Posts Tagged 'Indian'

Giant kelp-associated variation in coastal seawater chemistry across contrasting sites in Chile and Tasmania

Published 13 August 2026 Science Leave a Comment
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Background and Aims

Widespread shifts in seawater chemistry are occurring across spatial and temporal scales, with important consequences for coastal ecosystems. Giant kelp (Macrocystis pyrifera) forests elevate seawater pH and dissolved oxygen (DO) through photosynthesis, potentially providing short-term refugia from ocean acidification and deoxygenation. However, whether these effects persist across contrasting environmental settings remains unclear. Here, we assess how biological and oceanographic conditions regulate giant kelp-mediated modification of seawater chemistry across multiple sites.

Methodology

Hourly measurements of seawater pH, DO and temperature were collected during spring–summer 2022–23 using paired deployments inside and outside giant kelp forests at one site in central Chile and four sites in Tasmania, Australia. The influence of giant kelp density was also evaluated at two sites in southern Chile and three sites in Tasmania. An upwelling index was calculated for the central Chile site to assess the influence of regional oceanographic forcing on kelp-associated seawater chemistry patterns.

Key Results

Hourly pH and DO were higher inside giant kelp forests than outside at the central Chile site and at one Tasmanian site. At these locations, stronger daytime pH–DO relationships indicated that photosynthetic carbon uptake exceeded night-time respiration, generating a net positive metabolic signal. In Tasmania, giant kelp density was positively associated with hourly pH and DO, whereas no such relationship was detected in southern Chile. At the central Chile site, kelp-associated effects intensified during a strong upwelling event, reducing the severity of low pH and DO conditions.

Conclusions

Giant kelp forests can locally buffer short-term fluctuations in seawater pH and DO, but this capacity is highly site-dependent and influenced by giant kelp density and environmental conditions. Overall, our findings suggest that continued loss of giant kelp forests in Tasmania may reduce their potential to provide short-term refugia, while in Chile the strength of kelp-mediated seawater chemistry modification is likely to remain strongly influenced by variability in upwelling and freshwater inputs.

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Tracing the imprints of dual stressors: eco-physiological and genotoxic insights from Mystus gulio under acidification and warming scenario

Published 10 August 2026 Science Leave a Comment
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Global warming has caused a rise in ocean temperature and acidification, which collectively wreak havoc on marine animals. However, the combined ramifications of these stressors on energy metabolism, oxidative balance, and DNA integrity remain inadequately studied in teleosts. The present study aims to unravel the combined effects of warming (34°C) and acidification (pH 7.7) on energetic balance, antioxidant defense, and DNA stability in Mystus gulio at two intervals (15th and 45th day). Furthermore, the Effect Addition model was used to disentangle potential additive or non-additive effects of the combined stressors. Elevated temperature and acidification induced a marked energetic imbalance, as mirrored by suppressed ingestion and absorption rates, reduced scope for growth, and elevated excretion. Oxidative stress biomarkers revealed significant upregulation of SOD, CAT, and GST under combined treatment, which ultimately led to augmented lipid peroxidation. Genotoxic assessment delineated progressive upsurges in tail DNA (%) and olive tail moment, indicating compromised genomic stability. The integrated biomarker response consisted of six parameters, RR, ER, SOD, CAT, GST, and LPO, further corroborating that elevated temperature, alone and in combination with acidification, posed the highest cumulative physiological burden. Furthermore, the model assessment indicated that the interaction between warming and acidification varied among different biomarkers. Collectively, these findings corroborate that concurrent thermal and low pH perturbations destabilise physiological processes, oxidative instability, and DNA damage, which may undermine growth potential and jeopardise population structure and destabilise trophic relationships. The present study provides a mechanistic understanding of the impacts of multiple stressors and offers a robust integrative framework to predict fish vulnerability under future climate-driven oceanic vicissitudes.

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Global genome-wide patterns of genetic diversity and population structure in Coryphaena hippurus

Published 10 August 2026 Science Leave a Comment
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Highlights

  • Genome-wide SNPs reveal four discrete populations across ocean basins.
  • Mediterranean population forms a deeply divergent and isolated lineage.
  • Nuclear SNPs detect Pacific–Atlantic.
  • Environmental gradients (salinity, phosphate, pH, light) shape genomic structure.
  • Genetic connectivity varies within basins, revealing hidden substructure.

