The scale of the effort was itself remarkable. India’s reefs are dispersed across vastly different oceanographic settings: the coral atolls of Lakshadweep in the Arabian Sea, the fringing and patch reefs of the Gulf of Mannar, Palk Bay and Goa on the mainland, and the islands of the Maldives-adjacent Andaman archipelago in the Bay of Bengal, including the Mahatma Gandhi Marine National Park (MGMNP). Data on mass bleaching impacts across South Asia have historically been sparse, leaving the region largely invisible in global assessments. By mobilizing a network of scientists, dive operators and trained citizen observers, the research team assembled standardized observations of bleaching and mortality across most of the subcontinent’s major reef areas during a single, globally significant thermal anomaly. The result is a rare, region-wide snapshot of how a mass bleaching event unfolds across an entire nation’s reef estate.

The study’s central metric was degree heating weeks, or DHW, the standard satellite-derived measure of accumulated heat stress that underpins global bleaching forecasting systems such as NOAA’s Coral Reef Watch. As expected, bleaching intensity generally increased with accumulated heat stress. But the strength of that relationship varied dramatically from region to region, exposing the limits of DHW as a universal predictor of reef damage. Two reefs exposed to similar thermal loads could emerge with profoundly different outcomes, a finding that echoes a growing body of evidence that global forecast models need regional and temporal calibration if they are to serve as reliable early-warning tools for reef managers.

Nowhere was this regional variability starker than in the contrast between Lakshadweep and the Mahatma Gandhi Marine National Park. Lakshadweep emerged as the hardest-hit region in the country, with up to 37 percent of coral cover bleached or dead even at intermediate levels of heat stress. The oceanic atolls, which sit in the open Arabian Sea with little local protection from sustained warming, appear to have offered their corals no thermal escape. In sharp contrast, MGMNP in the Andaman Islands recorded only 11.5 percent bleaching or mortality, despite experiencing significant heat stress. The authors attribute this striking difference to regional oceanographic processes capable of creating mesoscale thermal refugia, localized zones where currents, internal waves or other physical mechanisms buffer reefs from the worst of the heat. Previous work in the Andaman Sea has shown that large-amplitude internal waves can deliver pulses of cooler water to reef surfaces during thermal stress, and the new findings lend further weight to the idea that such hidden refugia may be decisive in determining which reefs survive the coming decades.

The taxonomic story proved equally compelling. Across all regions, the branching and plating genera Acropora, Pocillopora, Galaxea and Montipora emerged as the most affected, consistent with the canonical hierarchy of bleaching susceptibility established in reefs worldwide. These fast-growing, structurally complex corals are the architectural engineers of Indo-Pacific reefs, and their disproportionate losses carry cascading consequences for fish habitat, carbonate production and reef growth. At the other end of the spectrum, the massive and encrusting genera Pavona, Platygyra, Goniastrea and Favites were among the least affected, their thick tissues and stress-tolerant symbionts once again proving their worth under thermal duress. For reef managers, this hierarchy has long served as a rough rule of thumb for anticipating post-bleaching community composition.

But Palk Bay broke the rule in spectacular fashion. In this shallow, turbid bay on India’s southeastern coast, the study documented a near-complete reversal of canonical genus-level susceptibilities. The ordinarily vulnerable Acropora colonies showed only modest bleaching or mortality, at 13.6 percent, while the ordinarily resistant Porites colonies suffered a staggering 61.8 percent mortality. Such a reversal is rare and scientifically provocative. It suggests that local conditions in Palk Bay, possibly including prior exposure to recurrent heat stress, the presence of thermally tolerant Symbiodiniaceae symbionts such as Durusdinium, or the moderating effects of turbidity, have reshaped the thermal tolerances of its coral communities in ways that global generalizations cannot capture. It also serves as a cautionary tale: resistance traits are context-specific, and a genus that survives in one reef region may collapse in another.

