Responses of corals and coral reef to the 1991 coral reef bleaching event in the Andaman Sea, Thailand

Abstract

Extensive coral bleaching occurred during abnormal seawater warming in the 1991 along the Andaman coast of Thailand. Mean sea water temperature was about 1 – 3 °C above the average normal summer ambient over two and a half months period before the onset of bleaching in late May. The timing of coral bleaching and the period of seawater warming were closely correlated at several localities suggesting a causal relationship. General description of the event was achieved by field surveys. At the PMBC reef site, establishment of tissue components analysis from coral – samples and coral-transect assessments provided some insight into the consequences of the event to coral colony and community levels, respectively. Conspicuous bleachings were observed among most of the zooxanthellae cnidarian hosts of which 94 taxa were recorded to have bleached. Acroporid corals were among the most susceptible species. By visual estimations, the percentage of living coral bleached ranged between 10% to 70% of the living coral on the reefs. Bleaching appeared in different degrees, unbleached, partially bleached and completely bleached, which also varied among individuals, species, and reef sites suggesting that the mechanism govern coral bleaching is c complex. Line transect assessment at the PMBC reef site showed decreases in several coral community components after the bleaching event. The reef decreased in total living coral cover from 65.88% to 18.92%, of which largely due to the decline of Acropora formosa covered. Furthermore, the number of species, number of colonies, and species diversity were markedly decreased. The reef was left predominant with corals of the small colony size-classed. Delayed and long term responses of the community to bleaching are also discussed. Deteriorated conditions of bleached corals included zooxanthellae population densities, chlorophyll-a contents and protein concentrations. Recovery of tissue biomass is a gradual process.