A first glimpse of corals: the story so far

by ZSL on

A team from ZSL and the Chagos Conservation Trust (CCT) have carried out a rapid reef assessment in the British Indian Ocean Territory (BIOT), funded by the Bertarelli Foundation, looking at how two years of extreme sea temperatures have affected the corals. CCT's Professor Charles Sheppard and Anne Sheppard share what they've seen from their dives. 

Coral in Chagos
The majority of corals have died with their soft limestone skeletons eroding

The deep green islands and beaches of bright, pale yellow look as enchanting as ever.  But beneath the water some profound changes have taken place in the last couple of years. The first clue comes from the water which is much more murky than it used to be, with a relatively large suspended sediment load. The reef condition itself shows why. Down to 15 metres depth, the large majority of corals are dead and their soft limestone skeletons are eroding. Skeletons of the once high cover of diverse corals have a film of very fine filamentous algae over them, and a covering of dusty sediment that comes partly from the eroding corals. It is not a pretty sight.  

A happier story develops below 15 or 20 metres deep. There, the percentage of reef surface that is covered with live corals increases to more healthy levels. There is a selection of species occurring though: some groups are more resilient or susceptible than others. To date, after six dives by both of us, we have yet to see a single live example of the brain coral Ctenella chagius. This is our endemic brain coral, which used to be common. Other corals, whole groups of them, are either missing or very scarce too. But a caveat is needed before we can make firm conclusions – we have dived only six times so far, but certainly the numbers of many are severely reduced.

Coral in Chagos
The water is murky with a relatively large suspended sediment load

This did happen before, in 1998, when water warmed and recovery after that took place, although it was about a decade before the scene on the reefs returned to its normal vibrant state. So, recovery could happen again. The reason for the mortality is warming water. We have so far recovered three of the temperature recorders we placed years ago and which have been recording sea temperature at various depth intervals every two hours since. They show clearly the warming that occurred through to last year when, around October, the temperatures dropped again, bringing respite. But much damage was done, and shallow water, unsurprisingly, warmed the most which is why the shallow 15 metres of reefs are the worst affected.

We are continuing to recover the deployed recorders, and to measure coral cover, to help form more accurate conclusions. We are also counting juvenile corals, to obtain estimates of how quickly and how easily corals may recolonise. More on that later…

Find out more about the expedition by following #BIOTExp17 on Twitter. 

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