Cyclone Oli deals major blow to French Polynesia's coral reefs

On 3 February 2010, tropical cyclone Oli passed by the Leeward Islands, to the west of Tahiti. The islands of Bora Bora, Raiatea-Tahaa, Huahine and Maupiti were subjected to waves six to seven meters high and to wind gusts of 170 km/hour. In the evening of 3-4 February, it was the turn of Tahiti and Moorea (Windward Islands), followed by the island of Tubuai (Austral Islands) to undergo the cyclone's impact, with mean wind speeds of 210 km/hour. Four days later, after repairing the facilities, CNRS's INSU Coral Observation Department based at CRIOBE (1) measured the effects of the cyclone after it had passed over the two reference sites on the north coast of Moorea.

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.

The damage varies according to depth. Thus it can be observed that:

• from 0 to 6 meters depth: a critical state of destruction. Most of the scattered live colonies have been broken off at the base. The area is totally covered with a fine pale yellow algal matting indicating an incipient algal bloom (a relatively rapid increase in concentration of algae). The percentage area covered with live coral is zero.
• from 6 to 10 meters depth: many live branched colonies are damaged but their base is still intact, which means that revival is possible.
• from 10 to 15 meters depth: the flanks of this area are in a critical state of destruction. The massive branched colonies (the vast majority of which were already dead following the Acanthaster episode but still intact before the cyclone) are no longer visible, although no algal matting is observed.
• from 15 to 30 meters depth: there is an abnormal covering of coral debris of small size (5 cm on average).
• apart from the coral, associated populations of fish, mollusks and sea urchins have also suffered considerably. For instance, many shellfish are in a state of decomposition between 6 meters depth and the surface.

Other observations carried out on the islands of Raiatea-Tahaa (Leeward Islands) and on Tubuai (Austral Islands) reveal an even more critical situation on the coasts that are most exposed to the action of cyclones.

Although it is still too soon to assess the impact of the cyclone on other species (fish, coral-eating starfish, etc), changes in the diversity and abundance of living organisms are to be expected. More precise data about fish populations are in the process of being collected, which will enable the real impact of the cyclone on these animal populations to be quantified. However, Oli appears to have been one cyclone too many for the reefs of some of the Polynesian islands (including Moorea, Tahiti, Raiatea, Tahaa, and Bora-Bora).

Two different scenarios are now possible: either the algae continue, dominating the system and getting the upper hand over the coral (leading to the death of the reef), which has happened in many reefs all over the world, or else the reef starts from scratch (its current state) and gets going again with assemblages of coral that are likely to be different (with regard to the species present, to abundance, etc.), as it has always done before.

Since the 1980s, scientists have observed the resilience of the coral reef (3). Although its past record shows that it has always got going again, the series of stresses which it has recently undergone (coral bleaching, cyclones, local pollution, etc) give less cause for optimism. We shall know ten years from now, which according to scientists is the time needed for its regeneration. In this context, the surveillance and long-term monitoring of reefs is essential in order to take the measure of the resilience of coral reefs in Polynesia today.

Notes

(1) For around thirty years, CRIOBE has had access to monitoring data for coral communities and fish populations, regularly collected in the area as part of a Central and Eastern Pacific network of which it is in charge at international level.

(2) The coral was nearly wiped out as a result of predation by Acanthaster, populations of which have exploded since 2006. Percentages of live coral fell by 96.1 ((± 2.0) % on the north coast of Moorea (values at 12 meters depth), leaving coral cover at a mere 1.0 (± 1.1) %.

(3) The resilience of the coral reef was observed after having undergone 7 episodes of massive bleaching (1983, 1987, 1991, 1994, 2002, 2003 and 2007), several cyclones (Orama, Reva and Veena in 1983, Wasa in 1991 and Martin in 1997), and two outbreaks of Acanthaster planci, the starfish that preys on coral. (from 1979 to 1984, and since 2006).