Coral Bleaching

This topic has been the subject of many documentaries of late. The coral animals which produce the immense limestone structures of the Great Barrier Reef can only do so with the assistance of tiny plant cells - algae - which live within the coral tissue.

Known as zooxanthellae, the algae convert the abundant sunlight which shines on the reef into food. Most of this - up to 98% - leaks from the algal cells and is used by the corals - in fact, corals get almost all their food from this source.

Although this appears to be daylight robbery, the corals, in return, provide the zooxanthellae with certain essential nutrients. Indeed, both corals and plants engage in a superbly efficient recycling scheme converting each other's wastes to products which can benefit them both. One of these is calcium carbonate - the building blocks which enable the corals to bold such impressive reefs.

This relationship works well in warm sunny waters - but when the temperatures rise above normal levels, it goes very sour. Instead of producing food from the sunlight, the zooxanthellae begin to poison the corals so, in order to save themselves, the corals spit out the algae. When they go, the zooxanthellae take the colour of the coral with them and the white coral skeleton becomes clearly visible through the transparent coral tissue so the coral looks as if it has been bleached.

When conditions improve, the few zoozanthallea left within the tissues may multiply again, but if stresses continue for too long the corals die. The coral skeletons then become covered with algae and eventually disintegrate.

In 1998, satellite maps showed a 2-3 degree increase above normal average sea surface temperatures over much of the Great Barrier Reef. This was followed by the worst coral bleaching event, both in scale and intensity, ever recorded: 88% of inshore reefs from Gladstone to Cape York were affected, one quarter of them severely, while 28% of midshelf reefs suffered, 5% severely.

It appears that corals and their zooxanthellae live close to the upper threshold of their temperature tolerance and just a small increase can cause bleaching. Scientists fear that global warming will continue to cause bleaching on the reefs throughout the world.