The mechanistic implications reach deep into coral biology. Bleaching occurs when heat-stressed corals expel or lose the photosynthetic symbiotic algae living in their tissues, starving the coral animal of its primary energy source. Whether a coral bleaches, recovers or dies depends on an interplay of factors: the symbiont types it hosts, its history of thermal exposure, the energy reserves it carries into the event, and the physical environment surrounding it. High-frequency temperature variability, for instance, is known to reduce bleaching risk by priming coral physiological responses, while chronic local stressors can erode resilience. The Indian study’s regional contrasts, Lakshadweep’s open-ocean exposure versus MGMNP’s wave-buffered refugia, Palk Bay’s turbid, heat-conditioned waters versus the clearer reefs of the Gulf of Mannar, provide a natural experiment in how these mechanisms play out at landscape scale.

The findings arrive at a sobering moment. The fourth global bleaching event has confirmed what many reef scientists feared: that warming-driven bleaching is no longer episodic but is ushering in an era of near-annual thermal stress, with global warming tripling the persistence of marine heatwaves and intensifying them by roughly a degree Celsius. For India, the stakes are concrete. Lakshadweep’s atolls support island communities whose shorelines depend on reef-derived sediment, and studies have warned that most atolls may become increasingly uninhabitable by mid-century as sea-level rise exacerbates wave-driven flooding. The loss of up to 37 percent of coral cover in a single event, at only intermediate heat stress, signals that these reefs may have less thermal headroom than their Maldivian and Chagos neighbors, and that recovery between now and the next event is far from guaranteed.

Yet the study is not simply a eulogy. The survival of MGMNP’s reefs, the unexpected toughness of Palk Bay’s Acropora, and the resilience of massive genera across the country collectively point to genuine, mappable refugia and resistance hotspots that could anchor conservation planning. The authors argue that local oceanography and context-specific resistance patterns must be integrated into national and global assessments of bleaching impacts, rather than relying on heat-stress metrics alone. Protecting the oceanographic processes that cool reefs, identifying and safeguarding thermal refugia, and monitoring the survivors of this event as seed populations for recovery are strategies that emerge directly from the data. The study’s underlying dataset has been made openly available through Zenodo, an act of transparency that should accelerate comparative analyses across the wider Indo-Pacific.

As the ocean continues to warm, the fate of India’s reefs will depend on how quickly science can move from global averages to local realities. This nationwide collaboration has shown that the answers lie not in a single number on a satellite map, but in the interplay of currents, waves, symbionts and history that makes every reef region unique. In the ruins of Palk Bay’s Porites and the survivors of the Andaman Sea, Indian reef science has found both a warning and a roadmap.

Subject of Research: Regional and genus-specific coral bleaching responses across India's reef regions during the fourth global coral bleaching event

Article Title: Regional and genus-specific factors underpin bleaching variation across India’s corals reefs during the fourth global coral bleaching event

Article References: Pinto, W., Deshpande, K., Lobo, A. S., Jamalabad, A., Hussain, A., Paul, A., Dutta, A., Arjunwadkar, C., Patel, F. D., Thareja, H., Nangia, I., Josh, J., Goenka, K., Manikandan, B., Namboothri, N., Kuwalekar, P., Nambiar, S., Jaishankar, S., Mahesh, S., … Arthur, R. (2026). Regional and genus-specific factors underpin bleaching variation across India’s corals reefs during the fourth global coral bleaching event. Coral Reefs. https://doi.org/10.1007/s00338-026-02919-7

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02919-7

Keywords: coral bleaching, fourth global bleaching event, degree heating weeks, Lakshadweep, Andaman Islands, thermal refugia, Acropora, Porites, Palk Bay, thermal stress, Indian Ocean reefs, bleaching susceptibility

Ocean acidification begins when carbon dioxide from the atmosphere dissolves into seawater. The gas reacts with water to form carbonic acid, which releases hydrogen ions and shifts the balance among dissolved carbon species. As hydrogen-ion concentrations rise, pH falls and carbonate ions become less available. Those ions are essential ingredients for organisms that construct aragonite or calcite, two crystalline forms of calcium carbonate. The aragonite saturation state, written as Ω_arag, summarizes how favorable the water is for forming aragonite. Values above one indicate supersaturation, while values below one indicate undersaturation, when dissolution can become thermodynamically favored. Yet remaining above one does not mean that all calcifying organisms are unaffected. Growth and calcification can become more difficult well before waters become undersaturated, especially when acidification occurs alongside heat, low oxygen, pollution or nutrient enrichment. Because Ω_arag responds to pH, carbonate-ion concentration, dissolved carbon dioxide and the ocean’s buffering capacity, it can reveal ecological stress that a pH measurement alone may not fully capture.

