Assessing two years of back-to-back reef changes in the Chagos Archipelago

In April 2017, Dom Andradi-Brown from the ORC group joined a team of 14 scientists from the Zoological Society of London, Bangor University, Warwick University and the Scottish Association of Marine Science for two weeks of coral reef surveys in the marine reserve of the British Indian Ocean Territory. While logistically and scientifically the expedition was a great success, the coral reef health surveys suggested there has been widespread coral death over the past couple of years, as Dom explains:

Last year during the April expedition, we recorded that there had been a coral die off in the shallows, particularly with the large plating and branching Acropora corals that was previously the dominant coral cover species on these reefs. The reefs were characterised in the shallows by many large upturned Acropora plates, with the few Acropora colonies still alive looking heavily diseased.

Dead Acropora plates being surveyed in Chagos

Many of the other branching corals, such as Pocillopora, were still alive but showing signs of bleaching.

Bleaching, which is caused by high sea temperature combined with sunlight exposure for a prolonged period of time, doesn’t necessarily kill corals. There are plenty of examples of corals recovering following bleaching events, and in fact these corals that do recover are the focus of research as they may hold the key to coral reef survival through climate change.

The positives from our 2016 trip were there were lots of healthy young corals, called recruits that had settled onto the reef, offering hope that the reefs of the archipelago could recover in time as they have after previous bleaching events.

We left the 2016 expedition slightly apprehensive about what would happen to the reefs next. Would the dead Acropora plates erode down destroying many of the coral recruits? Would the Pocillopora recover following the bleaching?

So it was with much trepidation we returned this year to see what further changes there had been. At first glance the shallow reefs looked fairly similar, after all, the plating Acropora that had previously died off had been the dominant coral at many sites.

However, as we got further into surveys we began to notice that the many other branching corals, such as the Pocillopora, that were bleaching as we left last year had now died as well.

In several places we encountered large rubble patches, most likely caused from the erosion and breakdown of the branching corals that had died over the previous two years.

Rubble patch from broken down branching corals

Unfortunately for the reefs of the Chagos Archipelago it seems they have had two years of back-to-back change. Despite this there are a few glimmers of hope.

Generally the deeper reefs below 20 m depth appeared reasonably healthy, with high coral cover. In the shallows, the remaining living corals (mostly Porites species) seem in good condition and there was no sign of further bleaching in progress while we were there.

Many of the coral recruits we observed last year have survived and grown. As part of our survey work this year we were interested in tracking reef recovery. So we have identified individual young Acropora colonies, measured their surface area and 3D structure to be able to track their growth over the next few years.

A healthy Porites colony on the reef

There is hope that the reefs of the Chagos Archipelago can recover, as a similar coral die-off happened back in 1998 from which the reefs recovered. However, the key question is the frequency with which bleaching occurs, and whether there will be time for recovery before the next big bleaching event.

Thanks to the Bertarelli Foundation for funding the expedition as part of the Bertarelli Programme in Marine Science. For updates from when we were in the field, please search for the hashtag #BIOTExp17 on Twitter.

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