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The reefs

Life on the reefs

Destruction of the reefs

Coral bleaching

Jean-Pascal Quod, ARVAM  et Loïc Charpy, IRD



Coral bleaching is a process whereby the coral colonies lose their colour, either due to the loss of pigments by microscopic algae (zooxanthellae) living in symbiosis with their host organisms (polyps), or because these zooxanthellae have been expelled.


Coral bleaching can affect hard coral  (madreporarians) as well as other symbiotic organisms such as soft coral, sea anemones, sponges and mollusca (giant clams, etc.).


Although coral bleaching generally occurs in the shallow parts of the reefs, in the most serious cases it can affect colonies located at depths of almost 40 metres.


Coral bleaching

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Why coral bleaching occurs


When exposed to natural constraints (such as an increase in the temperature, high tides or the proliferation of Acanthaster) or anthropic factors (such as pollution or the sedimentation of terrrigenous substances), coral  can respond in various ways, depending on  the species involved and on the extent of the disturbance. When the temperature of the sea is greater than the tolerance threshold for some time, the zooxanthellae carry out too much photosynthesis, and this leads the polyps to actively reject these algae.

When the atolls are hit by "El Niño " or "La Niña", the climatic conditions are disturbed for a short period and this leads to a change in the direction of the ocean currents, and in the case of " El Niño", to an abnormally large increase in the temperature in the region of the coral reefs. In the Pacific, one can say that the following 3 main situations occur (see scheme)

The global heating process which is affecting both the atmosphere and the oceans of the world and might involve increases of as much as 2°C over the next 50 years constitutes a serious threat to the survival of the coral, since this would mean that the tolerance limits would be overstepped.


The effects of the bleaching process

The loss of symbiotic algae is causing the primary functions of the coral colonies (their growth, their reproductive capacity, etc.) to regress, if not to stop altogether.

If this stress persists, the coral colonies will be at  least partly if not completely destroyed and invaded by algal meadows.

The disappearance of the coral would have the following consequences :

  • The fish populations associated with the coral would give way to herbivorous fish species, and the ichtyological biodiversity would thus be reduced;

  • The risk of ciguatera would increase, since this disease takes root in recently devastated necrosed surfaces, which are colonised first by algal meadows and then by Gambierdiscus toxicus: (see scheme

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  • The fishermen’s catches would decrease ;

  • The coastline would become eroded, since it would no longer be protected by the reefs from the sea swell;

  • The rate of frequentation by tourists would decrease, especially in the deep-sea diving sector.


For more details, a study about the Reunion island:




This page was based on :

Bigot L, Chabanet P, Charpy L, Conand C, Quod J-P, Tessier E (2000) CDROM : "Suivi des Récifs Coralliens" PRE-COI/UE

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update : 07/10/08