Rising temperatures trigger widespread coral bleaching in Gulf of Mannar & Palk Bay

The Gulf of Mannar and Palk Bay, located off the southeast coast of India, separate the country from Sri Lanka and are known as rich coral hotspots. Recently, they have experienced coral bleaching and die-offs similar to those seen in Australia’s Great Barrier Reef, an official told Down To Earth (DTE).

According to Purvaja Ramachandran, director of the Chennai-based National Centre for Sustainable Coastal Management, the bleaching and die-offs are taking place primarily due to the El Nino effect, which has led to increased sea surface temperatures in these regions.

El Nino is the warmer phase of the El Nino Southern Oscillation climate phenomenon.

“We are observing significant coral bleaching in the Andaman and Nicobar Islands, the Gulf of Mannar, Lakshadweep, and the Gulf of Kutch. This time, the die-off is different; some coral species, which are resilient, are surviving, while others that are not resilient are severely affected,” Ramachandran told DTE.

Corals are highly sensitive to environmental changes, especially temperature fluctuations.

R Raghuraman, Scientist C, highlights that between 1981 and 2010, the mean, maximum, and minimum atmospheric temperatures increased at a rate of around 0.2°C per decade, which is significantly higher than the rate for the entire century (1901-2010).

In tropical regions, mean sea surface temperatures (SSTs) have risen by almost 1°C over the past 100 years and are currently increasing at a rate of 1-2°C per century.

Read Coral bleaching grips Indian coasts; Lakshadweep, Andamans most affected

Sustained increases in SST cause the zooxanthellae, symbiotic microalgae that provide pigmentation and 90 per cent of the corals’ nutritional needs, to leave their hosts, resulting in the whitening of corals, known as ‘bleaching’. This phenomenon is a major threat to coral reefs worldwide.

When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality, according to the United States National Oceanic and Atmospheric Administration.

Since 1998, the reefs in both the Gulf of Mannar and Palk Bay have experienced multiple severe bleaching events, reflecting the ongoing and escalating challenges these critical marine ecosystems face.

The Gulf of Mannar consists of 21 islands divided into four groups based on their geographic proximity to the mainland: Mandapam (seven islands), Keelakarai (seven islands), Vembar (three islands), and Thoothukudi (four islands).

The coral reefs in this area face various threats, including pollution, sedimentation, destructive fishing practices, and biological invasion.

Major bleaching events have been recorded in 1998, 2010, 2016, and now in 2024, indicating a global pattern. The situation in the Gulf of Mannar has deteriorated due to overfishing and anthropogenic pressures, causing the reefs, once dominated by corals, to be overtaken by fast-growing seaweeds.

Raghuraman noted, “Previously, the reef in the Gulf of Mannar was healthy. But now, it is severely impacted due to overfishing and human activities. Coral growth is slow, at a maximum rate of 2.5 cm per year. When bleaching occurs, it takes about six months for corals to start recovering, during which time seaweeds quickly occupy the space. However, the Palk Bay reefs are still in relatively better condition.”

It is estimated that 30 per cent of the world’s coral reefs are already severely damaged, and about 60 per cent of the remaining reefs may be lost by 2030.

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