Marine heatwaves are more deadly for coral reefs than previously thought

A new study has found that coral reefs are much more threatened by marine heatwaves than what was previously realized.

A team of scientists led by the University of South Wales discovered that the skeletons of coral animals become so severely stressed from the heat that they begin to decay within weeks, which makes the entire 3D reef structure unstable. 

The research shows that just a 0.5°C increase in ocean temperature influences the extent of the mortality associated with coral bleaching.

The study is the first of its kind to demonstrate that marine heatwaves not only trigger the breakdown of symbiotic relationships in coral reefs, but can also lead to the heat-induced death of the coral animals themselves. The findings indicate that such mortality events should be distinguished as a previously unrecognized consequence of coral bleaching. 

“Until now, we have described coral bleaching as an event where the symbiotic relationship between coral and its microbes breaks down and corals lose their main source of nutrition, and the coral can die if the symbiosis is not restored,” said study co-author Professor Tracy Ainsworth.

“But what we are now seeing is that severe marine heatwave events can have a far more severe impact than coral bleaching: the water temperatures are so warm that the coral animal doesn’t bleach – in terms of a loss of its symbiosis – the animal dies and its underlying skeleton is all that remains.”

Study co-author Professor Bill Leggat explained that the skeleton itself is immediately overtaken by the rapid growth of algae and bacteria as a result of higher temperatures. 

“We were able to study the consequences of this process of rapid colonisation using CT scanning of the coral skeleton – as would be used in medical imaging. We show that this process is devastating not just for the animal tissue, but also for the skeleton that is left behind, which is rapidly eroded and weakened,” said Professor Leggat.

The team used new bio-optical techniques which allowed them to visualise and study the abrupt transition in the coral microbiome. Dr. Scott Heron from James Cook University said that this is the first time the rapid dissolving of coral skeletons has been observed.

“Climate scientists talk about ‘unknown unknowns’ – impacts that we haven’t anticipated from existing knowledge and experience. This discovery fits into this category,” said Dr. Heron. “As we begin now to understand this impact, the question is how many more of these ‘unknown unknowns’ might there still be that could bring faster and greater damage to coral reefs from climate change.”

The researchers hope that this new information will motivate the public to express how important coral reefs are to them, urging decision makers to address the immediate need to preserve coral reefs.

“Across the globe coral reefs are still a source of inspiration and awe of the natural world, as well as being critically important to the communities that rely upon them. Given that the degradation of coral reefs will result in the collapse of ecosystem services that sustain over half a billion people, we urgently need actions both globally and locally that protect and conserve these truly wonderful places.”

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: UNSW