Wildlife & Biodiversity

Coral bleaching influenced by location, other factors: Study

The study, conducted on coral reefs in the Indian and Pacific Oceans, found that heat was not the only driver of bleaching

By DTE Staff
Published: Thursday 03 October 2019
A young Acropora that was fully bleached during the 2016 bleaching event. Photo: Vardhan Patankar/NCF

Heat is not the only factor that determines the bleaching of corals, a study conducted by scientists has found.

Rather, many other factors could be responsible, including the location of the coral reef, according to a press statement.

‘Bleaching’ is a process in which corals, on getting stressed due to higher sea temperatures, get stressed and expel the very algae that produce food for them and give them their vibrant colours.

In the new study Temperature patterns and mechanisms influencing coral bleaching during the 2016 El Niño published in the journal Nature Climate Change, scientists studied how coral in the Indian and Pacific Oceans was affected by the El Niño of 2016, one of the warmest years on record.

The study was conducted in 226 reefs, stretching from east Africa to Fiji. It found that coral bleaching did not exactly depend on excess temperature or the distance the corals were from the equator.

Rather, bleached corals did or did not suffer the impact of temperature rise, with some reefs bleached up to 60 per cent and the others showing no impact.

The researchers also found that the geographical location of the coral reef between east Africa and Fiji played a major role in determining whether they would be bleached or not. 

With the strongest bleaching being observed in east Africa, reefs there were found to be more stressed than reefs in Indonesia and Fiji. Indian reefs suffered strong to moderate bleaching in the 2016 event depending on where they were. Thus, Lakshadweep reefs, which were closer to east Africa, bleached very badly while bleaching in the Andamans and Nicobars was slightly less intense. 

Different stress-tolerances

The researchers also found that since different coral reefs were composed of different species of coral and were exposed to varying levels of heat and stress, they had developed different tolerances to these factors, leading them to react differently to bleaching events.

Their past histories influenced how they responded to bleaching events.

“For instance,” said Rohan Arthur of Nature Conservation Foundation (NCF), Bengaluru, who took part in the study, “although Lakshadweep reefs bleached as badly as they did, the amount of subsequent mortality was actually lower than in previous El Niño events.”

He added: “This was most likely because these reefs had been subject to multiple events in the past two decades. The corals that made it through these events are likely to be relatively well adapted to heat stress.”

Arthur, and his colleague, Vardhan Patankar, working with Wildlife Conservation Society & National Centre for Biological Sciences, conducted research in Lakshadweep and the Andaman and Nicobar Islands to contribute data about Indian coral reefs in what was an international effort comprising twenty scientists.   

“Our results suggest that coral responses to global climate change may be changing as corals have different past experiences and tolerances to heat and stress,” said Tim McClanahan, lead author of the study. 

“The consequence is that management and policies need to be aligned with the locations and types of stresses if we are to identify potential refugia and other priority actions for coral reefs,” he added.

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