Coral Bleaching

Coral bleaching refers to the whitening of coral following the loss or reduction in the symbiotic single celled organism zooxanthellae, which provide nutrients to the polyp through photosynthesis. This was first described by Peter Glynn (1984) following the 1982-3 El Nino event that caused mass coral bleaching in the Panama and Galapagos regions of the Pacific. Before the 1980s, most coral mortality events were localised and largely linked to factors such as storms and exposure to air during low tides. However, since the ‘80s there has been a marked increase in coral mortality events that are spread over large areas and due to coral bleaching, as a response to increasing sea surface temperatures (Glynn, 1992). Although the monitoring of coral reefs has been more extensive in recent years, anomalously high sea surface temperatures (linked to anthropogenic climate change) have been more frequent and caused widespread coral mortality.

Figure 1. Bleached coral at Reunion Island in the western Indian Ocean. (Source: John Pascal/ Agence pour la Recherche et la Valorisation Marines).

The global rise in sea surface temperatures that has occurred and which is predicted to continue as a response to increasing atmospheric concentrations of greenhouse gases, is likely to result in widespread coral mortalities, due to the sensitivity of coral to periods of thermal stress. One of the best sources of information regarding coral bleaching is the book ‘Coral Bleaching: Patterns, Processes, Causes and Consequence’ (van Oppen and Lough, 2008). The book looks at all aspects of coral bleaching, as the title indicates, at a number of scales and includes attempts to predict future scenarios. In this book a comparison is made between coral bleaching events in the Caribbean and Indo-Pacific, with those in the Caribbean being much more significant. The Caribbean coral reefs are much more vulnerable ecosystems, since they contain less biodiversity than those in the Indo-Pacific and have been more negatively affected during past climate changes (Smith and Buddemeier, 1992).

Severe bleaching events occurred in the Caribbean in 1995, 1998 and 2005, but they all had different impacts to the ecology of the reefs affected. ‘Caribbean Coral Reefs after Bleaching and Hurricanes’ (Wilkinson and Souter, 2008) reviews that status of reefs in this region following the 2005 event, predicts future changes and assesses the susceptibility of reefs. The 1995 event resulted in minimal mortality, even though widespread regions of coral reef suffered from bleaching. 1998 was the most detrimental event, with a reduction in coral reef cover of 50% in Belize. While the 2005 event affected the most widespread area, minimal mortality was reported compared to the two previous significant events. This has been attributed to the number of storms during this period, which mixed the waters and prevented ‘doldrum conditions’ that were linked with previous events. Looking to the future, this book suggests that the climatic conditions that lead to coral bleaching events are projected to occur more regularly over the next 20-30 years. Whether or not bleaching events will occur depends on the adaptive ability of corals and their symbionts. Current scenarios indicate that more than 75% Caribbean reefs will have to increase their thermal tolerance by 1-1.5°C over the next half century to avoid bleaching events occurring more than once every five years. Many biologists believe this time frame is too short to prevent widespread loss of corals.

While many of these papers present a fairly depressing picture of the future for coral reefs, there is some evidence that they have the ability to adapt to changing sea surface temperatures. Baker et al (2004) present findings from a study investigating the thermal tolerance of coral reefs across the tropics. Their results suggest that corals containing thermally tolerant symbiotic algal species are more abundant on reefs after severe bleaching events. Adaptive shifts to these species will improve the resistance of affected coral reefs to future bleaching events, and therefore has the potential to prevent mass extinctions. However, the speed of change may well prevent these adaptations from occurring.