Respite for Gulf of Mannar’s coral reefs as marine heatwave subsides

Sea Surface Temperature drops by 3°C but officials fear damage may have been done already
Bleaching threshold temperature for corals is said to be over 30°C
Bleaching threshold temperature for corals is said to be over 30°C(Photo | Express)

CHENNAI: Coming as a major relief to the biodiversity-rich corals reefs of Gulf of Mannar, the marine heatwave has eased with the Sea Surface Temperature (SST) dropping to 30 from a peak of 33.7°C, mainly due to heavy pre-monsoon showers and change in oceanic conditions.

In the middle of April, mass bleaching of corals was reported across all islands in the Gulf of Mannar, due to high SST. From the first week of April till May 18, several areas in the gulf were issued a red warning by the Coral Bleaching Alert System (CBAS) of the Indian National Centre for Ocean Information Services (INCOIS).

TM Balakrishnan Nair, group director of Ocean Modelling, Applied Research & Services (OMARS), INCOIS, told TNIE, “This year, the number of marine heatwave days has increased three-fold along the Lakshadweep, Kerala and Gulf of Mannar, which has had an adverse impact on coral reefs and the marine ecosystem.”

According to sources in the forest department, although the SST has come down in the past 10 days, it is feared that the damage might already have been done. “A detailed underwater survey will be carried out shortly to assess the extent of coral bleaching and check whether there are any mortalities,” a senior official said.

The first rapid assessment was carried out in mid April in Thoothukudi, Mandapam and Palk Bay. The results indicated that mass bleaching had started. In particular, the massive coral genus Porites was found to be experiencing widespread bleaching. About 50% of Porites corals showed signs of bleaching; of this, 10% were found to be completely bleached. Fast-growing genera (branching corals) such as Acropora, Montipora and Pocillopora also showed early signs of bleaching.

Authorities stated that coral bleaching is reported every year in the Gulf of Mannar during summer, April to June. The bleaching threshold temperature for corals has been reported to be above 30°C.“This temperature is reached almost every summer, and so mild to severe bleaching is witnessed every year. But historically, corals in the gulf are known to be resilient and usually recover well. We hope this is the case this year too, unless there is significant coral mortality like in 2010 or 2016,” they said.

Scientists from NOAA and the International Coral Reef Initiative announced that the world was currently experiencing its fourth global coral bleaching event, the second one in the past decade. Mass bleaching of coral reefs, since early 2023, has been confirmed in at least 53 countries. The last major bleaching event was in 2016, during which the Gulf of Mannar cover diminished from 38.9% to 22.7%.

Gulf of Mannar and the adjoining waters of Palk Bay are among the most important regions in Tamil Nadu, as they are home to thousands of species, including the critically endangered dugongs. Recognising this threat, Chief Minister M K Stalin had declared a part this region as India’s first dugong conservation reserve. The state is also spending crores of rupees in saving the Gulf of Mannar islands from sinking by carrying out seagrass bed restorations.

Gulf of Mannar: A biodiversity hotspot

1. Considered as one of the world’s richest regions in terms of marine biodiversity, the gulf is historically known for its natural pearls and the surrounding industry of skilled pearl divers

2. The Gulf of Mannar features a total of 4,223 reported species, comprising of 181 species of seaweed, 158 species of arthropods and molluscs, 1,147 species of fin-fishes, 28 species of sea cucumbers, 11 species of sea snakes, 5 species of sea turtles, 290 species of birds, and 7 species of marine mammals, among others

3. The rich biodiversity of the region is primarily due to the presence of dynamic marine habitats such as coral reefs, seagrass meadows, mangrove forests, sand dunes, seaweed stretches, coral islands and oyster beds

4. In 1986, the Tamil Nadu government established the Gulf of Mannar Marine National Park (GOMMNP)

5. In 1989, the central government declared it as the first Marine Biosphere Reserve in India and obtained UNESCO recognition

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