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.
bleaching can affect hard coral (madreporarians)
as well as other symbiotic organisms such as soft coral,
sea anemones, sponges and mollusca (giant clams, etc.).
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 occurs
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.
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
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
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
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)
fishermen’s catches would decrease ;
coastline would become eroded, since it would
no longer be protected by the reefs from the
rate of frequentation by tourists would
decrease, especially in the deep-sea diving