Abstract

The origin and maintenance of marine biodiversity remain poorly understood, particularly in highly connected oceanic environments where extensive dispersal is expected to limit population differentiation. Evolutionary processes driving population divergence or speciation frequently depend on a proper knowledge of species’ life history and its interaction with major environmental variables. Using a genome-wide approach, we investigated the global population structure of the cosmopolitan pelagic fish Coryphaena hippurus Linnaeus, 1758, generating a dataset of 8.7 million SNPs and complete mitochondrial genomes.

Our results reveal, for the first time, four genetically differentiated populations, corresponding to major oceanic basins: Atlantic, Pacific, Indian and Mediterranean Sea. The Mediterranean population exhibited a distinct genomic signature, likely resulting from historical isolation and restricted contemporary exchange through the strait of Gibraltar. Despite this structure, genome-wide analyses uncovered extensive signals of historical connectivity among basins. D-statistics and f4-ratio tests detected significant excess allele sharing, particularly between Indo-Pacific and Atlantic populations, while phylogenetic network reconstruction in TreeMix supported multiple migration events, highlighting the role of ancestral gene flow in shaping global genetic patterns.

At finer scales, a limited but significant intra-oceanic structure was detected within both Atlantic and Pacific basins. Seascape genomic analyses revealed that environmental gradients such as salinity, phosphate concentration, light availability (PAR, Kd), and pH are significantly associated with genomic variation, suggesting that ecological factors contribute to population differentiation across heterogeneous marine environments.

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Localized biogeochemistry and seasonality govern carbonate chemistry in estuarine mangrove ecosystems

Published 3 August 2026 Science Leave a Comment
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Sundarbans, the world’s largest contiguous mangrove ecosystem and representing shallow coastal Bay of Bengal of the Northern Ocean, faces dynamic climate variations, including ocean acidification. To delineate ocean acidification from natural pH variations, it is crucial to perform long-term measurements of multiple carbonate chemistry parameters such as pH, total alkalinity (TA), and dissolved nutrients, among others. In the present study, surface water carbonate chemistry parameters, including TA, pH, and dissolved nutrients (o-phosphate and silicate), were analysed monthly between 2014 and 2022 in three pre-defined stations, namely Stn1, Stn2, and Stn3, part of Sundarbans Biological Observatory Time Series (SBOTS) located in Sagar Island, the largest island of the Indian Sundarbans. The observed deviation from the linear TA-Salinity curve in the studied sites of SBOTS showed the influence of freshwater in modulating TA. Generalized Additive Model (GAM) revealed substantial seasonal variability in the controls on TA. During monsoon, salinity was a dominant driver of carbonate chemistry, consistent with enhanced freshwater discharge. In contrast, during the post-monsoon season, primary productivity as indicated by the relationship with Chla, dissolved silicate, was found to exert a stronger influence on TA variability. Multilinear regression (MLR) analysis of calculated pCO2 further supported these seasonal trends. Overall, the findings highlight the importance of season-specific assessments, highlighting the critical role of freshwater discharge in shaping estuarine carbonate dynamics. These insights are vital for predicting the vulnerability and response of mangrove estuaries under future climate change scenarios.

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Freshwater forcing along the Indian Coastal Seas: impacts on productivity and acidification

Published 31 July 2026 Science Closed
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Freshwater fluxes from precipitation and river runoff play a critical role in modulating upper-ocean stratification, nutrient availability, and biogeochemical processes in the coastal waters of the Indian subcontinent. The formation of the barrier layer thickness (BLT) links freshwater input to vertical mixing, influencing both productivity and carbonate chemistry. High-resolution (5 km) MITgcm-BLINGv2 simulations are conducted for the Arabian Sea (AS) and the Bay of Bengal (BoB), and sensitivity experiments are performed to represent reduced and increased freshwater perturbations. We analyzed seasonal variability of buoyancy frequency (N2), mixed layer depth (MLD), net primary productivity (NPP), pH, and phytoplankton biomass across five coastal regions. Reduced freshwater scenarios weakened or eliminated BLT, leading to deeper MLD and N2 maxima, with subsurface nutrient-rich waters entrained upward. This enhanced nutrient availability increased NPP in the coastal regions. However, the upward transport of subsurface carbon also lowered surface pH by 0.03, indicating a trade-off between biological enhancement and increased surface acidification. In the increased freshwater scenario, the BLT strengthened, the MLD shoaled, and NPP decreased, while surface pH increased due to reduced vertical carbon exchange. Interestingly, stratification deepening under reduced freshwater input is more pronounced in the southeastern AS than in the BoB, contrasting conventional understanding. Vertical phytoplankton responses are consistent with these trends, with small and large phytoplankton biomass increasing under weaker BLT and decreasing under enhanced BLT. Freshwater-driven BLT modulation drives a complex interplay between carbon uptake and export along Indian coastal waters. These findings emphasize the importance of accurately representing freshwater fluxes in biogeochemical models to capture regional ecosystem responses.

