Archipelago-wide coral recovery patterns since 1998 in the Chagos Archipelago, central Indian Ocean

Patterns of coral recovery are analysed across reefs of the Chagos Archipelago, which spans 400 x 250 km in the central Indian Ocean. This archipelago mostly lacks direct human impacts and is subject only to global changes such as a rise in sea surface temperature. Following very heavy coral mortality (mostly > 90%) caused by the 1998 warming event, and despite 2 further sub-lethal bleaching events, the recovery of coral cover, colony numbers and juvenile recruitment has been good in many parts of the archipelago. There was little discrimination between atolls and depths, with a notable exception of 1 atoll where a repeat of heavy mortality had occurred in 2005. In 2006, coral cover was almost restored to pre- 1998 values at most shallow sites, but had recovered much less in deeper waters. However, in shallow water, coral cover values alone are a poor indication of recovery, because present, shallow cover mainly comes from Acropora palifera and other corals that are largely encrusting in juvenile form, in contrast to their mature condition, in which they provide a 3-dimensional 'forest' structure. Recruitment of juvenile colonies in 2006 ranged from 6 to 28 m(-2). Total juvenile density showed no significant pattern with atolls or with depth, but, taking each genus of juveniles in turn, many genera showed a marked depth preference. No shift was observed towards algal domination, or to assemblages dominated by Porites or faviids, as has been reported elsewhere. Recovery in Chagos 8 yr after massive coral and soft coral mortality is discussed in relation to an absence of other, locally manageable factors such as pollution, over-fishing and sedimentation.