Some coral reefs are keeping pace with ocean warming

Credit: Michael Fox. A healthy coral reef in Phoenix Islands Protected Area, 2018.
As ocean temperatures rise, some coral communities become more heat-tolerant. This is good news for corals living in changing climates.

A new study has found that heat stress is lessening over time after a series heatwaves in the middle Pacific Ocean.

PIPA was devastated by a heatwave in 2002-2003, but the reefs were able to recover and suffered minimal damage from a similar event in 2009-10. In 2015-2016, however, corals were subject to twice the heat stress from a heatwave, but the death rate was significantly lower than expected. This is according to research published in Geophysical Research Letters (AGU's journal for high impact reports that have immediate implications across all Earth and space sciences).

According to the authors of the new study, heat-tolerant offspring are being repopulated by corals that survived, which allows the community to keep up with rising sea levels at least temporarily.

This new study will help coral reef managers to identify the coral communities that are most likely to survive in warmer oceans, which could improve conservation and restoration results.

First author Michael Fox (postdoctoral scientist, coral reef ecologist at Woods Hole Oceanographic Institution) stated that it is easy to lose faith with coral reefs. The coral populations are performing better in PIPA than they expected, because the reefs are protected from local stressors and have sufficient time to recover between heatwaves.

Heatwaves underwater

As the world heats up, heatwaves under water are becoming more intense and frequent. This is putting pressure on ocean ecosystems. Coral reefs are particularly affected by high temperatures. This causes widespread bleaching, which results in corals expelled from their tissues. Further weakening of the organisms. Coral reefs are in dire straits if the ocean continues to warm.

In the new study, researchers monitored coral communities at four islands within PIPA, an area encompassing over 400,000-square-kilometers of coral reef and deep-sea habitat. PIPA was designated a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization in 2010. It was established by the Republic of Kiribati in 2008. Anne Cohen, a marine scientist from WHOI, said that the protected area provides a rare opportunity for us to study isolated coral reef ecosystems. She thanked Kiribati for this privilege.

To determine how each heatwave affected corals, the team used daily satellite data as well as temperature loggers. The team excluded 11 environmental factors, including greater cloud cover and more gradual warming, that could explain the corals' higher survival rates after the heatwaves of 2015-2016 and 2009-2010.

The coral cover at the surveyed sites was less than three quarters after the heatwave of 2002-2003. Although the reef was starting to recover after the 2009-2010 heatwave, there were fears of widespread bleaching. However, coral cover had increased more than 5% two years later. The "Super El Nio", which increased ocean temperatures by 3 degrees Celsius (5.4 degrees Fahrenheit), caused coral loss to be 40%, about half the amount of 2002 losses.

Coral reefs have a source of hope

Many reef-building species were able to withstand the heatwaves. Fox stated that areas once devoid of corals in 2002-2003 are now flourishing with the majority of the original species.

Sometimes, only a few species that are particularly hardy and fast-growing recover from bleaching events on other reefs around the world. Coral larvae can travel long distances on ocean currents. However, because of PIPA's isolation, researchers believe that heat-tolerant local individuals are repopulating reefs.

The researchers now need to find out how coral communities can keep up with the ocean warming.

These findings are important for giving hope for coral reefs' future and for supporting protection efforts, said Lizzie McLeod (Global Reef Systems Lead at Nature Conservancy), who was not involved with the study.

She suggests that reef conservationists prioritise the conservation of heat-tolerant corals because they can act climate refuges and repopulate sites destroyed by heatwaves.

The authors of the study warn that even corals as impressive as these have limits. It is crucial to reverse climate change. As heatwaves become more frequent or intense, even heat-tolerant communities could die out.

"We are in a race against the clock, so any news that increases the chances of corals making it is really good news," Nancy Knowlton, Emerita Sant Chair of Marine Science at the Smithsonian National Museum of Natural History said. She was not involved in the study. She said that the corals were doing their part. "We have to do ours."

More information: Michael D. Fox and co, Increasing Coral Reef Residualty Through Successive Marine Heatwaves. Geophysical Research Letters (2021). Information from Geophysical Research Letters Michael D. Fox and co, Increasing Coral Reef Residibility Through Successive Marine Heatwaves. (2021). DOI: 10.1029/2021GL094128