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Interplay between nutrient limitation and ocean acidification: plankton community shifts in the oligotrophic western Bay of Bengal

Published 30 July 2026 Science Closed
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Highlights

  • Nutrient limits the phytoplankton growth in the Bay of Bengal due to stratification.
  • Picoplankton dominated the phytoplankton community in the Bay.
  • An increase in phytoplankton biomass was observed in response to ocean acidification (OA) in nutrient-rich regions.
  • Decrease in phytoplankton biomass, bacteria and microzooplankton abundance was found in the nutrient-poor regions.
  • Nutrient limitation overshadow the ocean acidification impact in the Bay of Bengal.

Abstract

The perennial rise in atmospheric carbon dioxide (CO2) from anthropogenic activities increased oceanic carbon uptake, thereby lowering seawater pH. Rapid ocean acidification (OA) by 2 to 3 times than other regions of the same latitudinal belts was reported in the Bay of Bengal (BoB) due to the deposition of atmospheric pollutants, in addition to the dissolution of atmospheric CO2. Therefore, the impact of OA on phytoplankton composition is hypothesized to be greater in the BoB than elsewhere in the globe. On the other hand, the BoB experiences intense oligotrophic conditions due to strong stratification driven by nutrient-poor freshwater discharge from major rivers, which is the primary bottleneck for phytoplankton growth, rather than carbon. It is hypothesized that nutrient limitation may overshadow OA’s impact on plankton in the BoB. To test this hypothesis, six microcosm experiments were conducted in the western BoB between 13 and 20°N with different nutrient levels in the surface waters. The pH of the surface water was adjusted by 0.2 units below the ambient pH using CO2 gas bubbling to simulate the projected OA scenario for the year 2100. Following pH adjustment, incubations were carried out for three days. Picoplankton dominated the phytoplankton community (∼96%) at all stations, with prochlorophytes as the dominant group. Among the phytoplankton, procholorophytes, diatoms, and prymeniophytes increased due to OA across all stations, while other groups of phytoplankton were declined. The increase in phytoplankton biomass was observed in the nutrient-rich stations, while a decrease in phytoplankton biomass, bacteria and microzooplankton abundance was found in the nutrient-poor stations, suggesting that nutrient availability dominated over the OA. Under a business-as-usual scenario, an increase in stratification due to melting Himalayan glaciers is projected, leading to strong stratification and reduced nutrient inputs through vertical mixing in the future in the BoB. Under such a scenario, nutrient limitation may possibly overshadow the OA impact in the future, despite a rapid decline in pH driven by increased CO2 dissolution and deposition of atmospheric pollutants.

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A satellite-derived re-analysis of surface-ocean pH variability in the coastal waters of India

Published 22 July 2026 Science Closed
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Monitoring pH variations in coastal waters is essential for maintaining marine life and developing strategies to combat ocean acidification caused by climate change. The present study examines pH changes on interannual and seasonal scales along the northwest (NW), southwest (SW), southeast (SE), and northeast (NE) coasts of India over 30 years (1993–2022) using satellite data. Results reveal a significant decline in pH levels (R 2 = 0.727) across Indian coastal waters, with notable regional differences. The northeastern coast showed the greatest pH stability and the smallest decrease (R 2 = 0.798), while the southwestern coast experienced the highest variability (R 2 = 0.490). These regional pH fluctuations suggest varying resilience to ocean acidification. The NE and SW coasts, with higher variability, may be more susceptible to environmental changes, underscoring the importance of targeted monitoring and mitigation measures. In contrast, the more stable trends along the SE and NW coasts present opportunities to explore long-term resilience mechanisms. Seasonal fluctuations were evident everywhere, with winter consistently showing higher pH values and monsoon seasons the lowest. Principal component analysis indicated that the first two components accounted for 85.7% of the variance, highlighting factors such as seasonal river inflows, biological activity, and monsoonal freshwater input. Overall, these findings emphasise the importance of region-specific coastal management strategies to address climate change impacts and human pressures, while also providing a baseline for tracking future ocean acidification trends and assessing their effects on marine biodiversity and ecosystem services.

