Coral Reefs

In recent years coral bleaching has become a major threat to coral species. 16 % of the world's coral reefs were severely damaged as a result of the 1998 El Nino global coral bleaching event. Many coral reefs in the wider Caribbean were devastated in the Northern summer of 2005 when a series of major 'hot-spots' developed, with some experts reporting more than 50% bleaching on reefs, with more than half of these corals dying or succumbing to disease.

Coral bleaching is the loss of a polyp's zooxanthellae caused by a rise in sea surface temperature or a change in irradiance levels. The zooxanthellae often provide the coral with their pigment and so when the zooxanthellae are expelled the coral loose their colour, hence the term 'coral bleaching'. Polyps can recover by recruiting zooxanthellae from the water column but bleaching can also be fatal. The reasons for this are not yet fully understood but depend on several factors such as the level of temperature rise, its duration and the coral species temperature tolerance. However, even if a coral colony recovers the colony is often made more vulnerable to disease; this can be likened to when our immune systems are weakened by a cold making us more susceptible to other viruses.

Just 1°C above the long-term average can result in bleaching, and with tropical sea temperatures predicted to rise by 1 -3°C by 2100 (IPCC, 2007?), it is not known whether coral reefs will be able to adapt sufficiently in time.