|Tsunami Impact Assessment of Coral Reefs in the Andaman and Nicobar
Tsunami Impact Assessment of Coral Reefs in Andaman and Nicobar Islands
Tsunami Impact Assessment of Coral Reefs in Andaman and Nicobar Islands
Dr. Sarang Kulkarni
Reef Watch Marine Conservation
Funded by CORDIO
On 26th December 2004, an earthquake of 8.9 on the Rischter scale shook the entire Andaman Sea region. The quake affected the Andaman and Nicobar Islands very significantly. One effect is that low lying areas from South Andaman to Nicobar group of islands sunk between 1 and 4 meters and as a consequence large land areas have been submerged and is now to be found at 1 to 4 meters depth. However in the north Andaman Islands, large tidal and sub-tidal areas was uplifted and reefs here are now to be found on land.
Soon after the earthquake, a tsunami wave of 2 to 10 meters followed in south Andaman to Nicobar Islands. This caused huge loss of human lives and infrastructure in the islands. The impact destroyed roads, jetties and other basic infrastructure and continues to have a severe impact on peoples lives. Seawater has entered into paddy fields and houses and either remains there or, due to the sinking of the land, continues to enter into fields during high tides.
Reef Watch Marine Conservation (RWMC) has a Research Station and a Reef Research Program in these islands and the author was present during the tsunami. This Research Station is situated in Wandoor in the southwestern part of South Andaman Island. The Research Station suffered substantial damage due to the tsunami wave. After the tsunami, RWMC and the author was involved in relief work and the distribution of cloths, food, monetary help and medicine to affected families.
The impact of the tsunami is also very visible on coral reefs and coastal habitats such as coral reefs, seagrass and turtle nesting sites. Hence the tsunami washed away several important turtle nesting beaches in Nicobar Islands. However, several new beaches are formed in these islands and some turtle nesting in these new sites have also been reported after the tsunami event. Fortunately, one of the important Leatherback Turtle nesting sites in M. G. Marine National Park escaped from tsunami.
Due to the tsunami, it is expected that sand and sediment from land have been deposited on the sea grass beds. If this has taken place it will have long term impact on the population of dugongs. On 16th February, there was a sighting of 5 dugongs in one herd in the northwest coast of South Andaman. One observation of a salt water crocodile attacking a dugong was made in same area. It is feared that the population of Giant Robber crab have been washed away as this crab’s habitat is mainly coastal areas. The distribution of robber crab occurs mainly in Nicobar group of islands.
Impact on Coral Reefs
Initial surveys indicate that the coral reefs in several areas were affected by the tsunami. The impact was measured on a scale from severe to minimal. In most of the reefs, wood logs and other debris from land are found. After the tsunami, the average visibility of the seawater in the region was been reduced. As mentioned above, due to tectonic movement, the average depth has been increased from 1 to 4 meter in Nicobar and the southern Andaman Islands. This will affect the corals as some corals may not tolerate the reduction in light penetration. In most of the cases the larger colonies of Porites have been toppled. In last decade, coral reefs in shallow areas (1 to 2 m) of southern Andaman Islands, had been affected by UV radiation. Subduction may be good news for them as depth is increased so sunlight intesnsity has been reduced and this will provide suitable environment for corals to grow. Such reefs are found in M. G. Marine National Park and other parts of South Andaman Islands. Some islands such as, North Sentinel (interestingly this island situated around 40 kilometer from the west coast of South Andaman Islands), Parts of Middle and North Andaman islands have been uplifted causing large parts of reefs exposed. This reefs will not survive as either high tide water level don’t reach or depth is too shallow and they may not tolerate the increased sunlight intensity.
