Coral Bleaching

Corals, especially those which build reefs in tropical, shallow waters, are highly attuned to their environmental surroundings. Bleaching occurs when the corals are subjected to repeated and/or sustained stresses which exceed their tolerances. When this occurs, the symbiotic algae living in the coral tissue are ejected. The corals loose their colour and their white, calcerous skeleton shines through the transparent tissue. Corals can survive this condition for a short time and even take up their symbionts if the stresses subside. However, if the stresses persist, the corals will die. One well documented cause of bleaching is increase of sea surface temperatures (SSTs). A prolonged rise in SST during the hottest months of the year by as little as 1°C above the usual monthly average can result in a bleaching event (Glynn, 1996). The first major global bleaching event was recorded in 1998. Since then, several regional and local events occured, such as in the Caribbean in 2005 (Wilkinson, C. and Souter, D., 2008). Bleaching affects the majority of the tropical reefs around the World, with a large proportion dying. The rate of recovery is different from region to region, with healthy reefs (i.e. reefs not or only marginally stressed by other pressures) generally recovering and re-colonising quicker than reefs in poor condition. Some of the latter did not recover at all. The dead coral skeletons are broken down by wave activity and storms into coral rubble, leading to a change in the whole ecosystem from a rich and diverse coral reef into a much more impoverished community dominated by algae.

Figure 9:Projected areas of above normal sea temperature where coral bleaching is likely to occur for the SRES A2 scenario by two different models, the PCM (1.7°C increase in 100 years) and the HadCM3 (3°C increase in 100 years) by ca. 2035 (a) and by 2055 (b). Both models project severe annual bleaching in more than 80% of the Worlds coral reefs by 2080 (Donner et al., 2005).
  Figure 10:The impacts of coral reefs from rising sea temperatures. When coral reefs become heat-exposed they die, leaving the white dead coral, also known as bleaching. With even moderate pollution, the coral are easily overgrown with algae, or broken down by wave activity or storms, leaving only ‘coral rubble’ on the ocean bed (Donner et al., 2005).

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