Cyclones made coral recovery quicker in Gulf of Mannar this year

  • Cyclone disturbances in April and May this year assisted faster coral recovery in the Gulf of Mannar.
  • Studies showed that the corals were significantly healthier than what is normal for that part of the year and corals, which had experienced mild bleaching, had already recovered earlier than they usually do.
  • Corals are sensitive to temperature change and start to bleach when the sea surface temperature rises above a certain temperature. Recovery takes place when the temperature level drops.

Cyclonic disturbances between April and May in the Bay of Bengal caused massive destruction in West Bengal and Bangladesh in the form of Amphan in May this year. However, they seem to have had a positive impact on corals in the Gulf of Mannar, one of the major coral reef areas in India, by reducing bleaching and assisting faster recovery of the corals.

According to a recent study, researchers found that not only did the cyclonic and other mild disturbances reduce instances of bleaching in the south-eastern tip of the country, they also allowed the already bleached corals to recover, at least a month before schedule. This research is likely to be published as a part of a scientific study later this year.

When scientists from Suganthi Devadason Marine Research Institute (SDMRI) in Thoothukodi, along with Tamil Nadu Forest Department, conducted a survey in the Gulf of Mannar to check for coral bleaching in April-May, they discovered that the corals were significantly healthier than expected during that part of the year. During another study conducted in the last week of July the researchers found that the corals, which had experienced mild bleaching, had already recovered. Under usual circumstances, the corals recover in September, a couple of months after the monsoon sets in.

According to K. Diraviya Raj, assistant professor at SDMRI, annual coral bleaching and subsequent recovery during the summer months is common in Gulf of Mannar. “The corals start to bleach when the sea surface temperature rises by 2-3 degrees Celsius and rises higher than 30 degrees Celsius during April or May every year, while the recovery takes place when the temperature level drops below 30 degrees Celsius,” he said.

Certain species of corals such as Acropora, Pocillopora and Montipora are more sensitive to changes in temperature and are the first ones in the line of casualty when it comes to bleaching.

Healthy corals Gulf of Mannar. Photo by SDMRI
Bleached corals in June 2020. Photo courtesy SDMRI.

This year, however, when the team dove in to inspect the corals, they found that the extent and pattern of the present bleaching in Gulf of Mannar was not proportionate to the water temperature levels, Raj said. “That could be attributed to the prevailing high water turbulence, winds, rainfall and cloudy atmosphere caused due to the low pressure and tropical cyclonic storms in Arabian Sea and Bay of Bengal during April and May 2020,” he added.

Corals sensitive to water temperature

Explaining the phenomenon, R.S. Mahendra, a scientist with Indian National Centre for Ocean Information Services (INCOIS), an autonomous body of the Government of India, under the Ministry of Earth Sciences, said, “During the cyclonic disturbance, the turbulence in the waters must have been strong enough so that the warmer layer of water close to the surface could mix with the cooler waters from close to the bed,” adding that, “The movement of the air currents must have also been gentle enough so as to not let sediments block the corals’ access to sunlight.”

Corals thrive in clear, shallow water that lets in sunlight and those in the Gulf of Mannar are located between 0.5 metres to 6 metres below the sea level.

A table of low depressions in the Bay of Bengal shared by SDMRI
A table of low depressions in the Bay of Bengal. Table courtesy SDMRI.

Nayantara Jain, the executive director of Reefwatch Marine Conservation, agrees that mixing of cooler water from deeper parts of the Bay of Bengal with the warm waters of the gulf must have helped the corals escape bleaching. She said the reefs share a unique relationship with algae zooxanthellae which live in their tissues.

Elaborating on the relationship between rise in water temperatures and the coral reefs, Jain added, “The coral provides a suitable environment required by the algae for photosynthesis and in return, the algae supply oxygen and food. When the temperature of the water rises above around 30 degrees Celsius, the corals expel the algae – which is also responsible for the vibrant colour of the reefs – resulting in bleaching.”

Coral bleaching in June. Photo SDMRI
Bleached corals in June. Photo courtesy SDMRI.

The reefs can recover if the temperature returns to normal in two or three months, however, if the conditions prolong, both the coral and the algae die resulting in mass mortality or permanent bleaching, she added.

Interestingly, the temperature stamps in the summer months leading to the cyclonic disturbance indicated that the possibility of severe mortality in the Gulf of Mannar similar to what had happened in 2016. About 16 of 39 percent of live corals in the area had died due to the rise in water temperatures which persisted for more than three months.

Mahendra from INCOIS, who is part of a research team that collects and analysis satellite date on temperature on coral reefs around India, said that on two occasions water temperature of the gulf was the same as that of 2016 within the same time period. “Around April 25 and May 9 and May 16, the sea surface temperature had crossed 31˚C. It was the same as the readings we collected in 2016,” he said. INCOIS derived its data from the sensor Advanced Very High Resolution Radiometre (AVHRR) aboard NOAA satellite.

Satellite data of water temperature compares readings in 2020 and 2016, when there was mass mortality of corals. Graph courtesy INCOIS.

Cyclones preventing bleaching common in parts of the world

This phenomenon of a cyclonic disturbance preventing bleaching of corals may be an uncommon phenomenon in the Indian subcontinent, but Gabby Ahmadia, Director, Marine Conservation Science, Oceans, World Wildlife Fund (WWF) said that it is a usual occurrence in other parts of the world. Speaking to Mongabay-India, she said, “Even this year, even though the Great Barrier Reef in Australia suffered bleaching, parts of it experienced a weaker impact because of cyclonic disturbances in the region.”

The Great Barrier Reef is the world’s largest coral reef system and according to U.S. and Australian science agencies, it is still at risk of a widespread outbreak of coral bleaching despite a cyclone in the far west which helped to temporarily cool the heat-stressed corals.

The corals in the Gulf of Mannar, however, have steered clear of further risks since as per a survey conducted by SDMRI in the last week of July, more than 95 per cent of the corals have recovered. “There was no coral mortality this year. Normally, the recovery begins only by August and the corals heal completely September, but these cyclones and consequent winds have helped them to recover faster this year,” said Raj.

Read more: Corals in Gulf of Mannar show signs of recovery after bleaching, but new threats loom

Towards better prediction of coral bleaching

Banner image: Healthy corals in Gulf of Mannar. Photo courtesy SDMRI.

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