Coral reefs can adapt in response to mild marine heatwaves

A team of scientists from James Cook University (JCU) and the Australian Institute of Marine Science (AIMS) have found that some coral species can adapt to increasing temperatures that cause bleaching, but only when marine heat wave events remain mild.

Magena Marzonie, an AIMS@JCU Ph.D. candidate, is the lead author of a new paper examining how corals react to temperature extremes in isolated reefs of the Coral Sea Marine Park. She found that corals from different reefs showed considerable variation in their bleaching level in response to high water temperatures.

"Our experiments show that tolerance to heat stress differs between species of corals and between reefs," she said. "These differences suggest corals may be adapted to their local environmental regimes and are responding to changes in temperatures that have occurred due to climate warming over the past 35 years."

The team of scientists collected specimens of three coral species at nine reefs in the Coral Sea Marine Park during a 30-day voyage in 2020. Using a purposefully designed portable aquarium system, they conducted experiments at each reef to measure the upper temperature limits at which corals can survive. The ship-borne aquarium system is a miniaturized version of AIMS' National Sea Simulator, a world-class research aquarium in Townsville and allows scientists to undertake coral stress tests in the field.

The sampled reefs spanned 860 km and a range of environmental conditions including differences in sea surface temperature and the frequency and intensity of past marine heatwaves to identify how these can influence heat tolerance in corals.

Dr. Hugo Harrison, a Senior Research Fellow at JCU's ARC Center of Excellence for Coral Reef Studies and AIMS, is the lead of this study of adaptation to climate warming of corals in the Coral Sea Marine Park in partnership with Parks Australia.

He said, "Corals in this region regularly experience marine heatwaves, where they are exposed to temperatures near, or above, their upper limits. We wanted to test if reefs exposed to more frequent heatwaves in the past have developed enhanced capacity to withstand these events in the future. And this is exactly what we found."

"Coral reefs with the most heat tolerant corals have experienced more heatwaves over the past 35 years. Conversely, reefs which have escaped these heatwaves tended to have less heat tolerant corals," said Dr. Harrison.

Ms. Marzonie said while mild marine heatwaves may assist evolutionary adaptation to heat stress, she warns that such gains are undermined by the severe marine heatwaves such as those that led to mass coral bleaching in recent years.

"We also found that very severe heatwaves between 2016 and 2020 reduced corals' capacity to withstand higher temperatures. Coral can adapt—but only if they survive," said Ms. Marzonie.

Dr. Line Bay, Principal Research Scientist at AIMS and co-author and collaborator on this study said that marine heatwaves are now more frequent and last longer, which is a major threat to coral reefs globally. "It's imperative that we understand how corals can adapt to these events to better predict reef futures so we can develop the knowledge to support resilience-based management in a changing climate," said Dr. Bay.

The paper was published in the journal Global Change Biology.