The study area contains a dense mosaic of habitats. The Gulf of Mannar stretches between Tuticorin and Mandapam and includes 21 islands surrounded by coral reefs, mangroves and seagrass. Its waters support an extraordinary variety of marine life, including fishes, mollusks and reef-associated invertebrates. Palk Bay is a shallow, semi-enclosed basin connected to the Bay of Bengal and strongly influenced by river-borne sediments and freshwater. Both systems are shaped by the seasonal monsoon, but their depth, circulation, sediment transport and biological communities differ. Those differences can alter how quickly carbon dioxide accumulates, how efficiently waters mix and how much carbonate remains available. The researchers selected stations near coral reefs, seagrass meadows and mangrove ecosystems to capture this environmental range. They collected subsurface samples at depths of roughly 0.5 to 1 meter during the Northeast Monsoon, Post-Monsoon, Summer and Southwest Monsoon. Each station was sampled in triplicate, allowing the team to assess both regional patterns and the precision of its measurements.

The contrast between the regions was clearest in their average carbonate conditions. Palk Bay recorded a mean pH of 8.33 plus or minus 0.06, compared with 8.08 plus or minus 0.02 in the Gulf of Mannar. Its mean Ω_arag reached 3.22 plus or minus 0.57, while the Gulf of Mannar averaged 2.82 plus or minus 0.20. These values remain above the saturation threshold, but the lower Gulf of Mannar average indicates less favorable conditions for calcium-carbonate production. The researchers identified particularly low Ω_arag values, below three, during the Post-Monsoon season at the Kurusadai and Vedalai stations in the Gulf of Mannar. Palk Bay remained above three during the same season. The distinction is not a forecast of immediate reef collapse, nor does it establish a biological threshold for every species. Instead, it identifies a chemical gradient: organisms in the Gulf of Mannar may have less energy available for skeletal growth and less resilience when acidification is combined with warming or other disturbances.

Seasonal changes were driven by a shifting mixture of physical and biological processes. During the Northeast Monsoon, average surface temperatures were about 29.7 degrees Celsius in Palk Bay and 29.0 degrees in the Gulf of Mannar. By Summer, both regions approached 31.8 degrees. Salinity also rose during Summer, reaching an average of 35.83 practical salinity units in Palk Bay and 34.43 in the Gulf of Mannar, compared with lower values during the Northeast Monsoon. Monsoon winds alter circulation, freshwater delivery, sediment movement and vertical mixing. The Southwest Monsoon can transport upwelled, carbon-dioxide-rich water toward the Gulf of Mannar, while the Northeast Monsoon can carry lower-salinity water from the Bay of Bengal. Freshwater and nutrients can modify alkalinity and biological productivity, while respiration and the decomposition of organic material can add carbon dioxide to coastal waters. Photosynthesis can temporarily remove carbon dioxide near seagrass and algal communities, raising pH and carbonate availability. These competing influences help explain why the same coastline can experience sharp seasonal swings rather than a uniform, steadily declining signal.

Measurements of the carbonate system supported that interpretation. In Palk Bay, mean seawater partial pressure of carbon dioxide ranged from about 141 to 241 microatmospheres across seasons; in the Gulf of Mannar, the range was approximately 188 to 225 microatmospheres. The highest Palk Bay average occurred during the Northeast Monsoon, when freshwater inputs and mixing may have reshaped the local carbon balance. Carbonate-ion concentrations were generally higher in Palk Bay than in the Gulf of Mannar, while the Revelle factor, a measure related to the ocean’s resistance to absorbing additional carbon dioxide, ranged from 6.85 to 7.83 in Palk Bay and 7.39 to 7.74 in the Gulf of Mannar. A higher Revelle factor means that a given increase in dissolved inorganic carbon can produce a comparatively larger rise in seawater carbon dioxide. The researchers calculated carbonate variables with the CO₂SYS program using laboratory pH, temperature, salinity, total alkalinity, phosphate and silicate measurements. This approach allowed them to estimate pCO₂, carbonate ions, calcite saturation, aragonite saturation and buffering-related properties from a consistent set of chemical observations.

