When there are changes in the condition like temperature, light or nutrients the symbiotic linkage between algae and zooxanthellae gets deteriorated, causing them to turn white. In coral bleaching the density of zooxanthellae decline and the concentration of photosynthetic pigment decline which lead to decline in color. 20% of the coral reefs have already been destroyed so it is important to conserve them. There are various causes of coral bleaching which are summarised below:
Bleaching is mostly affected by the rising sea temperature. Corals have usually narrow range of temperature profile which increases their vulnerability whenever there is change in temperature. Upwelling and sudden drop in temperature is another cause of coral bleaching.
Another reason is sudden exposure to high tides or some major tectonic activity in the oceans. They are supposed to be one of the cause.
Sediment augmentation may be another reason. It can also be the cause of bleaching. Rapid dilution of sea water is another such cause.