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| Last Updated:: 26/05/2016

Cyclone saves corals, for now










Even as tropical cyclone Roanu battered Bangladesh, it brought rain and reduced temperature further down India’s east coast, helping control the worst case of coral bleaching in the Gulf of Mannar Marine Biosphere Reserve (GMBR) off southern Tamil Nadu.


With high heat and dry weather starting early this summer, the coral reefs off the 21 coastal islands in the GMBR started bleaching from March. The bleaching was intense and even the younger corals (recruits) were losing their colour. “These were dangerous signs and the trend was unprecedented,” J. K. Patterson Edward, Director of the Suganthi Devadason Marine Research Institute (SDMRI) based in Thoothukudi, Tamil Nadu, told


“Even after Cyclone Roanu cooled the water, we found that the surface temperature was 30.2 degree Celsius,” observed Edward. “Since it reduced from the earlier range between 32.60 C and 33.50 C, we are hoping that there would be no further bleaching. If the temperature intensity had continued, we may have had high coral mortality.”


SDMRI has been monitoring the health of the corals in the GMBR since 2005, and the trends of 2016 were the worst till now. The institute had recorded coral bleaching every year from 2005 to 2010. The worst in this series was in 2010, when elevated sea surface temperature (32.20 C to 33.2C) persisted for four months from April. This resulted in the coral cover decreasing to 33.2% due to severe bleaching and mortality. However, there was no bleaching between 2011 and 2015.


In 2016, the coral bleaching had started earlier and because of its high intensity, it was expected to result in high mortality. However, the current spell of unexpected rains could have come to the rescue of the corals. “We are hoping that there would not be further intensification of damage, and there could be improved recovery over time,” said Edward.


Considering the alarming trends this summer, the Tamil Nadu Forest Department had commissioned SDMRI to carry out rapid assessments of coral health once every 15 days. The last of the two assessment reports before Cyclone Roanu, prepared on May 2, 2016, presented an alarming picture.



One-third corals bleached



“The latest rapid survey conducted on reefs around 11 islands recorded the average coral bleaching in the GMBR as 33% which is more than a five-fold increase in bleaching intensity, while it was more than eight-fold in Palk Bay within a 15-day interval,” noted the report. “The average water temperature recorded in the GMBR is between 32.60 C and 33.50 C with highest in Mandapam group of islands.”


What caught the forest department and the scientists’ attention was the fact that despite the coral mortality of 2010, there had been no adverse impact on younger corals that time. But this year, even the younger corals started bleaching early, and that sent alarm bells ringing, since that could mean a lower chance of recovery.


“We initiated the process of monitoring coral health every 15 days after we realised that this year’s bleaching was more intense than all the previous years,” said a forest department official. “We hope that a good monsoon will start the recovery process.”


Though the cyclone, which started as a low pressure near Sri Lanka, brought rain and reduced the temperature in the GMBR, it cannot be said that the sea temperature may not rise once its affect passes. According to Skymet Weather, the eastern part of Tamil Nadu could get deficient southwest monsoon, which could mean there could be more warm months before the northeast monsoon can bring rain at the end of the year.


Bleaching of corals does not necessarily mean death. However, they indicate increased heat stress on the colonies, leaving them vulnerable to climate variability and change. Bleaching happens in those corals that host symbiotic algae zooxanthellae in their tissue. While the algae have a surface to anchor themselves, they produce food through photosynthesis and also feed the corals. They also give colour to the corals.


When the water temperature rises, the corals get stressed. In the process the symbiotic relationship breaks down – either the corals eject the algae or the algae leave their host. Either way, the corals lose part of their ability to get food and also become vulnerable to other stresses. Some of the coral colonies succumb to these vulnerabilities and die.


In the Gulf of Mannar region, bleaching usually starts when the sea surface temperature goes up above 30° C, according to Edward. The coral reef areas are shallow with their depth ranging between 0.5 metres and 3 m below surface, where they experience temperature from 28 to 290 C across the year.