Explained: How coral bleaching threatens Lakshadweep's marine ecosystem

A survey across various Lakshadweep islands revealed significant bleaching in a notable portion of hard coral species, mainly attributed to a prolonged marine heatwave since late October 2023

The ICAR-Central Marine Fisheries Research Institute (CMFRI) has found that coral reefs in the Lakshadweep Sea have undergone severe bleaching due to prolonged marine heatwaves since October 2023.

As outlined by R K Sreenath and Shelton Padua, senior scientists at CMFRI leading the investigation, coral bleaching poses a threat to the diverse marine ecosystems of Lakshadweep. Here’s what you need to know about coral bleaching, why it takes place and its impacts.

What are coral reefs?

Corals are sessile animals, meaning they permanently attach themselves to the ocean floor. They are categorised as either ‘hard’ or ‘soft’. According to the National Oceanic and Atmospheric Administration (NOAA) of the United States, “Hard corals have stony skeletons made out of limestone that are produced by coral polyps. When polyps die, their skeletons are left behind and used as foundations for new polyps.”

Over millennia, these resilient coral skeletons fuse to form intricate coral reefs, often referred to as ‘rainforests of the sea’ due to their vital role as habitats for myriad marine species and vibrant ecosystems.

Nearly all of Lakshadweep’s islands are coral atolls, with their soil largely derived from corals, and extant coral reefs around them.

What is coral bleaching?

Coral bleaching occurs when the temperature of the water rises above normal levels. Under such conditions, corals expel the microscopic algae living in their tissues, which are responsible for providing them with nutrients.

This expulsion renders the corals’ tissues transparent, exposing their white skeletons, a phenomenon termed ‘coral bleaching’. While bleached corals are not immediately dead, they face increased risks of starvation and disease. Experts estimate that without their algae, corals can survive for approximately two weeks.

Coral bleaching in Lakshadweep
 

The Lakshadweep region has been experiencing marine heat waves since October 2023, resulting in coral bleaching observed recently. Continued high temperatures could lead to the decline of corals in Lakshadweep.

The current temperature of the Lakshadweep Sea is of concern. Corals experience thermal stress when sea surface temperatures exceed 1 degree Celsius above the maximum mean temperature. This stress intensifies with prolonged exposure to elevated temperatures.

Scientists gauge accumulated heat stress using the Degree Heating Week (DHW) indicator over the past 12 weeks, measured in celsius-weeks. A DHW value exceeding 4 degree celsius-weeks signifies coral bleaching, a threshold already surpassed in Lakshadweep. Since October 27, 2023, the Lakshadweep Sea has consistently experienced temperatures exceeding the norm by 1 degree celsius.

Factors contributing to these elevated temperatures include not only atmospheric heat from global warming but also shifts in ocean currents.

Are issues of coral bleaching widespread?

A study conducted in 2022 by the Indian Institute of Tropical Meteorology (IITM), Pune, revealed a concerning trend of increasing marine heatwaves across the Indian Ocean. Led by Roxy Mathew Koll and published in the journal JGR Oceans, the study highlighted a significant rise in marine heatwaves attributed to the swift warming of the Indian Ocean and potent El Niño events. Once considered rare in the tropical Indian Ocean, these heat waves have now become a yearly occurrence.

The study also noted that the western Indian Ocean region experienced the most pronounced surge in marine heatwaves, increasing at a rate of approximately 1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.5 events per decade. From 1982 to 2018, the western Indian Ocean recorded a total of 66 events, while the Bay of Bengal experienced 94 events. An underwater survey conducted after the marine heatwave in May 2020 revealed that 85 per cent of corals in the Gulf of Mannar, near the Tamil Nadu coast, had undergone bleaching.

Impact of coral bleaching

Heat waves pose a significant threat to the livelihoods of coastal communities, as well as to the tourism and fisheries sectors. Moreover, they endanger vital marine habitats, including seagrass meadows. Like corals, seagrass meadows and kelp forests are experiencing adverse effects such as impaired photosynthesis, reduced growth, and hindered reproductive functions due to the heatwaves.

Lakshadweep, being formed by coral reefs, highlights the importance of reef health for the structural integrity of the islands. Coral reef loss could lead to the accumulation of organic matter, impeding the formation of corals in the future.