Biodiversity on the marginal coral reefs of South Africa: What does the future hold?

Southern African coral communities form a continuum from the more typical, accretive reefs in the tropics of Mozambique to the marginal, southernmost African distribution of this fauna in KwaZulu- Natal. While the latter are limited in size, they are gaining increasing attention as they provide a model for the study of many of the stresses to which these valuable systems are globally being subjected. Soft coral cover, comprising relatively few species, exceeds that of scleractinians over much of the southern reefs, and the coral communities attain a high biodiversity at this latitude on the East African coast. A long-term monitoring programme has revealed small yet significant changes in community structure on the reefs in recent years, concurrent with consistent increases in mean and maximum temperature. Insignificant bleaching was encountered during the 1998 ENSO event, unlike elsewhere in East Africa, but quantifiable bleaching occurred during an extended period of warming in 2000. Outbreaks of crown-of-thorns starfish (COTS) have caused longer-term changes in isolated areas. A study of coral larval dispersal and recruitment has been initiated to establish the capacity of the reefs to recover from the latter form of disturbance. The marginal nature of the reefs is further manifested by corals that generate aseasonal and atypical natural products and have a reproductive pattern that conforms with the pattern found on marginal reefs in western Australia. Calcium deposition on the reefs is also low due to physico-chemical factors that are related to latitude. Published projections on the long-term effects of climate change indicate that more reefs will become marginal as a result of global warming. Current monitoring on the South African reefs is being expanded to investigate the extent to which they will elucidate the future of more typical reefs.