Mesophotic Coral Ecosystems: The Chagos Archipelago

Abstract

The Chagos Archipelago, located in the central Indian Ocean and officially known as the British Indian Ocean Territory, contains some of the most remote reefs in the Indian Ocean. The Chagos Archipelago is comprised of a series of atolls, including the largest atoll in the world, the Great Chagos Bank. Records from surveys of mesophotic coral ecosystems (MCEs; reefs 30–150 m depth) in Chagos stretch back to 1905, with more extensive work conducted in the 1970s and post-2010. Coral and fish communities vary considerably with depth and among habitat types. Coral cover generally declines with increased depth across the shallow reef to MCE depth gradient, though in several locations close to 100% scler- actinian coral cover has been observed on MCEs. Consistent with earlier studies, we identify five coral spe- cies as indicative of Chagos MCEs. Recently collected fish community data are analyzed to illustrate, for the first time, patterns in reef fish species richness, abundance, biomass, and trophic groups across a shallow to upper- MCE depth gradient (0–60 m). Fish species richness, abundance, and biomass declined with increased depth, while richness, abundance, and trophic group patterns were also influenced by habitat type (seaward versus lagoonal reef). To date, the vast majority of MCE research in Chagos has focused on upper mesophotic depths. We recommend future work consider the full MCE depth range within the Chagos Archipelago. Keywords