Suppressed coral settlement following mass bleaching in the southern Persian/Arabian Gulf

Abstract

Coral reefs in the southern Persian/Arabian Gulf have become increasingly degraded in the past two decades, mainly due to recurrent mass coral bleaching events. The recovery of these reefs will be largely contingent upon the arrival and settlement of coral larvae and their post settlement growth and survival. Spatial and temporal patterns of coral settlement were quantified on 10 sites spanning >350 km of the southern Gulf using settlement tiles for two years when consecutive bleaching events occurred. Coral settlement was highly seasonal, with peak settlement occurring in summer each year (>95% of spat), with the remainder of settlement in autumn. Coral settlement was >2-fold greater in the first year (928 spat) compared to the second year (397 spat) representing overall settlement densities of 95 m−2 yr−1 versus 40 m−2 yr−1. The dramatic declines in larval settlement between years suggests bleaching-related impacts on fecundity occurred during the gametogenic cycle late in the first year, as well as impaired survivorship of larvae and/or spat during the second year when severe bleaching coincided with the peak settlement period. Poritids and merulinids (‘others’) comprised 4% and 94% of the spat, respectively, while acroporids were virtually absent (1 recorded spat), suggesting the continued extirpation of this formerly dominant group and a continuing shift towards more stress-tolerant assemblages. Settlement rates in the southern Gulf are low in comparison to other marginal reef environments, and the bleaching-related suppression of settlement observed here suggests that larval supply is unlikely to be sufficient to support recovery of these increasingly degraded habitats. Given the increasing frequency of bleaching events in the southern Gulf the prognosis for the future of regional reefs is grim.