Back-to-back heatwaves kill more than two-thirds of coral

Study shows dramatic loss of central Indian Ocean coral reefs between 2015 and 2017

Researchers from ZSL, by comparing reefs before and after two extreme heatwaves only 12 months apart, have found that hard corals in the central Indian Ocean plummeted by an estimated 70%. Despite this, their results suggest that some coral species are more resilient to rising temperatures which offers hope for these vital habitats. 

Close up underwater photo of bleached white coral with small silver fish swimming around

For nearly eight weeks in 2015, seawater temperatures surrounding reefs in the British Indian Ocean Territory (BIOT) were unusually high. The team compared surveys of the seafloor taken before and afterwards to map changes that this increased water temperature caused to the archipelago’s coral reefs. 

Their analysis, published today in the journal Coral Reefs, shows that the 2015 heatwave killed 60% of BIOT’s hard corals at depths of up to 10 metres, with some species more affected than others. 86% of Acropora corals, for example, previously the most abundant, perished. 

Before corals were given a chance to recover another heatwave struck BIOT just one year later, lasting for over four months. Although researchers were unable to assess its impact across all the islands, data they collected from the Peros Banhos Atoll showed that 68% of the remaining hard corals were bleached and 29% died, suggesting that approximately 70% of hard corals were lost between 2015 and 2017 overall. 

Underwater close-up photo of rock with new coral formations growing

Interestingly, although the second heatwave lasted longer, fewer of the surviving corals were killed. Researchers believe that the remaining corals are more resilient to rising temperatures and that their ability to endure and regenerate may be key to protecting reefs from climate change-induced rises in sea temperatures. 

Marine biologist and lead author, Dr Catherine Head of ZSL’s Institute of Zoology said: 

“We know it has taken about 10 years for these reefs to recover in the past but, with global temperatures rising, severe heatwaves are becoming a more regular occurrence, which will hinder the reef’s ability to bounce back. 

“Our data shows the event in 2016 was worse than in 2015, but it did less damage. We think this is because the 2015 heatwave killed off the more vulnerable species, and those that survived were more tolerant of hotter temperatures. Sadly, preliminary reports from April 2019 suggest another period of high sea temperatures has led to further coral bleaching in BIOT, though we don’t yet know how serious it is.

“It is encouraging that reefs may have some degree of natural resilience, though further research is needed to understand the mechanisms by which some corals are able to protect themselves. This may be our best hope to save these vital habitats from the catastrophic effects of climate change.”

Very close-up photo of a section of bleached coral in detail

Hard corals (Scleractinia) are animals closely related to sea anemones. Most form colonies where individuals, called polyps, grow together and each polyp builds a tough, calcium carbonate skeleton to protect their otherwise soft bodies. Hard corals are the building blocks of coral reefs, creating the habitats that sustain a quarter of all marine species. They are also incredibly sensitive to high temperatures which can cause them to bleach and even die. Although coral reefs are declining worldwide, it is often difficult to identify whether local disturbances – like fishing or pollution - or more widespread factors – like climate change - are to blame. By studying BIOT, which has been largely uninhabited since the 1970s, researchers were able to investigate the effect of climate change on reefs relatively undisturbed by local human activity. 

ZSL has been actively working in BIOT for more than 10 years to protect the 55 islands that make up the archipelago and the 60,000 square kilometres of coral reefs that surround them. We were instrumental in establishing the largest no-take Marine Reserve in the Indian Ocean in 2010 and coordinate the Bertarelli Programme in Marine Science which comprises a team of 50 researchers from 14 institutions across the world. This study was funded by the Bertarelli Foundation. On 1 April 2019 ZSL launched a new project funded by the UK government to understand the impact of plastic pollution on the BIOT natural environment.

Learn more about our work in BIOT

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