Brain coral with sedimentation damage, MBC 2010, Masbat Bay, Dahab.

Bleaching happens when the coral is under stress. Many stressors can cause coral bleaching, the primary cause being high water temperature, but also disease, sedimentation, pollutants and changes in salinity. Divers doing skills close to corals or with poor buoyancy control may stir up sediment which may settle on the coral surface blocking light for the hard coral’s symbiotic algae, feeding and respiration. When a stressor persists for a longer period of time, the symbiotic algae living inside the coral’s tissue is expelled and the corals look white (the calcium carbonate skeleton is visible) or lighter in colour than normal. The symbiotic algae provide the coral with more than 90% of their food. If the stressor persists over a longer period of time, the coral will eventually die.