Rising sea temperatures spell death knell for coral reefs

The future of coral reefs around India is hanging in the balance with the rise in sea surface temperatures, a study revealed. A 16-year study (2003-2019) by wildlife

The future of coral reefs around India is hanging in the balance with the rise in sea surface temperatures, a study revealed. A 16-year study (2003-2019) by wildlife research body Bombay Natural History Society (BNHS) and Wildlife Institute of India (WII) recorded that coral reefs in the Lakshadweep Islands were not only undergoing recurring bleaching, but are at the risk of widespread mortality owing to a significant rise in average sea surface temperatures (SST).

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The impact of rising temperatures is applicable to the coral reefs along India’s coastline, threatening the marine ecosystem, said researchers. The study was published in the Ecological Indicators journal by Elsevier.

Corals are marine invertebrates that attach themselves to rocky intertidal regions or the ocean floor, and are carnivorous animals that eat zooplanktons, apart from zooxanthellae (algae) living within them.

They have the same protection status as that of a tiger or an elephant (schedule 1) under the Wildlife Protection Act, 1972. Coral reefs are created by millions of tiny polyps, forming large structures along ocean floors. They are home to a large marine biodiversity and considered rainforests of the ocean as they sequester large amounts of carbon. Rising sea temperatures due to global warming leads to whitening or bleaching of corals, reducing their capability to photosynthesize and they begin dying of starvation.

Historically, corals have a narrow range of distribution and mostly found between the Tropic of Cancer and Capricorn as ocean temperatures are relatively stable, ranging between 18-24° C. “Beyond 29° C, continuous incidents of bleaching will take place. This is a serious issue as the temperature threshold of more than 30° C will lead to large-scale coral reef mortality,” said Apte, adding that temperature was the key parameter for the sustenance of any marine ecosystem.

Owing to high species diversity in corals, which could give varying results, researchers used giant clams (Tridacna maxima) as surrogate species to study the impact as mercury rose. Over 14 years (2003-17), the group conducted field surveys across three reefs in Lakshadweep archipelago – Agatti (17.5 sq. km), Bangaram-Tinakkara (46.25 sq.km), and Kavaratti (4.96 sq. km), and analysed annual population changes in response to changing SST. “The coral ecosystem is likely to change and it has a huge economic implication on the coastal livelihood. They also act as a defence against rising sea levels. Corals are the best agents of carbon sequestration on earth. The moment all this is gone, it will spell doom to one of the finest and richest ecosystem on earth,” said Apte.

Dr Patterson Edward, director of the Tuticorin-based Sugandhi Devadasan Marine Research Institute, who was not part of the study, said, “Findings of this study are accurate as we also experienced increased bleaching and mortality of corals at Gulf of Mannar, Tamil Nadu, twice (2010 and 2016) over the past decade with a loss of 25% corals. Even this year, we experienced bleaching but fortunately there was no mortality. However, increased anthropogenic impacts combined with the effect of climate change pose a severe threat to the future of corals in India. It is an alarm for us to initiate restoration at the earliest.”