2005: A Bad Year To Be A Caribbean Coral

Coral bleaching is a well described anomaly in great reef ecosystems around the world. It has been described as the loss of intracellular endosymbionts, known as zooxanthellae, through expulsion. Zooxanthellae give coral reefs their high colouration and are arguably responsible for the beauty of the coral reefs. Once a coral has expelled its zooxanthellae it becomes a lighter or completely white appearance, hence the term "bleached". Coral bleaching can be caused by several stimuli, most prominently these include environmental triggers such as water temperature change, change in solar radiation received, changes in water chemistry and bacterial infection.

Mass Bleaching Events, where 100 km squared or more of coral is bleached, have been reported to have occurred on many occasions around the world in the last 30 years. However the most prominent of these occurred in 2005 in the Caribbean Sea. Whilst still under research, this event is reportedly caused by an anomalously warm water level and high surface light intensity from the months of June to October.

The authors of this paper use several methods to detect sea temperature variations. Most prominently these temperatures were detected by "satellite-based sea surface temperature" observations (SST), but also through manual methods which were subsequently checked with the corresponding location or pixcel from the satillite image. The sea temperature was reportedly increased by +1.2° C on average, with some areas peaking at +16° C. This elevated temperature lasted for many weeks and caused a 50% mortality rate in all Caribbean regions inhabited by coral. This is the worst case of thermal stress related mortality to date (however as mortality cannot be directly linked to the warm water anomaly, it is easy to critically disagree with the authors on this statement).

The increased temperature of seawater can lead to a loss of resistance to pathogenic disease by the coral, and an increase in the abundance of microbial pathogens in the surrounding waters. The authors speculate that opportunistic diseases can easily arise in this type of situation and more extensively bleached corals are immunocompromised. This is backed up by the strong correlation between disease outbreak and bleached corals.

Hurricanes passing within 100 km of coral reefs serve to call surrounding water and prevent this type of event, however in the year of 2005 several hurricanes passed too close to coral reefs and in addition to the long-lasting warmer water caused long-term damage to coral reefs that has lasted decades. When comparing this long-lasting damage to the remarkable stability of coral reefs during the prior 22,000 years, the authors conclude that warm water anomalies are human-caused but that the direct cause is unknown. However it is easy to dismiss this statement as no references or evidence is given to back this up.

Eakin C.M. et al (2010) Caribbean corals in crisis: record thermal stress, bleaching, and mortality in 2005, PLoS One, Published Online. Available from [http://hubs.plos.org/web/biodiversity/article/10.1371/journal.pone.0013969]