Climate-driven declines in 3D structure of shallow reef slopes in the central Indian Ocean

The physical structure of coral reef habitats is known to be a strong driver of the abundance and diversity of associated marine organisms, and furthermore gives a clear indication of a reef’s health and resilience to... [ view full abstract ]

The physical structure of coral reef habitats is known to be a strong driver of the abundance and diversity of associated marine organisms, and furthermore gives a clear indication of a reef’s health and resilience to disturbance. Alongside the morphological structure of the reef, the composition of reef benthos gives an indication of the level of carbonate accretion or erosion occurring and therefore how well the ecosystem functions. 

The Chagos archipelago lies approximately 500 km south of the Maldives archipelago. Less than 1% of the 650,000 km² Exclusive Economic Zone is inhabited, and the remaining area has been a UK managed no-take MPA since 2010, making this area one of the least disturbed coral systems in the world. In early 2015 to late 2016 the region was affected by an intense El Nino heating episode which caused much of the region's corals to bleach. We used ‘Structure from Motion’ (SFM) photogrammetry to quantify and measure subsequent changes to reef forms. The SFM methodology allows reefs to be analysed quantitatively using a number of relevant metrics such as rugosity, surface texture, volume, and coral growth-form composition. 

This work quantitatively shows how the physical structure of these reefs have changed through time due to climate-induced mortality, away from the various confounding anthropogenic impacts normally experienced in such systems. The effects of the recent bleaching episode have been severe, causing widespread die-off of the shallow-lying coral reef areas across the archipelago, followed by rapid loss of structure through physical and biological erosion.