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Damaged Caribbean reefs under attack
Rhett A. Butler, mongabay.com
April 10, 2006


After experiencing one of the most devastating coral bleaching events on record during September and October of 2005, reefs in the Caribbean are under attack from deadly diseases according to Reuters.

Warnings of the onset of the bleaching event were first reported by the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch Satellite Bleaching Alert monitoring system in late August in the Florida Keys. The bleaching spread through much of the eastern Caribbean in September and October.

According to scientists in Puerto Rico, bleaching was both widespread and intense with colonies representing 42 species completely white in many reefs. Surveys show 85 to 95 percent of coral colonies were bleached in some areas, while reefs in Grenada suffered close to 70 percent bleaching in some areas. Reefs in the British and American Virgin Islands were affected to a lesser extent.

Coral bleaching is associated with a variety of physiological stresses, the most important of which is elevated sea surface temperatures. Bleaching causes coral to expel symbiotic zooxanthellae algae living in their tissues -- algae that provide corals with nourishment. Losing their algae leaves coral tissues devoid of color, and thus appearing to be bleached. Corals can recover from short-term bleaching, but prolonged bleaching (over a week) can cause irreversible damage and subsequent death.

Presently Caribbean reefs are suffering from "black band disease, white plague and other ailments" according to a Reuters article which says that researchers from the National Park Service and NOAA are unsure of the extent of the disease outbreak.

Black band disease, associated with several species of cyanobacteria, is characterized by the degradation of coral tissue which causes coral to blacken with the formation of dense white patches of filaments. These disease is not to afflict 42 species of coral worldwide.

Bleaching events leave coral more susceptible to such infections and scientists are concerned that higher sea temperatures could contribute the ever greater reef die-offs in the future.

Caribbean reefs. Image by R. Butler

The first coral bleaching on record occurred in 1979. Since then, there have been six events (not counting 2005), each of which has been progressively more frequent and severe. In the El Niño year of 1998, when tropical sea surface temperatures were the highest yet in recorded history, coral reefs around the world suffered the most severe bleaching on record. 48% of reefs in the Western Indian Ocean suffered bleaching, while 16% of the world's appeared to have died by the end of 1998. 2002 was even worse: 60 to 95 per cent of individual reefs of the 110,000 square mile (284,000 square kilometer) Great Barrier Reef suffered some bleaching, while reefs in Palau, the Seychelles, and Okinawa suffered 70-95% bleaching. While most of these reef ecosystems have recovered to some degree, warmer water temperatures in the future may have a more lasting impact.

Scientists have recently warned that the world's coral reefs face a grim future should global temperatures and atmospheric concentrations of carbon dioxide continue to rise. Higher ocean temperatures will produce increasingly severe bleaching events, while elevated levels of carbon dioxide could further acidify the world's seas.

NOAA satellite image for larger view of the regions of high thermal stress as of Oct. 25, 2005, from NOAA's Degree Heating Week (DHW) satellite-based product that accumulates high temperature events. Image courtesy of NOAA

Ocean acidification is of particular concern to scientists because it is crucial to the formation of coral. Coral and other marine organisms use free carbonate ions in sea water to build calcium carbonate shells and exoskeletons, but as atmospheric carbon dioxide levels rise and more carbon dioxide is absorbed by the world's oceans, sea waters become increasingly acidic by stripping out carbonate ions. Lower carbonate ion concentrations make it more difficult for organisms to form shells, leaving them vulnerable to predators and environmental conditions. In the past, changes in ocean acidity have caused mass extinction events. According to a study published in the September issue of Geology, dramatically warmer and more acidic oceans may have contributed to the worst mass extinction on record, the Permian extinction. During the extinction event, which occurred some 250 million years ago, about 95% of ocean's life forms became extinct. The same fate could befall modern day marine life. In September 2005, a team of scientists writing in Nature warned that by 2100, the amount of carbonate available for marine organisms could drop by 60%. In surface ocean waters, where acidification starts before spreading to the deep sea, there may be too little carbonate for organisms to form shells as soon as 2050.

Ove Hoegh-Guldberg, head of the University of Queensland's Centre for Marine Studies, believes that Australia's Great Barrier Reef -- the world's largest reef -- could lose 95 percent of its living coral by 2050 should ocean temperatures increase by the 1.5 degrees Celsius projected by climate scientists.

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This article used media materials provided by NOAA, NASA, and Reuters in addition to previously released information from mongabay.com.