Coral Bleaching

These bleached corals at North Keppel show the true colour of the coral animal without their symbiotic algae Photo: AIMS

AIMS is developing new and more sophisticated models for predicting bleaching threats and corals’ potential to recover and adapt to new climatic conditions. These models are based on our study of the relationships between environmental conditions, coral health and past bleaching events.

Coral bleaching occurs when corals are stressed by a change in environmental conditions. They react by expelling the symbiotic algae that live in their tissues and then turn completely white.

The symbiotic algae, called zooxanthellae, are photosynthetic and provide their host coral with food in return for protection. Their presence gives the corals a distinctive brown hue, and fluorescent pigments produced by the host add the vibrant colours that characterise corals.

Acute or prolonged stressful environmental conditions cause a breakdown in this symbiotic relationship, first revealing the fluorescent pigments and then leaving the white calcium carbonate skeleton visible through the coral tissue.

Bleached corals can no longer gain energy from photosynthesis, and if bleaching persists for an extended period, corals will starve and die.

For those that survive, bleaching can deplete the corals’ energy resource to the extent that corals do not reproduce for one or two years. The threat to corals increases as the bleaching events become more frequent because they have no time to recover.

Stress can be caused by unusually high or low sea temperatures; high or low light levels; and the presence of freshwater or pollutants. But the extent of coral bleaching depends on above-average temperatures, the duration of high water temperatures, the coral species in question and the environmental history of the reef where the corals are found.

Recent decades have seen several major bleaching events worldwide, including on the Great Barrier Reef.