Cyclone Oli Deals Major Blow to French Polynesia's Coral Reefs

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.

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). CNRS (2010, March 28). Cyclone Oli deals major blow to French Polynesia's coral reefs. ScienceDaily

Fish bounty hope after Aila churns bay

Cyclone Aila, which left a trail of death and destruction in Bengal in May last year, may lead to a boost in fish catch in eastern India over the next few months, a top ocean scientist said today.

Shailesh Nayak, secretary with the ministry of earth sciences, told a gathering at the 97th Indian Science Congress that the cyclone which killed nearly 140 people and affected 65 lakh in Bengal, caused an unprecedented spurt in the production of a marine organism which forms the bottom of the aquatic food chain.

"Following the cyclone, the production of phytoplankton (microscopic plant-like marine organisms) doubled in the Bay of Bengal," Nayak said. This bloom that lasted a month was spread over an area of 3,000sqkm in the Bay of Bengal, he said.

While such algal blooms are commonly observed in India's western coast, it rarely happens in the Bay of Bengal because its waters are highly compartmentalised. As a result, there is a limited scope for the circulation of the nutrient-rich water from the bottom to the surface and vice versa.

The algal bloom may have beneficial effects on fish catch from the sea, fishery experts said. "Fish availability increases considerably after a phytoplankton boom," said B. Madhusoodana Kurup, the director of the school of industrial fisheries at the Cochin University of Science and Technology in Kerala. This is because phytoplankton are eaten by another set of marine organisms called zooplankton which forms the feed for fish. When zooplankton are abundant in the spawning season, fish production goes up, Kurup said.

Such a spurt in potential fish production is normally witnessed six months to 18 months after the phytoplankton boom, depending on the type of fish harvested from a particular area. Considering that some of the prominent marine fish varieties harvested in Bengal are perch, mackerel and carangid, the effective increase in yield can be expected one year to 18 months after the disaster, he said.

Nayak said a passing cyclone triggers a churning process in the ocean, which in turn brings up the nutrients lying at the bottom of the sea surface. The increase in nutrient availability prompts the plankton to work overtime to produce more.

Supercomputer nodels provided twenty four hours in advance of cyclone sidr

Early on the morning of Nov। 16, Cyclone Sidr hit Bangladesh and showed no mercy. The death toll continues to rise even today. Hundreds of thousands of people were left homeless. But, nearly 24 hours in advance of the storm, Hassan Mashriqui, assistant extension professor of coastal engineering with LSU, the LSU AgCenter and the Louisiana Sea Grant College Program, gave Bangladesh emergency officials storm surge maps so detailed that area agencies were able to take action, saving countless lives.

"It's nice to know that LSU's capabilities helped people there before disaster struck," said Mashriqui. "It's the practical application of all of our theoretical research."
On Nov. 12, he saw that the cyclone had developed in the Bay of Bengal. Watching its progress closely, he contacted Imtiaz Hossain, assistant to Robert Twilley, the very next day. Twilley, associate vice chancellor of research and economic development at LSU, director of the Coastal Systems and Society Agenda, professor of coastal sciences and leader of the Shell Coastal Environmental Modeling Laboratory, or CEML, immediately gave Mashriqui access to a large portion of CEML's supercomputing capabilities to facilitate the development of storm surge models.
The following day, Mashriqui went to Tampa, Fla., to give a lecture at a hurricane conference. It was from his hotel room that he was able to access the LSU supercomputing network and run the first model. What he saw sent him scrambling to contact Bangladesh officials.
"These models are incredibly accurate and highly detailed," Mashriqui said. "You can pinpoint events down to small counties and towns. We were looking at a 10 -- 12 foot storm surge that would devastate anything in its path."
Through an LSU student whose father is employed at the Bangladesh Ministry of Food and Disaster Management in the Office of Disaster Management and Relief Bhaban, a unit that operates much like FEMA, Mashriqui was then able to communicate his findings to dozens of agencies who could then act by raising the danger signal to its highest level, moving people out of harm's way and concentrating relief efforts before the storm even hit.
A native of the area, Mashriqui first began running storm surge models on the Bay of Bengal several years ago in conjunction with LSU's Center for Computation & Technology, or CCT, forming the Bay of Bengal Cyclone Surge Modeling Program. This project provides modeling support for the Bay of Bengal basin and strives to build partnerships with appropriate agencies.
"The advance notice we were able to provide certainly saved lives and helped to lessen the devastation," said Mashriqui. "When you can pinpoint the areas of impact and determine the level of storm surge that far ahead of landfall, it provides critical time for agencies and officials to focus energy and resources to the areas that will need them most."
Adapted from materials provided by Louisiana State University.