https://www.newscientist.com/article/2209750-double-heatwave-killed-two-thirds-of-coral-in-central-indian-ocean/
[links in online article]
Double heatwave killed two-thirds of coral in central Indian Ocean
12 July 2019
By New Scientist Staff and Press Association
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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