Exploring the twilight zone in the Chagos Archipelago

For the past couple of weeks Catherine Head and I from Oxford’s Ocean Research and Conservation (ORC) group have been lucky enough to take part in the Berteralli Foundation Chagos Archipelago expedition. The Chagos Archipelago, officially known as the British Indian Ocean Territory, is the largest continuous no-take (i.e. no fishing) marine protected areas in the world, covering 397,667 sq miles. On the expedition we’re involved in studying the health of the reefs, particularly in the face of widespread coral bleaching currently occurring (see Catherine’s recent blog post). In addition to this reef health monitoring, a major focus for my work is to conduct some of the first exploration of the twilight zone reefs of Chagos.

Trevallys (left) and large, fragile sea fans (right) on twilight zone reefs at 58m in Chagos.

The twilight zone, known scientifically as mesophotic coral ecosystems, includes coral reefs from 30m to 150m depth. These reefs are characterised by light dependent ecosystems, but adapted to very low levels of light. Due to the remote nature of the archipelago, in recent times diver surveys have been limited to a maximum depth of 25m, so most twilight zone reefs in Chagos have never been scientifically surveyed.

So why are we interested in the twilight zone?

Many of the impacts that cause most damage on shallow reefs in Chagos, for example processes such as coral bleaching and direct storm damage, are believed to decline in severity at greater depths. This means that twilight zone reefs may act as a refuge for shallow reef life.

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Dom setting up the ROV unit.

We’re using a remote operated vehicle (ROV) to survey the upper twilight zone around the Chagos Archipelago in the 30-60m depth range. Already we’ve had many exciting findings! For example, the charismatic Chagos Clownfish (Amphiprion chagosensis), found only in Chagos, had previously been found down to 25m, we’ve extended that known depth range down to 37m after documenting several individuals in an anemone off Peros Banhos in the north of Chagos earlier in the expedition.

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The endemic Chagos Clownfish (Amphiprion chagosensis) adjacent to an anemone at 37m.

The structure of the reef changes a lot in the twilight zone. One of the most common corals found on the shallow reefs of Chagos belong to the genus Porites. On shallow reefs these corals have distinctive rounded boulder shapes. At twilight depths we’ve documented very flattened plate-like Porites colonies. We think this change in shape is an adaptation to the lower light levels on these deeper reefs, as this pattern has been observed on twilight reefs elsewhere in the world. However, researchers are still trying to understand the advantages to corals of becoming flatter, particularly at the fine scale (something Jack Laverick in the ORC group is actively working on).

Shallow reef Porties (left) is much more rounded, whereas Porties found in the twilight zone forms flattened plates (right).

As well as the seabed reef-specific twilight zone surveys, when deploying the ROV we’ve often found lots of sharks at twilight depths. Mostly these have been grey reef sharks (Carcharhinus amblyrhynchos) that have been interested in the ROV unit, circling in closer to look. On a couple of occasions, during ROV surveys in one of the Chagos atoll lagoons we found black-tip reef sharks (Carcharhinus melanopterus). What is clear from the ROV surveys is that sharks in Chagos are regularly visiting twilight reefs, further reinforcing the importance of these deeper reef habitats to larger mobile predatory species in the marine reserve.

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Grey reef sharks at 30m on a ROV twilight reef survey.

In the Middle of the Indian Ocean

Dom and I are lucky enough to currently be at sea in the middle of the Indian Ocean on this year’s Bertarelli Foundation expedition to the Chagos Archipelago – known as the British Indian Ocean Territory. Chagos has the world’s largest no-take marine protected area and I have been involved in research here on Chagos’ reefs for the last 5 years. This year’s multi-disciplinary expedition team includes scientists from the Zoological Society of London, Stanford University, Bangor University, University of Western Australia and the University of Oxford. The team’s priorities include tagging of pelagic animals such as sharks and manta rays, maintenance of the array system to gather information on the movement of these pelagic populations in and around the archipelago, and monitoring the health of the reef ecosystem with a particular focus on the coral bleaching event that is predicted this year.

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The reef team! Left to right: Luke (crew member), Dan Bayley, Heather Koldewey, Hans Dejong, Rob Dunbar, Dominic Andradi-Brown, Ronan Roche and Catherine Head

The reef team’s work is focusing on reporting the affects of coral bleaching on the reef life here. Coral bleaching is when the coral cells expel their symbiotic zooxanthellae, which are single-celled algae that live inside the coral cells and photosynthesise providing the coral with energy and in return the zooxanthellae gain a secure environment to live. This bleaching is known to be a stress response to increases in sea surface temperature, caused by El Nino climate patterns and exuberated by human-induced global climate change. Wide spread bleaching has been reported in the archipelago in the past, most severely in 1998 and 2005 which resulted in the die-off of many corals. Importantly though Chagos reefs are so resilient, due to the lack of direct human impacts, that they recovered quicker than any other reefs in the Indian Ocean. More recently the archipelago experienced bleaching last year, which our surveys suggest has caused mortality to many of the table corals (Acropora), and as expected the reef is now beginning to bleach again this year.

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A still mostly healthy reef largely unaffected by the bleaching to date (Brothers Reef, Chagos)

As part of the reef team Dom and I have been quantifying the 3D structure of the reef itself and to see if this has any relationship with the number of bottom-dwelling reef fish, such as damselfish. We do this by filming quadrates of the reef that will later be converted into 3D models using a specially developed pipeline, designed by Grace Young in the ORC group. We also put out GoPro cameras on the quadrate locations to record the fish life, so we can compare the two.

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Dom undertaking reef complexity surveys

In addition we have been undertaking surveys to quantify the health of the reef and the extent of bleaching. One of the positive things we are seeing is high numbers of coral juveniles, giving hope for the recovery of Chagos reefs.

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High numbers of juvenile coral recruits on the reef