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A Quarter Century of Data Reveals Nearly Half of Corals Lost to Marine Heatwaves

Healthy reef. Coral cover sites such as these are rare. Credits: Wenzel Pinto

A landmark long-term scientific monitoring effort shows that reefs in the northern Indian Ocean have lost nearly half of their coral cover since 1998. The research, led by the Nature Conservation Foundation (NCF) and the Centre for Advanced Studies of Blanes (CEAB-CSIC), attributes this decline to repeated marine heatwaves, increasingly intense and frequent due to climate change, which have struck corals without giving them the time they need to recover.

The scientific study has now been published in the journal Diversity and Distributions. It analysed data collected continuously from 1998 to 2022 in three atolls (Agatti, Kadmat and Kavaratti) of the Lakshadweep archipelago, a territory of India located in the Arabian Sea.

This 24-year monitoring, the longest ever carried out in the area, has recorded major marine heatwaves such as those during El Niño events in 1998, 2010 and 2016, as well as other less severe episodes. It documented the varying degrees of coral impact after each event and, in some cases, their subsequent recovery, highlighting the importance of local factors—such as depth—for their ability to withstand and recover.

In figures, the scientific monitoring revealed that coral cover in Lakshadweep reefs fell from 37.24% in 1998 to 19.6% in 2022, representing the loss of nearly half (47.3%) of the live coral covering the reefs.

The monitoring also shows that although the proportion of corals severely affected immediately after a marine heatwave (visible through bleaching) has decreased in recent years, their recovery rate is slowing. Data indicate that, on average, a minimum of six years without new bleaching events is needed for a reef to recover significantly—a period increasingly difficult to achieve due to the growing duration and frequency of heatwaves in the region. The lack of time for coral regeneration translates into a progressive loss of live coral cover year after year.

Another major milestone of this scientific study is the identification of six distinct coral response patterns to marine heatwaves, going far beyond species level. The researchers found that damage and recovery differ, inferring six response types that combine factors such as the depth at which corals live, whether they are exposed to wave action or sheltered, and their reaction to bleaching. Interestingly, different species with very different life cycles ended up in the same group, suggesting that local conditions—more than biological traits—can be decisive in determining whether a coral survives and later recovers. This finding challenges the simplistic idea of ‘winner’ and ‘loser’ coral species under climate change, and allows the creation of a system to predict which reefs are more vulnerable and which more resilient, moving beyond such reductionism.

For the study’s authors, this provides a valuable tool for marine conservation, both for local actions (not only in the Indian Ocean, but also in other vulnerable areas worldwide) and for global measures.

Lead author of the scientific article, Mayukh Dey, of NCF’s Oceans and Coasts Programme and a doctoral researcher at the Manipal Academy of Higher Education, points to some of the study’s worrying findings: ‘Our research shows that while some reefs recover strongly, with each new event they are losing this recovery capacity. Many are now dominated by slow-growing species that fulfil only part of the ecological functions. They may be unable to keep pace with sea-level rise—growing at the same rate and thus maintaining their role as natural barriers for the islands. This is especially concerning for low-lying atolls, where reefs play a key role and make human habitation possible. We hope our study will help Lakshadweep plan for a future where the effects of climate change are cushioned by careful planning and development.’

CEAB-CSIC and NCF researcher and head of the Oceans and Coasts Programme, Rohan Arthur, says: ‘The Lakshadweep reefs have been home to me since I first dived here as a young researcher. Seeing these diverse ecosystems decline before my eyes is devastating. Our findings are bittersweet. On one hand, we have seen that reefs are more complex and resilient than I could ever have imagined. On the other, it is clear that we have a limited window to act and give them the time they need to heal. We can change the narrative on tropical reefs and avoid the extinction of reefs as necessary and beloved as those of Lakshadweep.’

CEAB-CSIC researcher and study co-author, Teresa Alcoverro, emphasises the need to act: ‘The results of this research make us question the future of shallow marine ecosystems everywhere and leave us with a feeling of helplessness in the face of seemingly unending losses. I am often asked what we can do about this and whether proactive management can help, given the scale of climate change. What the Lakshadweep reefs show is that there is still some resilience in marine systems, and our efforts should really be focused on supporting it. We should all be pressuring our governments to take serious action on climate change. It has never been more important.’

The study makes it clear that, despite the resilience still present in some reefs, the future of tropical corals will largely depend on urgently reducing global emissions and slowing ocean warming. Without such action, the authors warn, local actions and corals’ natural recovery capacity will be insufficient to prevent the progressive loss of ecosystems that are key both to biodiversity and to the survival of millions of people who depend on them.

Reference article: Local Environmental Filtering and Frequency of Marine Heatwaves Influence Decadal Trends in Coral Composition. Authors: Mayukh Dey, Teresa Alcoverro, Carmen Gómez, Nachiket Kelkar, Jordi F. Pagès, Wenzel Pinto, Shreya Yadav, Rucha Karkarey, Mayuresh Gangal, M. K. Ibrahim, Aaron Lobo, Elrika D’Souza, Vardhan Patankar, K. K. Idrees Babu, Rohan Arthur. Journal: Diversity and Distributions. Open access link: https://doi.org/10.1111/ddi.70043

 

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