CHENNAI: The tsunami that hit the Tamil Nadu and Andaman and Nicobar coasts last year has irrevocably altered the marine ecology of the Bay of Bengal region, says a preliminary tsunami impact assessment report prepared by the Zoological Survey of India for the Union Ministry of Environment and Forests.

The Andaman and Nicobar islands are the worst hit, though the coastal morphology of eastern peninsular India has also changed permanently. However, the exact extent of devastation can be ascertained only after sustained long-term investigations, the report says.

The brunt of the attack was borne by the coral reef ecosystem, which, in most cases, helped reduce the impact on land. "Most of the coral reefs in the Great Nicobar islands have been reduced to rubble. Sand and silt have been deposited on the reefs, choking them. Many reef-dependant species have lost their habitat and their breeding cycles have been disrupted," says K. Venkatraman, Joint Director, ZSI.

The massive sea ingress and submergence of Nicobarese coastal flatlands have uprooted several corals, increased water turbidity and destroyed habitats.

The tsunami, however, has been kinder on the fragile Gulf of Mannar biosphere, shadowed by Sri Lanka. Though the change in coastal morphology is striking, with the ecosphere showing "geomorphic changes," the reefs have been only "partly affected by the breakage of some of the branching corals and uprooting of table corals." The hardier "boulder corals," which also act as wave-brakes, have suffered less damage.

Islands submerged

Many islands such as Kurusadai,Valai-Thalayari and Appa have been divided into two as the waves have submerged the connecting sand tracts. Other islands have been eroded and their size reduced.

The seaweed and seagrass ecosystem between Rameswaram and Kanyakumari have either been uprooted or submerged, "dislocating many associated organisms and changing the species composition."

The worst affected is the benthic ecosystem comprising the invertebrate animals. A huge population of sponges have been affected and animals such as crabs, lobsters and stomatopods displaced from their coral homes. "The tsunami has changed the breeding area by dumping silt and debris and relocating the breeding population to other areas which may not be conducive to their survival. Also, increased water turbidity may lead to mass mortality of fish. The breeding cycle of marine turtles has already been affected and the loss of the seagrass which is the main food for endangered marine mammals like the dugong may also lead to a change in breeding habitat," the report says.

However, the tsunami has benefited river estuaries: the mouths of the Adyar and the Cooum in Chennai, for example, have been desilted, opening up the choked rivers to flushing by seawater which may positively impact riverine biodiversity.

"Only a comprehensive assessment using standardised methodology can generate the real extent of damage. Also, there should be regular monitoring to assess further changes and recovery," says Dr. Venkatraman.

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