Statistical analyses pointed to carbonate chemistry, rather than any single physical measurement, as the principal control on Ω_arag. A two-way analysis of variance found significant effects of both season and region, as well as a significant interaction between them, meaning that the magnitude of seasonal variability differed between Palk Bay and the Gulf of Mannar. Pearson correlations showed a moderate positive relationship between Ω_arag and pH, with a correlation coefficient of 0.672, and a much stronger relationship with carbonate-ion concentration, with a coefficient of 0.959. Ω_arag was negatively related to pCO₂ and the Revelle factor. The team also used structural equation modelling to examine direct and indirect links among 15 environmental variables. In that model, pH and carbonate ions exerted strong positive influences on aragonite saturation, while pCO₂ exerted a negative influence. Temperature, salinity and nutrient concentrations played smaller or indirect roles. The analysis reinforces a basic chemical principle: when excess carbon dioxide shifts carbonate ions toward bicarbonate, calcifying organisms face a reduced supply of the building blocks needed for aragonite.

The ecological consequences could reach across the food web and into coastal economies. Coral reefs create three-dimensional habitat for fish and invertebrates, shelter young organisms and support fisheries. Mollusks and echinoderms also depend on calcium-carbonate structures, and previous experimental work has shown that tropical sea urchins can be sensitive to carbon-dioxide-driven changes in calcification and physiology. A weaker balance between reef construction and erosion could gradually reduce habitat complexity, even if seawater remains technically supersaturated. The study suggests that Palk Bay’s extensive seagrass meadows may help moderate local conditions by taking up carbon dioxide during photosynthesis and storing carbon in biomass and sediments. The Gulf of Mannar has less seagrass coverage in some areas, and habitat degradation and sedimentation may reduce this potential buffer. Its deeper waters and exposure to monsoon-linked upwelling may further increase the delivery of carbon-dioxide-rich water. The researchers therefore describe the Gulf of Mannar as more vulnerable than Palk Bay, while emphasizing that both systems require continued observation. Their recommended next step is long-term monitoring that combines Ω_arag, pH, pCO₂, alkalinity, nutrients, temperature, oxygen and biological surveys, providing managers with an early-warning system for changing coastal conditions.

These results should be interpreted as a baseline rather than as a long-term trend. The investigation was a pilot assessment covering 24 locations during 2023–2024, so repeated observations over many years will be needed to distinguish persistent acidification from normal coastal variability. That distinction matters particularly in semi-enclosed waters, where river discharge, sediment movement, monsoon circulation and biological carbon cycling can change carbonate chemistry over short distances and time periods. A single regional average may therefore conceal conditions experienced by organisms living near an island reef, seagrass meadow or sediment-influenced shoreline.

Aragonite saturation is also best viewed alongside measurements of total alkalinity, dissolved inorganic carbon, oxygen, nutrients and temperature. Together, these variables can help identify whether low saturation reflects atmospheric carbon dioxide uptake, respiration and organic-matter decomposition, freshwater dilution, upwelling or changes in alkalinity. Biological surveys are equally important because species differ in their sensitivity and capacity to acclimate. Tracking coral growth, mollusk recruitment, echinoderm abundance and seagrass condition with carbonate chemistry would link chemical exposure to ecosystem response. Such integrated observations could help separate areas that are naturally variable from emerging hotspots where local stressors amplify the broader influence of rising carbon dioxide.

Subject of Research: Seasonal ocean acidification vulnerability in the Gulf of Mannar and Palk Bay

Article Title: Aragonite saturation state indicates emerging ocean acidification hotspots in the Gulf of Mannar and Palk Bay along the Southeast Coast of India

Article References: Rangesh, K., Rajan, R. S. P., Dineshbabu, M., Dhayalan, R. E., Johnson Arun Kumar, C., Tharmadurai, S., Anand, M., & Panda, U. S. (2026). Aragonite saturation state indicates emerging ocean acidification hotspots in the Gulf of Mannar and Palk Bay along the Southeast Coast of India. Discover Oceans, 3(1), Article 52. https://doi.org/10.1007/s44289-026-00165-x

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00165-x

Keywords: ocean acidification, aragonite saturation, Gulf of Mannar, Palk Bay, coral reefs, seagrass meadows, carbonate chemistry, Indian Ocean, Aragonite, saturation, state, indicates