Over two days, 3 – 4 February 2010, tropical cyclone Oli passed by the west of Tahiti subjecting the islands of Bora Bora, Raiatea-Tahaa, Huahine and Maupiti to waves six to seven meters high accompanied by wind gusting to 170 km/hour. Following this, it was the turn of Tahiti and Moorea followed by the island of Tubuai to undergo the cyclone’s impact, experiencing mean wind speeds of 210 km/hour. This was classed as a severe tropical cyclone, category 4, the second highest storm classification.
Centre National de la Recherché Scientifique, CNRS, the largest governmental research organization in France, has a Coral Observation Department based on Moorea which has been regularly collecting data on coral communities and fish populations in the area. Four days later, after repairing damage to their facilities, they undertook an inventory of the cyclone’s effects after it had passed over two reference sites. Their scientists discovered the extent of the damage to the coral reef, already been made vulnerable by the invasion of a coral predator, was one of almost complete destruction. Their observations revealed that cyclone Oli had flattened the coral population finishing off a reef that was already vulnerable.
The Crown of Thorns Sea Star, Acanthaster planci, notorious for preying on coral, had already nearly wiped out the coral populations on the outer slopes of Moorea. Since the start of an explosion in Acanthaster populations in 2006, the percentages of live coral coverage at 12 meters depth has fallen by around 96 % on the north coast of Moorea, reducing coral cover to roughly 1.0 %. Although this invasion has been a cause for concern, the physical structure of the reefs, particularly the outer slope, the most favourable area for reef growth because of the water’s high level of oxygenation, had been little affected as the skeletons of the dead colonies were still in place, holding out the promise of recovery.
However, once the cyclone had passed, the physical structure of Moorea’s outer slopes, especially 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 were found to have fallen by 50% at all depths down to 30 meters. Rugosity is an important coral reef parameter that describes the amount of “wrinkling” or roughness of the reef profile. It is an index of substrate complexity. Areas of high complexity are likely to provide more cover for reef fishes and more places of attachment for algae, corals and various sessile invertebrates. A large number of coral colonies previously present were torn off by the wave action and broken up by boulders. The three-dimensional structure of the reef has been badly affected, which may be detrimental to long-term recovery.
Damage to the reef varies with depth:
- From 0 to 6 meters there’s severe destruction. Most of the scattered live colonies being broken off at the base. The area is now totally covered with fine pale yellow algal matting of an algal bloom and there’s no live coral coverage remaining.
- From 6 to 10 meters although many live, branched colonies are damaged their bases are intact, which means recovery may be possible.
- From 10 to 15 meters the flanks of this area are in a critical state of destruction. The large branched colonies, most of which were already dead following Acanthaster predation but intact before the cyclone, are no longer visible, no algal growth is observed.
- From 15 to 30 meters depth there is an abnormal covering of small coral debris, 5 cm on average.
- The populations of fish, molluscs and sea urchins associated with the reefs have also suffered considerably with many shellfish being seen in a state of decomposition between the surface and a depth of 6 meters.
- Either the algae will increase and continue to dominate the system by overgrowing the substrate, leading to the death of the reef, as has happened to many reefs around the world.
- Or the reef will start from scratch recruiting new assemblages of coral from larval settlement resulting in a reef likely to be different from the pre-existing one regards species present, and bio-diversity.
As to the future of the reef, there seem to be two possibilities:
Given that algae are already encroaching on the remains of the reef, I find the second possibility remote, although one can always hope.
Scientists have been monitoring the resilience of these reefs since the1980s. During this period, the reefs have been suffered seven episodes of massive bleaching (1983, 1987, 1991, 1994, 2002, 2003 and 2007), several cyclones, and two outbreaks of Acanthaster planci, the starfish that preys on coral.
Although in the past these reefs have always recovered, this recent series of stresses, coral bleaching, cyclones, local pollution, and predation gives little cause for optimism. It’s too soon to make an accurate assessment of the impact of the cyclone on other species such as fish, and non-coral invertebrates but changes in their numbers and diversity are to be expected. Data about fish populations is being collected, which in time will provide a clearer picture of the extent of the damage caused by the cyclone. It could take up to ten years before the reefs recover, if they are able to, making long-term monitoring of reefs essential in order to take the measure of the resilience of coral reefs in Polynesia today.
It would appear though that cyclone Oli may have been one cyclone too many for the reefs of some of the Polynesian islands, including Moorea, Tahiti, Raiatea, Tahaa, and Bora-Bora.
This incident serves as an example of the plight of many tropical reefs around the world. If a reef is healthy, say in a MPA not subject to manmade pollution and over fishing, it can weather natural disasters such as a cyclone and recover over time. However, where a reef has been constantly affected by stressors such as pollution unbalanced ecosystem owing to over-fishing, damage from shipping etc, there comes a time when it can no longer recover. It becomes added to the statistics as one of the increasing number of reefs lost over the last 50 years, joining the estimated 19% of the world’s coral reefs already lost and the 35% seriously threatened (Wilkinson, 2008), a process which is continuing with little sign of abatement.