, which is protected from local stressors, and where reefs have enough time to recover between heatwaves, the coral populations are doing better than expected."

Just like on land, heatwaves underwater are becoming more frequent and intense as the world warms, putting stress on ocean ecosystems. High temperatures hit coral reefs hard by causing widespread coral bleaching events, where corals eject the symbiotic algae from their tissues, further weakening the animals. With continued ocean warming, many coral reefs face a dim future.

Dan Thornhill, a program director in NSF's Division of Ocean Sciences, added, "This study provides strong evidence that corals adapt rapidly to warming ocean temperatures. The resilience of corals in the Phoenix Islands Protected Area is certainly encouraging. But it remains to be seen whether increased temperature tolerance in coral populations can match the rapid warming and extreme events we are likely to see with climate change."