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Dangerous algal bloom on TN coast

india Updated: Nov 11, 2008 23:53 IST
MR Venkatesh

Put it down to global warming or whatever.

At least two of the 21 islands in the Gulf of Mannar area off the Ramanathapuram coast in South Tamil Nadu, declared by the Centre as a National biosphere reserve, have been skimmed off their rich biodiversity recently in the wake of what scientists term a stunningly unusual “harmful algal bloom (HAB)” wave.

The marine park, stretching off the coast from Mandapam on the mainland up to Tuticorin has suffered an ecological dent in a 30 km stretch from near Pamban to Keezhakarai.

Scientists at the Mandapam Regional Centre of the Central Marine Fisheries Research Institute (CMFRI), told HT that the sudden multiplication of the micro-organism Dino Flaggelate Noctiluck “in huge numbers” had rapidly depleted oxygen levels in the sea water killing tens of hundreds of fishes along the coast and attacking the coral reefs as well.

“Corals got bleached due to lack of oxygen while many fishes and sea animals also died. This type of mass mortality of bio-diversity has taken place for the first time in the Gulf of Mannar,” Dr. G. Gopakumar, Principal Scientist-in-charge here said. The HAB phenomenon has been reported from other parts of the world, but the “intensity here has been severe,” he said.

On being alerted by local fishermen that hundreds of fish were dying at many places along the coast, a CMFRI team rushed to the area. The wild spread of the algal bloom, fortunately not toxic, happened just before the outbreak of the northeast monsoon and in unusually high temperatures, he said.

CMFRI estimated that nearly 13.9 tonnes of commercially important species have died in this stretch from October 5 to October 9.

According to the scientists, during the bloom phase, the coastal water in that stretch had turned dark green in colour. The phosphate and ammonia levels were also very high. At several landing points, the bloom resulted in “very low oxygen levels”, below 1 ml per litre of water against the norm of 5 ml/litre. In the reefs around those two islands, even as late as October 19, “no fish were observed” as sea grass and seaweed had also been wiped out, they said.

Fortunately, with the onset of the monsoon, temperatures dropped and the ‘DFN’ algal bloom virtually disappeared by October 15, leaving an unbearable stench from a huge quantity of dead fish, and pale white damaged coral reefs mainly in Vaazhai and Mulli Islands.