BIOT covers approximately 650,000 km2, in the central Indian Ocean, with a central core approximately 400 x 300 km across which contains five islanded atolls and numerous submerged atolls and banks. These are all built of limestone and rest on a limestone platform mostly 0.5 to 2 km thick, overlying a volcanic substrate. The shallow areas relevant to this BAP therefore are composed entirely of biogenic limestone reefs, mainly of coral and algal origin. This biogenic limestone is a relatively soft rock, whose continued creation to offset natural erosion is essential for maintenance of the reefs and of the islands supported by them.
In terms of biodiversity, this is a rich site in Indian Ocean terms for several groups of benthic biota, with over 300 coral species and with fish counts exceeding 800 species, although seagrasses show an impoverished biodiversity. However, all this is based on older data, and there has been a recent, substantial decline in the condition of shallow water habitats since the surveys were carried out. At present, any biodiversity assessments are in a state of flux; there have been several multi-annual rises and falls in condition but a multi-decadal decline since at least the 1970s.
Threats:
Threats are predominantly from ocean warming which is severely affecting the reef-building corals and hence endangering the main shallow structures of the archipelago, including islands. Warming episodes are now being called ‘marine heatwaves’. In addition to an important background rise in seawater temperature measured in the archipelago of nearly half a degree over the last decade, there are warming pulses that are significantly greater which last for several months, usually between March to May. These marine heatwaves cause bleaching and then usually mortality not only of the reef building corals but of all benthic biota dependent on an algal symbiosis such as soft corals and “false” corals that occupy a substantial proportion of the reef. Other threats exist, but all are minor in comparison over most of this uninhabited archipelago. Biodiversity is supported by corals and soft corals, and the vast range of fauna and flora they support, feed and shelter, and loss of this habitat negatively impacts associated biodiversity. These large losses are relatively recent, so to date there has been little quantitative study of their magnitude, though some extinctions and functional extinctions have been recorded.