Saturday, March 27, 2010

Polynesia's coral reefs wiped out by Cyclone Oli

Science Daily: On 3-4 February 2010, tropical cyclone Oli hit western French Polynesia. From 7 February 2010, the Coral Observation Department at CNRS's National Institute of Earth Sciences and Astronomy (INSU), based at the Centre de recherches insulaires et observatoire de l'environnement (CRIOBE, CNRS/EPHE) in Moorea, rapidly undertook an inventory of the cyclone's effects after it had passed over two reference sites. The scientists were soon to discover the extent of the damage: the coral reef, which had already been made vulnerable by the invasion of a starfish that is a coral predator, had been almost completely destroyed.

…The results left no room for doubt: cyclone Oli had planed down the coral populations and finished off a reef which was already vulnerable. In fact, Acanthaster, a starfish that preys on coral, had already decimated the coral populations on the outer slopes of Moorea (2). Although this was a cause for deep concern, the physical structure of the reefs, and especially that of the outer slope (which is the most favorable area for reef growth due to the well-oxygenated water) had nevertheless been little affected, since the skeletons of the dead colonies were still in place, holding out the promise of a possible revival.

However, after the cyclone had passed, the physical structure of Moorea's outer slopes (especially on the northern side) were found to be seriously and lastingly affected. Comparison of data before and after the cyclone struck reveals a very significant reduction in the relief of the outer slope. The rugosity indices (linear distance of developed reef/ linear distance of flat reef) have fallen by 50% at all depths down to 30 meters, as shown by statistical tests carried out at the sites studied. A large number of colonies present, even if dead as a result of predation by Acanthaster, were torn off by wave action and broken up by boulders. This time, it was the three-dimensional structure of the reef which was affected. This determines the habitat of much of the fauna associated with the coral, including many species of fish….

Tropical Cyclone Oli swirled over the South Pacific in early February 2010, some 200 nautical mies (370 kilometers) west-northwest of Bora Bora

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