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Acute low pH associated with coastal acidification is detrimental to larval development of the Cape urchin Parechinus angulosus

Published 26 June 2026 Science Closed
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Acidification in coastal habitats is increasing in duration and amplitude under the continued influence of ocean acidification and contributing coastal processes. The impacts of low pH conditions on calcifying organisms, especially echinoderms, is well established, with the early developmental stages being especially vulnerable. This is the first study to assess the impact of locally relevant coastal acidification scenarios on the early development of the Cape urchin Parechinus angulosus. Our findings suggest that the early larval stages of this species are unlikely to survive when exposed to low pH conditions, specifically during the onset of skeletogenesis. In our laboratory experiments, larvae that were exposed to the low pH treatment (pH 7.32) showed significantly reduced growth (GLMM, Time × Treatment interaction: β = −0.361 ± 0.019, z = −19.06, p < 0.001) and developmental regression compared with those from the control treatment (pH 7.95). Substantially slower growth rates were observed in the low pH treatment (length = 72.3 hpf0.18) compared with in the control treatment (length = 24.24 hpf0.54). There was also evidence of abnormal and delayed development and potential dissolution of skeletal structures under the low pH condition. However, fertilisation success and larval survival did not differ significantly between the experimental treatments, suggesting that developmental impacts of low pH over short durations, even though substantial, may be sublethal. The developmental impacts are likely to impair the transition of larvae to the adult stages, which may ultimately affect populations of this ecologically important species under future coastal acidification scenarios.

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Identification of the source of carbonaceous aerosols using stable carbon and nitrogen isotopes and the implications of its deposition on the coastal ocean

Published 24 June 2026 Science Closed
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Highlights

  • Aerosols and their major sources are seasonally variable at Visakhapatnam.
  • Total suspended matter was higher during winter than during summer.
  • Biomass burning is a dominant source of aerosols during winter.
  • Fossil fuel and coal combustion are the major sources during summer.

Abstract

The continuous rise in anthropogenic aerosol emissions degrades ambient air quality, and their deposition onto the surface ocean alters chemical and biological characteristics. Identifying the sources of aerosols is crucial for taking appropriate measures to minimize their impacts. Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) are promising tools for identifying sources of carbonaceous aerosols. The objective of this study is to identify the dominant sources of carbonaceous aerosols over an urban region using stable carbon (δ13C) and nitrogen (δ15N) isotope ratios, and to evaluate their potential influence on surface ocean acidification in the coastal Bay of Bengal. Aerosol samples were collected between March 2016 and February 2017 at a fortnightly interval, over an urban region, to examine the sources of carbonaceous aerosols and to evaluate the possible impacts on surface ocean acidification. Significantly high concentrations of total suspended particulates (TSP) during winter (112 ± 26 μg m−3) compared to summer (58.8 ± 8 μg m−3), associated with an insignificant seasonality in δ13CTC (−26.9‰ to −22.9‰), indicating ageing of organic aerosol through oxidation. In contrast, higher δ15NTN during winter (2.2‰ to 12.1‰; 5.4 ± 2.9‰) than summer (−12.9‰ to −1.8‰; −4.7 ± 3.3‰) indicate different sources. Based on source characteristics of δ13CTC, δ15NTN and the isotope mixing model, biomass burning and coal combustion are the major sources of carbonaceous aerosols during winter, whereas coal and fossil fuel burning contributed during summer. Since biomass burning contains higher concentrations of acidic aerosols, such as sulfates, and its deposition over the surface ocean results in higher level of pH levels compared to coal ashes. A higher decline in pH of the coastal waters during winter than summer was reported in the coastal Bay of Bengal. This study confirms that the deposition of higher sulphate and nitrates due to biomass burning in the Indo-Gangetic Plain (IGP) region is responsible for a greater decline in pH of the surface ocean during winter than summer. Taking appropriate measures to reduce biomass burning in the IGP region would decrease ocean acidification and allow the atmospheric CO2 sink into the coastal Bay of Bengal to achieve net zero carbon emissions in the future.

