Seychelles Reefs Permanently Damaged by Global Warming
NEWCASTLE Upon TYNE, UK, May 16, 2006 (ENS) - When coral reefs are bleached out, they may never recover, according to the first report on the long-term impact of a 1998 global warming event in the Indian Ocean that damaged the reefs of the Seychelles' Inner Islands. Fish species that depended on the damaged reefs are already locally extinct, the study found.
From autumn 1997 to spring 1998 the Indian Ocean, and many of the world's other tropical oceans, experienced a rise in sea water temperature. In the Indian Ocean this was attributed to an El Nino Southern Oscillation event that scientists view as part of an overall pattern of global warming.
Situated in the Indian Ocean four degrees south of the equator and a thousand miles off the east coast of Africa, the Seychelles are a group of some 115 islands scattered across 500,000 square miles. The Inner Islands in the northern part of the archipelago are one of three main Seychelles island groups.
During the 1997-1998 autumn, winter and spring, branching coral species on the reefs surrounding the Seychelles inner islands bleached and died, particularly corals such as staghorn, elkhorn and table corals.
The found that more than 90 percent of the Seychelles Inner Islands coral was killed by bleaching.
Lead researcher Nick Graham, of Newcastle University's School of Marine Science and Technology, said, "We have shown there has been very little recovery in the reef system of the inner Seychelles islands for seven years after the 1998 coral bleaching event."
While the 1998 event was devastating in the short term, the main long-term impacts are that the damaged reefs are largely unable to reseed and recover. "Many global warming bleaches simply collapsed into rubble which became covered by unsightly algae," Graham's team writes in their study, published Monday in the "Proceedings of the National Academy of Sciences," a U.S. publication.
"Reefs can sometimes recover after disturbances," said Graham, "but we have shown that after severe bleaching events, collapse in the physical structure of the reef results in profound impacts on other organisms in the ecosystem and greatly impedes the likelihood of recovery."
Bleaching occurs when corals encounter stressful environmental conditions, explain scientists at the Australian Institute of Marine Science in Townsville, a partner on the study team. High water temperature is the most prevalent stress leading to coral bleaching.
Stresses cause coral to expel zooxanthellae, the small algae living inside the bodies of coral. Because the zooanthellae give the coral their color, when they dissipate the corals suffer a loss of color, hampering the photosynthesis process. This eventually leads to death of the corals.
The survey showed that four fish species are possibly already locally extinct - a type of butterfly fish, two types of wrasses and a type of damsel fish.
Six species are at critically low levels, the scientists found - a type of file fish, three types of butterfly fish and two damsel fish. Their decline probably started to happen soon after 1998, they said.
The survey also revealed that species diversity of the fish community had decreased by 50 percent in the heavily impacted sites. "Reduced biodiversity results in a more fragile and less stable ecosystem," the scientists wrote.
Smaller fish have been reduced in number more quickly than larger species but their decreased availability has started to have a more lasting effect on the food chain, the researchers found, and they projected that this effect is likely to be amplified as time goes on.
They observed a decrease in herbivorous fish, worrisome because these fish species control the spread of algae.
Researchers speculate that the reefs' inability to reseed is due to their relative isolation surrounded by vast stretches of ocean. A lack of nearby reefs to provide larvae which could settle and grow into new coral structures and the absence of favorable sea currents to transport the larvae could be responsible for the lack of juvenile corals, they believe.
A socio-economic study conducted in 2004 by the Khaled bin Sultan Living Oceans Foundation projected that the total recreational benefits of seven Seychelles sites in 2005 would be worth approximately US$52 million.
Graham said the Seychelles still holds healthy coral reefs to attract divers in the outer islands, although the condition of the inner islands' reefs is "bleak." Early results from diver tourist surveys in the inner islands suggest that diver satisfaction is high with granite reefs, wrecks and whale sharks.
"Unfortunately," said Graham, "it may be too late to save many of these reefs, but this research shows the importance of countries tackling greenhouse gas emissions and trying to reduce global warming and its effect on some of the world's finest and most diverse wildlife."
Graham's team included researchers from the University of Newcastle upon Tyne, the Australian Institute of Marine Science in Townsville; the Centre for Environment, Fisheries and Aquaculture Science, Lowestoft; the Seychelles Centre for Marine Research; and the Seychelles Fishing Authority.
The work was supported by grants from the British Overseas Development Administration, now the DFID, the Leverhulme Trust and the Western Indian Ocean Marine Science Association, a nongovernmental and nonprofit regional organization.