Vasco da Gama, Goa: The corals in the Gulf of Mannar suffered severe losses during the global coral bleaching event that occurred between March and October 2016, according to new data.
A joint team of researchers from the Suganthi Devadason Marine Research Institute (in Thoothukudi), the Gulf of Mannar Marine National Park and the Department of Environment of Tamil Nadu, which conducted intensive underwater surveys, has found evidence of a high extent of coral bleaching and subsequent mortality. The study’s results were published in the June issue of Current Science.
Coral bleaching refers to the loss of zooxanthellae, a single-celled organism that gives corals their colour. Its absence makes corals appear white or as if bleached. Coral bleaching is what happens when sea surface temperature rises by 1-2ºC, according to Patterson Edward, the director of the Suganthi Devadason Marine Research Institute.
“Coral will recover if normal temperature returns within a short period, but if the temperature stress continues for a long term, the corals will eventually die,” Edward told India Science Wire.
Corals are marine animals, typically living in compact colonies; on the other hand, coral reefs are the diverse underwater ecosystems built by coral colonies. Reefs have the highest biodiversity of any marine ecosystem, and provide important and direct economic benefits to the people. In the last few decades, coral reefs have undergone dramatic degradation due to various natural and human-induced activities.
Almost a fourth of all corals were bleached, with a mortality of 16%, in the Gulf of Mannar between March and June 2016. As a result, the total live cover of corals fell by 23%. The water temperature was recorded to be between 31.2º and 32.6º C; it started decreasing in July 2016 and reached around 27º C in October 2016.
The rate of mortality varied with coral species. Fast-growing species such as Acropora, Montipora and Pocillopora experienced severe mortality while massive and slow growing species of corals, like Porites, Favia and Favitesare, were more resistant to the bleaching and escaped death.
“The local annual variation of coral cover and coral bleaching occur during late April, when the sea surface temperature rises by 2-3º C, but they tend to recover within a period of three months when the normal temperature returns,” Diraviya Raja, a member of the research team, told India Science Wire. “In general, annual bleaching event does not cause mortality. The coral bleaching and severe mortality during 2010 and 2016 are due to global coral bleaching events.”
The Gulf of Mannar is one of the four major coral reef areas in India, with 117 corals species, formed mainly around 21 uninhabited islands situated between Rameswaram and Thoothukudi. The coral reef area declared as the Gulf of Mannar Marine National Park has been monitored since 2003.
The first global bleaching event occurred in 1998. An increase in average global surface temperature over the last two decades has become a major threat to reefs.
Ravi Mishra writes for India Science Wire and tweets at @Ravimishra1970.