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.
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 research team, led by scientists at the University of Newcastle upon Tyne, surveyed 21 sites and over 50,000 square meters of coral reefs in the inner islands of the Seychelles in 1994 before the bleaching event and again in 2005.
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 colour, when they dissipate the corals suffer a loss of colour, hampering the photosynthesis process. This eventually leads to death of the corals.
The collapse of the Seychelles reefs removed food and shelter from predators for a large number of diverse marine species. In their 2005 survey, the scientists found the average coral cover in the area to be just 7.5 percent.
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.