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.