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Marine invertebrates and fishes exhibit inconsistent body size responses to ocean acidification

Published 18 June 2026 Science Closed
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Body size is a fundamental characteristic of all living organisms that determines physiological functions and life-history traits. Ecological theory predicts that ocean acidification can cause body size reductions, confirmed by several studies reporting miniaturization in ectotherms. Based on this prediction, we would expect a broad suite of species to show similar plastic body-size responses to elevated CO2. Using four natural climate change analogues of ocean acidification across the northern and southern hemispheres, we quantified body size alterations across 18 marine invertebrate and fish taxa to test for climate-driven miniaturization. Only three species consistently showed body-size reductions under ocean acidification: one urchin and two fish species. In contrast, 15 other species, ranging from highly calcified to non-calcified, displayed unchanged or increased body sizes or inconsistent miniaturization. If body-size miniaturization responses were consistently reproducible across taxa we would have observed it more frequently, suggesting that species responses to ocean acidification are more variable than previously thought and likely vary depending on a species’ physiology and life history. Thus, rather than entire communities undergoing miniaturization, species are likely to display a spectrum of responses, with some exhibiting size reductions, others demonstrating physiological resistance to elevated CO2, and others potentially benefiting from the indirect effects of ocean acidification.

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Pteropod vulnerability to ocean acidification in the eastern Arabian Sea

Published 18 June 2026 Science Closed
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Highlights

  • First study on pteropod response to ocean acidification in the eastern Arabian Sea.
  • High pteropod abundance during fall inter monsoon season due to food availability.
  • pH in the Arabian Sea was low during south west monsoon with pHT upto 7.75
  • Pteropod shell dissolution was observed under acidified conditions
  • Protrusions through the pteropod shell were observed under acidified conditions

Abstract

The rapid rise in atmospheric CO2 and its subsequent uptake by the oceans has led to ocean acidification and other associated changes in the marine ecosystem. The recent reports of the shoaling of the aragonite saturation horizon in the northern Indian Ocean are particularly alarming, as they pose a serious threat to the survival of calcareous organisms. Pteropods, also known as sea-butterflies, are believed to be highly susceptible to ocean acidification due to their thin aragonite shell. In our study in the eastern Arabian Sea, we found low pH conditions with surface pHT as low as 7.751 during late South-west monsoon (SWM). The pteropod abundance is high during the fall inter-monsoon (FIM), suggesting that the system continues to sustain productivity even after the cessation of peak monsoon activity. This also implies that the food availability regulates pteropod abundance in the eastern Arabian Sea. As pteropods are key components of food sources for many marine species, such as fish, any changes in their abundance can have cascading effects on the marine food web. To show how pteropods will be affected in futuristic elevated CO2 conditions, a CO2 manipulation experiment was conducted in the eastern Arabian Sea during December 2024. Pteropods belonging to Creseis acicula from the eastern Arabian Sea were subjected to pHT = 7.470, and pCO2 = 1734 μatm under controlled conditions. Our findings suggest that acidification led to the dissolution of pteropod shells. Acidification also led to protrusion through the shells, and these protrusions varied in length up to 88 μm. These structural alterations represent an acute response of pteropod shells to reduced pH, highlighting their rapid vulnerability to acidification stress. These observed protrusions need to be assessed further to determine if they provide any competitive advantage in combating or minimizing the impact of ocean acidification.

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Temperature, but not acidification, influences the growth and lipid profile of juvenile sand whiting, Sillago ciliata (Cuvier 1829)

Published 18 June 2026 Science Closed
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Seafood provides an essential source of macro- and micronutrients for coastal communities worldwide. Climate change is a key threat to seafood security, altering the sizes, abundances, distributions, physiology and ecological interactions of fisheries species, and increasingly, there is evidence of impacts to seafood nutritional quality. In a 12-week mesocosm experiment, we tested the influence of projected ocean warming and acidification scenarios on the growth and lipid quality of juvenile sand whiting (Sillago ciliata), a popular fisheries species in eastern Australia. The growth of S. ciliata significantly increased (by 61% body weight) under elevated temperature (+3°C) but was not affected by acidification treatment levels. Lipidomic analysis revealed no influence of temperature or acidification on total lipid content or the composition and total proportions of lipid classes and subclasses. However, elevated temperatures significantly impacted the overall composition of fatty acids, including a shift toward higher saturation and a decline in important omega-3 fatty acids. Fish exposed to elevated temperature treatments had more saturated fatty acids than those at control temperatures, along with reduced levels of the valuable omega-3 eicosapentaenoic (C20:5) and docosahexaenoic (C22:6) fatty acids. Despite impacting fatty acid composition in S. ciliata, the increased growth of the juvenile whiting, if sustained into adulthood, under elevated temperatures, may help compensate for the overall availability of essential polyunsaturated fatty acids to support consumer nutritional requirements. These findings contribute to the growing body of evidence on variable climate resilience in nearshore species to future environmental conditions and the implications for the trophic transfer of nutrients in estuarine ecosystems.

