Taxonomic patterns of bleaching within a South African coral assemblage

In 1998, the Indian Ocean coral reefs suffered a severe and extensive mass bleaching event. The thermal tolerances of corals were exceeded and their photosynthetic symbionts (zooxanthellae) lost. Mortalities of up to 90% were recorded on the reefs of Seychelles, Maldives, Kenya and Tanzania. South African coral reefs were among the few that largely escaped the 1998 mass bleaching event, but may be threatened in the future if global warming increases. This study assessed the extent of coral bleaching and partial recovery at Sodwana Bay, South Africa during 2000 and 2001. Bleaching levels in this study varied over the course of a year, which suggested that seasonally varying parameters such as sea temperature were the most likely cause of bleaching. Bleaching levels were highest at the shallowest site. However, these bleaching levels were very low in comparison with those of reefs elsewhere in the Indian Ocean. The greater volume of water over the relatively deeper reefs of Sodwana Bay may have protected the reefs from severe bleaching. Field measurements on the three reefs indicated that, although the reefs at Sodwana Bay are still healthy, bleaching increased from <1% in 1998 to 5-10% in 2002. Bleaching occurred in 26 coral genera. The Alcyonacea were highly susceptible to bleaching, especially Sarcophyton sp. Among the hard corals, Montipora spp. were the species most susceptible to bleaching. The sensitivity of these genera to early and slight increases in temperature suggests that they can forewarn of a possible greater bleaching event. In contrast, the coral genera Turbinaria and Stylophora were most resistant to bleaching.