The impact of two bleaching events on the survival of small coral colonies (Okinawa, Japan)

This study addresses the question whether the density and composition of small coral colonies (≤ 3 cm diameter) were affected by the high sea surface temperature anomalies in the summers of 1998 and 2001 in western Okinawa, Japan. While the percentage of coral cover decreased dramatically at depths < 4 m in 1998, there were more subtle changes in the abundance of small coral colonies. Change in the latter was attributed mainly to mortality of Acropora, Montipora, and pocilloporid colonies on shallow reefs, although the populations recovered within 2 yrs because of recruitment. Small coral colonies were less affected by the 2001 thermal anomaly than by the 1998 event, although they were of similar intensity and duration. The density of small Porites colonies changed little in 1998, but showed a steady decline after 1998, while there was no obvious change in the density of faviids. The limited impact of elevated temperatures and high irradiance on small coral colonies and the significant impact on large colonies concur with mass transfer theory, which suggests that small colonies have higher mass transfer rates than large colonies and are hence more likely to survive in times of heat and irradiance stress.