India’s Coastal Waters Reveal Emerging Ocean Acidification Hotspots

Two ecologically important coastal waters along southeastern India are showing markedly different chemical conditions that could shape the future of coral reefs, seagrass meadows, shellfish and fisheries. A study of the Gulf of Mannar and nearby Palk Bay found that the Gulf of Mannar had lower average pH and lower aragonite saturation state, a combination that signals greater vulnerability to ocean acidification. The research examined seawater collected during four seasons from 24 stations across the two semi-enclosed marine systems in 2023 and 2024. Although neither region had reached the chemical point at which aragonite dissolves outright, the Gulf of Mannar repeatedly approached less favorable conditions for organisms that build shells and skeletons from calcium carbonate. Palk Bay, by comparison, generally retained a larger chemical margin for calcification, although its carbonate chemistry varied more strongly through the year. The findings place local ocean chemistry at the center of conservation planning for two habitats whose ecological value extends well beyond their shorelines. They also demonstrate why broad global averages can miss the early warning signals emerging in dynamic coastal seas.

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