The Caribbean: Coral Reef Bleaching
Picture Coral reefs
Coral Reef - Bleaching - Short Introduction
Coral Reefs - Functions
Coral reefs represent one of the most important resources of tropical islands. They perform valuable functions, they:
  • supply sand to beaches
  • play a critical role in the formation and maintenance of reef islands
  • are habitants for a variety of marine communities
  • serve as spawning and nursery grounds for numerous species of reef fish
  • function as protective barriers for beaches and coasts by reducing incident wave energy through the processes of wave reflection, dissipation, and shoaling
  • are significant contributors to the economic resource base of many small island states
Coral Bleaching
Picture Bleaching Corals are small animals. Most live in immense colonies, harvesting nourishment and energy from microscopic algae (plants called zooxanthellae) which inhabit their cells by thousands. The algae are golden brown and combine with other pigments to lend their coral hosts a spectacular appearance. Coral bleaching is the whitening of coral colonies due to the loss of the symbiotic zooxanthellae. This loss exposes the white carbonate skeletons of the coral colony. There are a number of stresses or environmental changes that may cause bleaching including
  • disease
  • excess shade
  • increased levels of ultraviolet radiation
  • sedimentation
  • pollution
  • salinity changes
  • increased temperatures
The exact mechanism by which corals bleach or the trigger that includes bleaching is unknown
The effect of increasing seawater temperatures
Picture No Bleaching Corals have narrow temperature tolerances (approximately 25 - 29°C) and salinity tolerances (about 32 - 36 ppm). In some islands, some species of corals currently live at or near their threshold of temperature tolerance. Corals respond to the combined effects of irradiation and water temperature elevation by paling in color, or bleaching. However, corals generally do not bleach simply as a result of rapid fluctuations in water temperature but rather as a result of departures in temperature above their seasonal maximums. If the temperature elevation is substantial over an extended period (e.g., 3 - 4°C for more than 6 months), significant coral mortality is likely. On the other hand, if the temperature increase is relatively small (e.g., 1 - 2°C) for a short period, bleached corals may recover, though with reduced growth and impaired reproductive capabilities.

That already happened:
  • Caribbean islands experienced an increase in mean annual temperature of more than 0,5°C during the period 1900 - 1995.
  • Coral reef bleaching is spreading
Future consequences of climate change
In many parts of the tropics (e.g., the Caribbean Sea, the Pacific Ocean) some species of corals live near their limits of temperature tolerance. Elevated seawater temperatures (above seasonal maxima) can seriously damage corals by bleaching and also impair their reproductive functions, and lead to increased mortality.

Further informations:
Intergovernmental Panel on Climate Change (IPCC) (1998).
The Regional Impacts of Climate Change: An Assessment of Vulnerability. Cambridge University Press.
Bureau of Oceans and International Environmental and Scientific Affairs, U.S. Department of State
University of the Virgin Islands