Thresholds Database > Coral bleaching, Great Barrier Reef, Australia

Background

The Great Barrier Reef stretches 2,000km along the north-eastern coastline of Australia. Coral bleaching events have occurred infrequently on the reef throughout history, but since 1979, the number, scale and intensity of reported coral bleaching events has grown dramatically (Berkelmans et al. 2004).



The worst coral bleaching event on the Great Barrier Reef (GBR) on record occurred in 1998 with only 13% of inshore reefs remaining unbleached, although significantly less offshore reefs were affected (Berkelmans and Oliver 1999). Bleaching severity ranged from minimal to >60% of cover. This event was followed in 2002 by another large bleaching event with the inshore reefs again experiencing greater bleaching than offshore reefs.

Alternate Regimes

1. Coral reefs

2. Bleached and possibly dead coral

Mechanism

Coral polyps cannot survive in water temperatures above about 30oC for long (Bowen and Bowen 2002), but the threshold level and duration of that temperature varies between species (Berkelmans et al. 2004). In temperatures above the threshold, the coral polyps expel the symbiotic algae that provide both essential nutrients and the rich colouration of the coral, leaving the coral with a bleached appearance. If the sea temperature does not drop soon enough for the algae to return, the coral polyps die.



Berkelmans and colleagues (2004) attempted to further explain the relationship between coral bleaching and sea surface temperature They monitored environmental conditions throughout the bleaching event in the summer of 2002. Presence/absence data for 1998 and 2002 were modelled and they found that the best predictor for bleaching was the maximum temperature over any 3-day period (max3d SST) during summer (Berkelmans et al. 2004). They found that the timing and distribution of this maximum temperature varied along the length and breadth of the GBR resulting in the patchiness of the bleaching. They also found that some species are more sensitive to temperature increase than other species.



The modelling predicted that a 1oC increase on 1998/2002 temperatures would raise the bleaching occurrence to 82%, a 2 oC increase would see 97% of the reefs bleached and a 3 oC increase would see 100% of reefs bleached (Berkelmans et al. 2004). They proposed that with average temperatures on the GBR expected to rise by 1.2-1.9oC by 2050 (Done et al. 2003), the corals are vulnerable to depletion.

Management Decisions in Each Regime