Back-to-back heatwaves in the central Indian Ocean killed more than two-thirds of corals in two years. But some corals were more resilient to the high sea temperatures, which could provide hope for the important habitat as the planet warms
Catherine Head at the Zoological Society of London and her colleagues studied reefs in the remote Chagos Archipelago of the British Indian Ocean Territory before and after two ocean heatwaves with unusually high sea temperatures, which came 12 months apart.
In 2015, seawater temperatures around reefs in the territory were unusually high for nearly eight weeks, and sea-floor surveys before and after the heatwave saw live healthy coral cover fall by 60 per cent.
Before the corals could recover, they were hit by another ocean heatwave in 2016, lasting for more than four months. Although the team was unable to assess the impact of the second heatwave across all the islands of the archipelago, data from the Peros Banhos Atoll show that 68 per cent of the remaining corals there were bleached and 29 per cent died.
This suggests that about 70 per cent of hard corals were lost between 2015 and 2017. But while the second heatwave lasted longer, fewer of the surviving corals were killed.
The team suggests that the remaining corals are more resilient to rising temperatures and their ability to survive may be key to protecting reefs from rises in sea temperatures driven by global warming.
Hard corals are the building blocks of reefs, which provide a home for about a quarter of all marine species and food, protection and income for some 500 million people worldwide.
Similar coral death and changes to the make-up of species in the reef were seen in the Chagos Archipelago following global coral bleaching in 1998, from which recovery took 10 years.
This relatively rapid recovery suggests that the reef is highly resilient and the lack of disturbance it has from humans – a result of the UK’s controversial removal of local people to make way for a US military base – increases the probability the reefs will recover again over time.
But as these kinds of heatwaves become more frequent, the ability to recover will become “increasingly compromised”.
“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,” says Head. “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.”
She said preliminary reports from April 2019 suggest another period of high sea temperatures has led to further coral bleaching in the British Indian Ocean Territory, although it isn’t yet known 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,” says Head. “This may be our best hope to save these vital habitats from the catastrophic effects of climate change.”
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