The Coral Bleaching Event

The 1998 coral bleaching event

The summer of 1997-1998 was the hottest recorded on the Great Barrier Reef in the 20th Century. Mild bleaching began in late January and intensified by February/March . Extensive aerial surveys of 654 reefs by the Great Barrier Reef Marine Park Authority 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 on the Cairns to Whitsunday coast, and offshore reefs in the Keppel Island group. 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 Keppel Island group.

The 2002 bleaching event

The summer of 2001-2002 was a mass bleaching event that was more severe than the 1998 event . In response to this event, the Great Barrier Reef Marine Park Authority implemented the world’s most comprehensive survey of coral bleaching in collaboration with AIMS, CRC Reef and NOAA. Aerial surveys observed bleaching in almost 60% of the 641 reefs observed . This made the event the worst on record . The first signs of substantial bleaching were reported in January 2002. The worst of the bleaching event was over by April 2002. Underwater surveys found that few reefs had completely escaped the effects of coral bleaching. However, the majority of reefs appear likely to survive the bleaching event with only minimal coral death. Extensive mortality was recorded on only a few of the inshore reefs surveyed, where up to 90% of corals were dead. Inshore reefs were more severely affected by bleaching, as was the case in 1998. However, in 2002 many offshore reefs were also affected .

Research results

Bleaching and mortality were most evident in shallow sites. 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 events. Records of water temperature show that sea temperatures were 1 – 2°C higher than long-term average values in the central and southern Great Barrier Reef during the bleaching period .

Surveys of reefs on the Great Barrier Reef by the Australian Institute of Marine Science in 1999 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. The majority of reefs offshore had changed very little as a result of bleaching one year after the event.

Bleaching around the world

The 1998 and 2002 bleaching did not only affect the Great Barrier Reef. In 1998, 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 the effect of this ENSO event on coral reefs is strong evidence that its impact was strengthened by an underlying trend for global warming.

Bleaching Survey by Ray Berkelmans for 1997/8 Summer	Bleaching Survey by Ray Berkelmans for 2001/2 Summer	Modelled incidence of bleaching for 1997/8 Summer	Modelled incidence of bleaching for 2001/2 Summer
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