Previously the coral reefs in the area were dominated mainly by the genus Porites followed by Acropora. If there is substantial damage to Porites, then entire reefs in that particular region are likely to become unstable and may be unable to withstand further environmental stress. Presently rapid assessment is underway, initially covering reefs in South Andaman. In a second phase, surveys will be carried out in the Nicobars and North Andamans. In a rapid assessment, two to three dives of 60 minutes up to 20 meters and 2 hours of snorkeling are carried out. Information on type and extent of damage and video-photo documentation of coral reefs is also being carried out. This rapid assessment is to be done over large areas of Andaman and Nicobar Islands with the intention of having finished the survey before the monsoon which starts in May.
Brief review of the outcome of surveys
Jolly Buoys: Coral reef of this island has been extensively damaged. In the reef flat sand and silt has been deposited on the coral reefs. Several large colonies (larger than 2 m. in diameter) have been uprooted and scattered all over the reef, coral colonies occurring on the reef edge have been pushed to the deeper water up to 20 m. Broken branches of Acropora and Hydnophora rigida have been scattered all over the reef, the visibility is reduced, fish abundance and diversity has decreased, land logs and other debris originated from the land is found over large parts of the reefs
Redskin: As in Jolly Buoys, the reef here is largely destroyed. This reef is mainly dominated by massive Porites lutea. On the reef flat, corals showed little damage. However the colonies on the reef slope were more damaged. Some large colonies have been broken loose and are now found at the depth of 15 meter. The visibility of the water has been reduced and the beach width has been reduced and beach slope has increased.
Alexandra: The damage on the reefs are comparable to those in Jolly Buoys and Redskin and the visibility of the water has been reduced substantially. The corals deeper than 15 m have been sedimented over by sand and silt. This reef earlier was dominated by Porites and Acropora. Now the most affected species was of Acropora. Only a few colonies of Porites lutea have been uprooted.
Grub: Interestingly this area showed very minimal impact of the tsunami wave. Large colonies of Acropora (fig 1) are still in excellent condition. Earlier Porites, Acropora and Echinopora lamellosa dominated the reef. Presently it appears that the abundance of Acropora has been increased. Only a few colonies of Acropora, Porites, Echinopora and Psammacora have been broken/toppled as a result of the tsunami.
North Bay: This area is closer to Port Blair and showed very little impact on the reefs due to the tsunami wave. However the topography and composition of beach has changed. This reef is dominated by Porites lutea, Porites nigrescens and Acropora. There were few sightings of brakeage of the Porites nigrescens and Acropora.
Conclusion and Recommendations
The surveys have shown significant damage to the reefs over extensive areas. However, the tsunami wave is a natural disaster and from a geological perspective, coral reefs have suffered from such damage many times before and have recovered. However from a human perspective there might be immediate and long-term impact on human activities such as fisheries. In addition the devastated coral reef means reduced natural protection from turbulent seas during the monsoon and this may lead to increased erosion of coastal areas. The destruction of reef will also have negative impact on tourism in the archipelago as much of the tourism is related to diving on the coral reefs. To speed up the recovery certain initiatives must be taken such as removal of debris from the reef, overturning toppled colonies and transplantation of broken branches of corals like Acropora. This may be initiated in M. G. Marine National Park and then continued to other areas.
1. In some reefs where tsunami was very active corals such as Acropora managed to withstand
2. Acropora cytherea survived after tsunami in Grub Island
3. Uprooted colony of Acropora in Jolly Buoys
4. Over large area is covered with broken branches of Acropora and still able to survive after two months
5. Due to tsunami few areas are covered with sand and corals are buried underthem, here Acropora branch is partially buried understand and some portion is died and some portion still survives
6. Plate coral Echinopora lamellosa uprooted
7. In several reefs in Marine National Park, large colonies of Porites lutea are uprooted and scattered all over the places this making entire coral reefs unstable as it is the major coral reef building coral in the island
8. Psammacora digitata also broken down showing that most of the coral species have been affected and this species is uncommon in the Andaman and Nicobar Islands
9. Branch of Acropora competing with Porites lutea for survival. Wherever Acropora branch had a contact with Porites had killed the portion of Porites. On killed portion Acropora will cement itself firmly and grow it back.
10. Broken Acropora cemented itself firmly on bare hard substratum and started growing.