Heat stress: Gulf of Mannar coral reefs began to bleach, TN undertaking rapid assessment

In particular, the massive coral genus Porites experiencing widespread bleaching. About 50℅ of Porites show signs of bleaching.
Healthy acropora (R), Partially bleached montipora (L)
Healthy acropora (R), Partially bleached montipora (L)

CHENNAI: Alarm bells have started to ring inside the biodiversity-rich Gulf of Mannar Marine Biosphere Reserve as the coral reefs begin to show early signs of mass bleaching. The State forest department has ordered rapid underwater surveys to assess the gravity of the situation.

In March, TNIE had reported about NOAA issuing a red alert for Gulf of Mannar as "above normal" sea surface temperature (SST) is likely to trigger mass bleaching and coral mortality. NOAA had forecasted that bleaching would occur between the last week of May and the first week of June.

But, bleaching has started as early as the third week of April and is likely to worsen.

Jagdish S Bakan, director, Gulf of Mannar Biosphere Reserve told exclusively to TNIE that coral bleaching was spotted in some pockets of the reserve last week.

Healthy acropora (R), Partially bleached montipora (L)
Harsh summer beckons: NOAA issues red alert for Gulf of Mannar, mass mortality of corals likely

"Because there was an alert from NOAA, we were constantly monitoring. Thoothukudi-based Suganthi Devadason Marine Research Institute (SDMRI) and Chennai-based National Centre for Sustainable Coastal Management (NCSCM) are roped-in to carry out field and modelling studies," Bakan said.

NCSCM has expertise in undertaking ocean circulation modeling to understand the distribution of ocean temperature around the coral areas of Gulf of Mannar and Palk Bay. "MODIS Satellite data would be used to monitor the SST from April to June 2024," the official said and added that necessary work orders to SDMRI and NCSCM were already issued.

The SDMRI officials told TNIE the rapid survey was planned in 11 randomly selected islands in Gulf of Mannar and some sites in Palk Bay to get first hand information.

Fully bleeched Porites
Fully bleeched Porites

"The rapid survey is being conducted from April 22 to 27. Based on the survey conducted in Thoothukudi, Mandapam and Palk Bay, the temperature in the Gulf of Mannar has gone higher than the bleaching threshold and is around 33 degree Celsius. Corals have started to bleach. In particular, the massive coral genus Porites experiencing widespread bleaching. About 50℅ of Porites show signs of bleaching. Of this, 10℅ are totally bleached and the rest are partially bleached.10 to 20% of other massive coral genera such as Favites, Dipsastraea, Goniastrea and Platygyra experience partial bleaching.

Fast growing genera (branching corals) such as Acropora, Montipora and Pocillopora show early signs of bleaching though not yet bleached," K Diraviya Raj, associate professor, SDMRI, told TNIE.

Healthy acropora (R), Partially bleached montipora (L)
Live Coral cover in Gulf of Mannar down to 27%

Bakan said the next two to three weeks are crucial and monitoring is underway. "If the region receives some good rains, bleaching would ease out. If temperature continues to remain about threshold then it would be a cause of major concern. We are planning to continue the underwater survey once every week or a fortnight. In the next survey, all 21 islands in the Gulf of Mannar and 5 reef sites in Palk Bay will be covered to get detailed information on bleaching prevalence," he said.

Earlier this month, NOAA scientists and International Coral Reef Initiative network scientists announced that the world was currently experiencing its fourth global coral bleaching event, which is the second in the last 10 years. 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 Gulf of Mannar cover dropped from 38.9% to 22.7%.

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