THE SCIENCE OF BLEACHING
There’s ‘bleaching’ and then there’s ‘mass bleaching’.
BLEACHING OCCURS when polyps expel their symbiotic algae – zooxanthellae – as self-defence against stressful conditions. It’s an evolved response that works well most of the time because it occurs when the algae produce too much oxygen, toxic to coral polyps.
Under normal conditions, the stress would go away quickly, the coral would repopulate with zooxanthellae and recover. Minor bleaching happens frequently on reefs, whenever conditions slip outside the range in which a coral is comfortable.
Mass bleaching, however, is a modern phenomenon that was first seen in the late 20th century. It occurs where a stress is profound and protracted – such as an extended period of high water temperatures. During a mass bleaching the stress is so widespread that most or all corals on a reef will be affected. When a mass bleaching occurs and corals must try to survive too long without their zooxanthellae, polyps will die, leaving behind their white calcium carbonate skeletons. The stark white appearance of reef that’s been killed during a mass bleaching event lasts for no more than a matter of weeks, because the reef will then quickly become smothered by algae.
The first mass bleaching documented on the GBR was in 1998 and struck mostly the reef’s southern end. The most recent mass bleachings, in 2016 and 2017, more severely affected the northern and central sections of the GBR. In between, several other events have occurred across different stretches of the reef, but none has been so severe or widespread as those in 2016 and 2017.