The 1998 coral bleaching event

The summer of 1997-1998 was the hottest recorded on the Great Barrier Reef in the twentieth century. Mild bleaching began in late January and intensified by February/March. Extensive aerial surveys by the Great Barrier Reef Marine Park Authority of 654 reefs showed great variation in the extent of bleaching between inshore (<10 km from the coast) and offshore reefs, and between reef regions along the coast. While only 14% of offshore reefs surveyed were reported to have high levels of bleaching, 67% of the inshore reefs surveyed had high or extreme levels of bleaching. The most severely affected reefs were inshore reefs in the Cairns to Whitsunday coast, and reefs in the Keppel Island group. Most coral bleaching and mortality was seen in shallow water.

This research concluded that high sea temperatures combined with periods of calm seas, high light, and in some areas, reduced salinity as a result of flooding, caused the major bleaching event. During the bleaching period, sea temperatures were 1 – 2°C higher than long-term average values in the central and southern Great Barrier Reef.

On the most severely bleached areas of reef, subsequent coral mortality was patchy. On some reefs, up to 80% of corals died in the next few months, for example, some reefs in Palm Island group north of Townsville. On other reefs, mortality was low to negligible, including most reefs in the Keppel Island group.

Coral bleaching Photo:GBRMPA

In 1999, surveys by the Australian Institute of Marine Science of reefs on the Great Barrier Reef were used to track the extent of reef recovery from bleaching. These surveys showed that coral cover declined as a result of bleaching on only a small percentage of the 48 reefs surveyed. The reefs where coral cover declined were in the most severely affected inshore area between 17 – 19°S. One year after the event, the majority of reefs offshore had changed very little as a result of bleaching.

In 1998, the bleaching did not only affect the Great Barrier Reef - the most severe bleaching event ever recorded occurred in every coral reef region in the world. In many cases, the bleaching events were predicted by advanced reports of water temperature ‘hot-spots’ from satellite records of water temperature patterns. Reports of major water temperature anomalies (deviations from usual values) were recorded throughout the Indo-Pacific as a result of a very strong El Nino Southern Oscillation (ENSO) event. Many reports have argued that the seriousness of this ENSO event on coral reefs is strong evidence that its impact was strengthened by an underlying trend for global warming.