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New research in the Indian Ocean shows how two heatwaves between 2015 and 2017 have killed off as much as 70 percent of the coral reefs in the region.

Corals take about a decade to recover from events like this, but UK zoologists suggest global warming means these events could become more frequent.

STORY-LINE:

A kaleidoscope of life and colour set in the crystal clear waters of the Seychelles in the Indian Ocean.

Coral reefs cover less than one percent of the world's ocean beds, yet they are home to a quarter of all marine life on the planet.

However between 2015 and 2016 record temperatures in the southern hemisphere exposed corals around the world to bleaching.

According to many scientific reports it was caused by El Nino Southern Oscillation (ENSO) weather system.

This is an occasional shift in the ocean-atmosphere system in the tropical Pacific. It affects weather around the world.

During the back to back heatwaves scuba diving researchers from ZSL (Zoological Society of London) closely examined the waters ten metres deep off the Chagos archipelago (British Indian Ocean Territory) near the Seychelles archipelago.

The team was focussed on the hard corals which the 2015 heatwave depleted by 60 percent.

Other corals were also affected, 86 percent of the Acropora was killed, previously it had been the most abundant species.

ZSL's Dr Catherine Head has been part of a team which has been investigating the corals over the past seven years.

She says she saw extensive bleaching which showed the corals were under stress.

"Corals take on this white appearance and that's as a result of corals basically expelling their zooxanthellae which are single celled algae which they live in symbiosis with, and these algae provide the corals with a lot of their energy budget. So when they expel these algae they're able to survive for a period of time, but it depends how long the temperatures stay high and how high they go," says Head.

She says there was particular concern about the staghorn coral:

"The staghorn corals for instance which are known to be more weedy species were highly impacted. So 86 percent of the Staghorn corals were actually affected by these heat waves, whereas some of the more robust corals were less affected," she says.

According to Head the hard corals are like the vital infrastructure of an environment, upon which other species are able to live and thrive.

Head says: "So hard corals are really the foundations of the reefs. They're what build the reefs themselves because they lay down a calcium carbonate skeleton and that actually forms the physical structure of the reef and allows many other creatures to inhabit them, provides them with homes and a food source."

Over the two heatwaves the ZSL researchers found seventy percent of the hard coral structures had been killed.

The findings are reflected in studies carried out by other researchers earlier this year.

Leo Barret is the Project Leader for Marine Conservation Society in the neighbouring Seychelles archipelago (MCSS).

He says: "We lost up to 50 percent of all corals, now three years ago in 2016. This is mainly the branching corals, so we're talking about acropora, for example."

Barret and his colleagues are establishing two several coral nurseries with the aim of rearing stronger corals.

Corals are very important habitats, they support a vast number of other species, some according to Head rely on particular corals to survive:

"The different species of fish that you see for instance would reduce because some fish species are reliant on just one particular coral. We have species of butterfly fish for instance which are corallivours, they eat certain corals, so certain species of coral. So some of these species are likely to die out or at least become locally very rare. So we will see a reduction in the diversity of the fish and the invertebrates which live on the reefs and perhaps also a reduction in the structural complexity of the reef. So some corals contribute more to the calcium carbonate structure than others. And that's what causes this really complex, really interesting, integral structure. So we could see a reduction in that as well."

Scientists are concerned the heatwaves are increasing in regularity as well as becoming more severe.

Head says: "So if these heatwaves become more frequent as they are predicted to do, then the reefs simply won't have a chance to recover. So there are predictions which say perhaps reefs may go functionally extinct by about 2050 should the (carbon) emissions continue as they are at the moment, but really it depends how much we can reduce our carbon emissions as per the Paris Climate Change Agreement to keep sea surface temperatures below 2 degrees."

The areas around the Chagos are becoming protected, development surrounding the area is restricted as it is on other coral reefs across the world.

But scientists like Head say this alone is not enough and cutting carbon emissions is key.

Archive : Cerf Island, Seychelles - 20 February 2019

1. Various underwater shots of coral reef off coast of Cerf Island

2. Wide of beach

3. Close of crab crawling on sand

London, UK - 11 July 2019

4. Various of ZSL researcher Dr. Catherine Head looking at coral exhibit in the aquarium of London Zoo

5. SOUNDBITE: (English) Dr. Catherine Head, researcher, ZSL (Zoological Society of London)

"Corals take on this white appearance and that's as a result of corals basically expelling their zooxanthellae which are single celled algae which they live in symbiosis with, and these algae provide the corals with a lot of their energy budget. So when they expel these algae they're able to survive for a period of time, but it depends how long the temperatures stay high and how high they go."

6. Various of staghorn coral and fish in aquarium tank in London Zoo

7. SOUNDBITE: (English) Dr. Catherine Head, researcher, ZSL (Zoological Society of London)

"The staghorn corals for instance which are known to be more weedy species were highly impacted. So 86 percent of the Staghorn corals were actually affected by these heat waves, whereas some of the more robust corals were less affected."

Archive : Cerf Island, Seychelles - 20 February 2019

8. Various underwater shot of Indian Ocean coral reef

London, UK - 11 July 2019

9. SOUNDBITE: (English) Dr. Catherine Head, researcher, ZSL (Zoological Society of London)

"So hard corals are really the foundations of the reefs. They're what build the reefs themselves because they lay down a calcium carbonate skeleton and that actually forms the physical structure of the reef and allows many other creatures to inhabit them, provides them with homes and a food source."

Archive : Cerf Island, Seychelles - 20 February 2019

10. Pan right of Leo Barret, Project Leader, Marine Conservation Society Seychelles (MCSS), entering water

11. Close of Barret in water

12. SOUNDBITE (English) Leo Barret, Project Leader, Marine Conservation Society Seychelles (MCSS):

"We lost up to 50 percent of all corals, now three years ago in 2016. This is mainly the branching corals, so we're talking about acropora, for example."

13. Various of diver cleaning coral nursery

London, UK - 11 July 2019

14. SOUNDBITE: (English) Dr. Catherine Head, researcher, ZSL (Zoological Society of London)

"The different species of fish that you see for instance would reduce because some fish species are reliant on just one particular coral. We have species of butterfly fish for instance which are corallivours, they eat certain corals, so certain species of coral. So some of these species are likely to die out or at least become locally very rare. So we will see a reduction in the diversity of the fish and the invertebrates which live on the reefs and perhaps also a reduction in the structural complexity of the reef. So some corals contribute more to the calcium carbonate structure than others. And that's what causes this really complex, really interesting, integral structure. So we could see a reduction in that as well."

Archive :Cerf Island, Seychelles - 20 February 2019

15. Various underwater shots of coral reef off coast of Cerf Island

London, UK - 11 July 2019

16. SOUNDBITE: (English) Dr. Catherine Head, researcher, ZSL (Zoological Society of London)

"So if these heatwaves become more frequent as they are predicted to do, then the reefs simply won't have a chance to recover. So there are predictions which say perhaps reefs may go functionally extinct by about 2050 should the (carbon) emissions continue as they are at the moment, but really it depends how much we can reduce our carbon emissions as per the Paris Climate Change Agreement to keep sea surface temperatures below 2 degrees."

Archive :Mahe, Seychelles - 20 February 2019

17. Various underwater shots of coral reef off coast of Cerf Island

18. Wide of sunset on beach

19. Close of waves on beach