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 in the Department of Zoology, in collaboration with the Zoological Society of London (ZSL) and other institutes, have found that hard corals in the central Indian Ocean plummeted by an estimated 70% after two extreme heatwaves only 12 months apart. Despite this, their results suggest that some coral species are more resilient to rising temperatures which offers hope for these vital habitats. 

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 Acroporacorals, 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. 

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 climage change-induced rises in sea temperatures.

Dr Gwilym Rowlands, from the Department of Zoology at the University of Oxford, and one of the study’s authors said: “The paper illustrates the importance of considering historical exposure when assessing or predicting the response of biological communities to climate change. By examining five years of satellite data for the region, our study illustrates how variable marine heatwaves can be, both in terms of spatial extent and intensity. By monitoring coral reef systems across a wide geographic area and across multiple years, we demonstrate how the most substantive changes to coral community were associated with the stress event monitored in 2015, rather than more severe or prolonged period of heat stress that followed."

Marine biologist and lead author, Dr Catherine Head of ZSL’s Institute of Zoology (formally of the Department of Zoology and now a Research Associate), expands on this view adding: 

“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."  

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.