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Reef 'Stress Test' Aims to Preserve Threatened Corals
A 'high priority' coral reef near southern Tanzania.
Credit: T.McClanahan/Wildlife Conservation Society.

Researchers have developed a coral reef "stress test" in hopes the system will act as a kind of marine biodiversity triage, allowing for better management of the most diverse and hardiest of corals in threatened and quickly dwindling ecosystems.

The test is a model that looks at environmental factors that stress corals — mainly rising sea temperatures — and how these stresses affect overall coral and fish diversity. Acidification of ocean waters and overfishing of reefs can also stress coral communities.

"The future is going to be more stressful for marine ecosystems, and coral and their dependent species top the list of animals that are going to feel the heat of climate warming," said the Wildlife Conservation Society's Tim R. McClanahan, lead author of the study, published in the online edition of the journal Global Change Biology.

The model uses layers of historical data, satellite imagery and field observations to produce a composite map on the status of reefs in the western Indian Ocean, in addition to an index of coral communities, their diversity and their susceptibility to bleaching.

Bleaching occurs when the corals are stressed and release the symbiotic, single-celled algae that live inside them, turning the coral bone white. The algae provide food for the corals, and bleached reefs can't survive for long.

The study encompasses a wide swath of the western Indian Ocean, ranging from the Maldives to South Africa, an area already deeply affected by bleaching events and coral mortality.

The model identified the coastal regions stretching from southern Kenya to northern Mozambique, northeastern Madagascar, the Mascarene Islands, and the coastal border of Mozambique and South Africa as having the most promising characteristics of high diversity and low environmental stress.

The authors say these biologically diverse and hardy reefs are therefore a priority for implementing management that will reduce the human impact, while alternative strategies for adaptation are necessary in areas with lower chances of long-term survival.

"The study provides us with hope and a map to identify conservation and management priorities where it is possible to buy some time for these important ecosystems until the carbon emissions problems have been solved," McClanahan said.

Carbon emissions contribute to greenhouse gas levels in the atmosphere, causing some of Earth's warming; they also cause ocean waters to become more acidic, which can threaten coral reefs.

The coral reefs of the western Indian Ocean represent a significant portion of the overall biodiversity of tropical reefs worldwide.

In addition, the region's reefs are a crucial testing ground for management responses to climate-driven events such as coral bleaching. For instance, an estimated 45 percent of living coral in the Indian Ocean was killed during an uncharacteristically warm stretch in 1998.

Caleb McClennen, director of the WCS's Marine Program, said the study shows there's still a window of opportunity to save coral reefs, the ocean's most biodiverse ecosystem.

"Reducing human impacts to minimize the multiple stressors on these globally important reefs will give corals a fighting chance in the age of global climate change," McClennen said.