Definition

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.

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.

The disappearance of the coral would have the following consequences :