Patterns in coral bleaching

The patterns and amount of coral bleaching can vary greatly between coral species, between reefs, and even over a distance of a few metres on the same reef.

Water temperatures will be more extreme in the shallow waters of a reef flat than in deeper waters on the reef slope. Reefs close to the mainland are also generally warmer than more offshore reefs influenced by the open ocean. Reefs in the northern Great Barrier Reef also tend to be a few degrees warmer than those 2000 km to the south.

Coral bleaching

The factors with the greatest influence on whether coral bleaching will occur, are the increase in temperature above the normal level experienced by that coral, and the length of time the coral is exposed to that temperature. For example, corals in the northern Great Barrier Reef might tolerate exposure to temperatures of 31°C for one week, while those at Heron Island in the southern Great Barrier Reef might not. Coral might tolerate a 2°C rise about normal temperatures for a day, but not for a week.

There is also a great difference in how coral species react to high temperatures. In the 1998 bleaching on the Great Barrier Reef, the worst affected families were the Acroporidae (which includes the staghorn corals) and the Pocilloporidae (including brown-stem coral). Other species, such as many massive coral species, were more resistant to the damage. Even different parts of the same colony could be affected differently, with the shaded sections of colonies less prone to bleaching than their upper surfaces in some places.

Most mortality occurred in faster growing corals species about 2 - 30 years old. There was good survival of very old corals (> 100 years old), even in badly damaged reefs. Fortunately for the tourism industry, the majority of reefs offshore - those where most tourism and diving are conducted - suffered negligible bleaching or death.