Caribbean Coral Crisis

Coral bleaching has become a major threat to coral reef ecosystems worldwide. Stress to the coral-algal symbiosis can cause corals to expel their endosymbiotic algae which may result in coral mortality. The majority of mass coral bleaching has only occurred when unusually warm ocean temperatures exceed corals’ physiological tolerances.

From June – October 2005, a warm water anomaly occurred and developed across the tropical Atlantic Ocean and greater Caribbean Sea region. The elevated temperatures persisted for many weeks and fuelled the most severe and extensive mass coral bleaching event observed in the Caribbean. The National Oceanic and Atmospheric Administration (NOAA) warned scientists of the anomalously warm conditions as they developed and spread. Maps of thermal stress indicated areas in which mass coral bleaching could occur and were used as a guide for the timing and location of researchers’ field observations. As a result, scientists from 22 countries undertook the documentation of basin-scale bleaching.

For this study, sea surface temperature data from sensors aboard Polar-orbiting Environmental Satellites were used. Different researchers carried out field surveys of coral reefs in the Caribbean for bleaching and mortality. The surveys included the following; measures of coral bleaching, number of coral colonies bleached, total colonies surveyed, average observation depth, observation date and location.

After the anomalous water temperature increase, bleaching started off in Colombia. It then extended across the entire Caribbean, with intense bleaching along the Antilles. Several species and sites were reported to bleach for the first time. This included the first known bleaching at Saba, the first documented mass bleaching of the Flower Garden Banks, involving at least partial bleaching of all Millepora alcicomis and Montastraea cavemosa colonies. The first mass bleaching of Acropora palamata in Virgin Islands National Park was also reported.

Some surveys conducted from the peak of thermal stress were analysed to assess coral mortality. Monitoring revealed that a combination of bleaching and other disease outbreaks killed coral colonies stressed by high temperatures. In parts of the Caribbean, temperatures stayed elevated into mid-2006, although it was below the bleaching threshold, many corals remained bleached and disease and mortality remained in this period. Mortality exceeded 50% in several locations making it the worst stress-related mortality documented in the Caribbean. Comparison of satellite data with field surveys showed a strong coherence between thermal stress and widespread bleaching and mortality.

Overall, bleaching isn’t the only cause of reef decline in the Caribbean. The mass bleaching and mortality event of 2005 warming further disturbed Caribbean ecosystems. Coral bleaching is expected to be an even greater threat to coral reefs in the future. Mass coral bleaching from thermal stress followed by outbreaks of contagious or opportunistic diseases have become a threat to coral reefs globally. There is no real sign of recovery after bleaching in Caribbean reefs and so repeated events like the one in 2005 are likely to have caused reef decline that will last for many years.

The data from this study will help researchers and resource managers as they develop actions to protect against thermal stress. New studies identifying ways in which reductions of other sources of stress can increase reef resilience could also gain knowledge from this paper.

I think that this a significant paper as it shows how a small change in water temperature can have such an extensive effect on coral health. However, I found the methods quite confused as there were so many different researchers involved, doing different methods.

Eakin et al., 2010. Caribbean Corals in Crisis: Record Thermal Stress, Bleaching, and Mortality in 2005. PLoS ONE. Volume 5. Issue 11.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013969