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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.
According to Dr R K Sreenath and Dr Shelton Padua, senior scientists at the CMFRI heading the research, coral bleaching threatens Lakshwadeep’s diverse marine ecosystems. Here is what you need to know about coral bleaching, why it takes place, and its impacts.
Corals are sessile animals, i.e. they permanently attach themselves to the ocean floor. They are classified as either ‘hard’ or ‘soft’. According to the United States’ National Oceanic and Atmospheric Administration, “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.”
These stonly coral skeletons, over thousands and millions of years, form complex coral reefs, which have often been referred to as “rainforests of the sea” — home to thousands of marine species, and vibrant ecosystems.
Almost all of Lakshadweep’s islands are coral atolls, with their soil largely derived from corals, and extant coral reefs around them.
Coral bleaching takes place when the water is too warm. In such conditions, corals expel the microscopic algae which live in their tissues, and, in simple terms, produce food for them.
Without these algae, corals’ tissues become transparent, exposing their white skeleton. This is called coral bleaching. Bleached corals are not dead, but run the risk of starvation and disease. According to experts, without their algae, corals can survive for about two weeks.
While Lakshadweep has been gripped by marine heat waves since October 2023, coral bleaching was spotted only last week. If the water does not cool down, the bleaching can eventually lead to the death of Lakshwadeep’s corals.
Lakshadweep Sea has previously seen coral bleaching events in 1998, 2010 and 2015, but the scale of the current one is unprecedented.
Corals experience thermal stress when sea surface temperatures exceed 1 degree celsius above the maximum mean temperature. This stress worsens if the high temperatures persist over a period of time.
Scientists use the Degree Heating Week (DHW) indicator to measure accumulated heat stress in an area over the past 12 weeks, by adding up any temperature that exceeds the bleaching threshold during that time period. This is calculated in celsius-weeks.
DHW values above 4 degree celsius-weeks cause significant coral bleaching — a threshold that Lakshadweep has now crossed, CMFRI found. However, Lakshadweep Sea has been consistently experiencing temperatures 1 degree celsius above the norm since October 27, 2023.
Apart from excessive atmospheric heat (caused due to Global Warming), shifts in ocean currents also lead to unusually high water temperatures.
A 2022 study by Indian Institute of Tropical Meteorology (IITM), Pune, had reported that marine heatwaves are increasing in the Indian Ocean. The study, headed by Roxy Mathew Koll and published in the journal JGR Oceans, reported a significant increase in marine heatwaves, due to the rapid warming in the Indian Ocean and strong El Niños. Earlier, these heat waves used to be rare in the tropical Indian Ocean, but have now become an annual affair.
The western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of about 1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.5 events per decade. During 1982–2018, the western Indian Ocean had a total of 66 events while the Bay of Bengal had 94 events. An underwater survey showed that 85% of the corals in the Gulf of Mannar near the Tamil Nadu coast got bleached after the marine heatwave in May 2020.
The heat waves threaten livelihoods of coastal communities, tourism and fisheries sectors, and critical marine habitats, including seagrass meadows. Similar to corals, seagrass meadows, kelp forests are experiencing detrimental impacts such as impaired photosynthesis, reduced growth, and hindered reproductive functions due to the heatwaves.
Lakshadweep is formed by coral reefs and hence the health of reefs are important for the very structure of the islands. Death of coral may also lead to accumulation of organic matter, preventing the formation of corals later.
