Coral Bleaching - El Nino Southern Oscillation

Coral bleaching seems to be influenced most strongly by the El Nino Southern Oscillation (ENSO). This is a cyclic phenomenon of sustained SST of more than 0.5˚C, occurring across the central Pacific Ocean, caused by complex interactions between atmospheric pressure systems (Huppert & Stone, 1998). It occurs irregularly but recent patterns have been more regular and stronger than in the past. Mass coral bleaching events have been closely correlated with the occurrence of ENSO, as it can push SSTs above the threshold for the symbiosis between coral and zooxanthellae. The symbiosis can usually resist short term stress but as the oscillations typically last for months, the sustained stress leads to expulsion of zooxanthellae (Douglas, 2003).

An unusually strong ENSO back in 1982-1983 led to severe mass coral bleaching. Air temperature rose by 2-3˚C leading to a significant rise in SST (Glynn & Colgan, 1992). It affected mainly the Eastern Pacific, from Northern Costa Rica to Southern Ecuador. Mortality was seen to correlate with the increase in sea surface temperature. Many areas have been analysed since, with 70-95% mortality being seen due to bleaching in reef communities around Panama, Columbia and Costa Rica (Glynn & Colgan, 1992). The worst coral mortality was recorded at 97% in the reefs of the Galapagos Islands, Ecuador. Rapid recovery via reproduction and immigration of new species took place but the reefs have not fully recovered.

Coral Bleaching - Most Severe Bleaching Event Recorded

The most severe mass bleaching event recorded took place in 1998. This coincided with a strong ENSO at the same time, which caused SSTs around the Pacific Ocean to rise by up to 0.9˚C (Goreau et al., 2000). For example, studies conducted on the effects in the West Pacific found that some areas of the inner and southern Great Barrier Reef were hit by 80-90% mortality rates as a result of bleaching (Goreau et al., 2000), far higher than any levels previously recorded. Again research shows that recovery is taking place however the timescale for a full recovery, or whether or not a full recovery is possible, is not known. However due to the extent of the bleaching, recruitment of new corals was limited in the short term as most source regions of corals were also badly damaged in the incident.

In order to gain a better biological understanding of bleaching, the process can be split into three subdivisions: triggers, mechanisms and symptoms. These will be considered below in more detail.