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Large CO2 seeps and hydrate field on the seafloor offshore Mayotte Island

Published 16 June 2026 Science Closed
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Gas hydrates modulate methane and carbon dioxide benthic fluxes into the ocean and usually occur embedded in the sediment. Here we use acoustic surveys alongside optical and geochemical observations from remotely operated vehicles to show that CO2 hydrate mounds are forming directly on the seafloor atop a large liquid CO2 vent field offshore Mayotte Island. The venting, which initiated following volcanic activity in 2018, deleteriously impacts surrounding coral communities due to local acidification.

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Long term variability of temperature and pH in the Bay of Bengal: an investigation on acoustic perspective

Published 28 May 2026 Science Closed
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This study comprehensively assesses the long-term variability of temperature, ocean acidity changes, and their implications on sound absorption and acoustic propagation in the Bay of Bengal. The analysis reveals a persistent warming trend in the Indian Ocean over the past 50 years, with a significant increase in temperature observed during the Sagar Maitri cruise in 2019. Thermal structure analysis using HadleySST EN4 data indicates warming in the upper 50m but a cooling trend in the 100-200m depth range. Oceanic Heat Content analysis highlights an increasing tendency of heat storage in the upper 50m, indicative of global warming.

In the context of surface ducted propagation, Sonic Layer Depth (SLD) and gradients in the Sound Speed Profile (SSP) were crucial factors influencing acoustic energy behavior. The study revealed a decreasing trend in in-layer gradient (Gr_SL) since 1990, intensifying after that period. The below-layer gradient (Gr_BL) also exhibited a decreasing trend, implying complex dynamics in the sonic layer with potential implications for sound propagation in the surface duct.

The investigation into pH changes spanning 65 years demonstrates a declining trend, particularly since the 1990s, attributed to increased atmospheric CO2 dissolution. The study linked this decrease to anthropogenic activities, aligning with global trends. The analysis of sound absorption illustrated a nonlinear relationship between absorption, frequency, and pH, emphasizing a significant impact of ocean acidification on sound absorption in the Bay of Bengal. The acoustic propagation modeling further highlighted a decrease in transmission loss with reducing pH, leading to increased sound travel and potentially noisier oceans. Salinity variations play a more significant role than temperature in influencing sound absorption.

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Aragonite saturation state and coral reefs health assessment in Sri Lanka

Published 11 May 2026 Science Closed
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Ocean acidification (OA) and nutrient enrichment can separately or together threaten coral reefs by reducing calcification efficiency and increasing physiological stress, ultimately weakening reef resilience. Therefore, the study evaluates the prevailing OA level over the Sri Lankan coral reef areas using the aragonite saturation state (ΩAr) and assesses the nitrate (NO3−), and phosphate (PO43−) concentrations over the coral sites. The study was conducted on coral reefs on the eastern coast (EC), southern coast (SC), northern coast (NC), and west coast (WC) of Sri Lanka from April to June 2024. A total of 63 seawater samples were collected around each coastal site for analysis. The ΩAr were supersaturated (ΩAr > 1) and ranged from 2.98 ± 0.04 to 4.92 ± 0.12. Throughout the study period, the study sites had ΩAr values exceeding 2.92 ± 0.16, indicating that the nation’s corals were resilient to deterioration, and the comparative analysis demonstrates that these sites were not vulnerable to OA. However, the NC exhibited significantly (P < 0.05) the lowest ΩAr values (3.2 ± 0.64), positioning the regions near the lower bound of optimal calcification conditions. While ΩAr values indicate low OA stress during sampling, elevated NO3− concentrations (2 – 5 μmol L−1) in SC (2.19 ± 1.28 µmol L−1) and WC (3.52 ± 1.48 µmol L−1) may exacerbate coral bleaching during thermal stress events, representing a co-stressor rather than OA effect. Coral bleaching HotSpot (HS) identification emphasizes how spatially distributed HS are from January to June. The OA risk assessment confirmed that climate change will bring high risk to the coral calcification, reproduction, and damage to the breeding ground, which impact on the ecology and economy of Sri Lanka.

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Light and tidal inundation and exposure regulate the sensitivity of estuarine benthic greenhouse gas fluxes to warming and ocean acidification

Published 28 April 2026 Science Closed
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Coastal sediments are globally significant sources and sinks of greenhouse gases (GHGs), yet their contributions to climate feedbacks of warming and ocean acidification remain uncertain, in part due to limited understanding of short-term variability. Here, we use a fully factorial laboratory experiment to disentangle how diel light–dark and tidal inundation and exposure interact with warming and elevated pCO2 to regulate benthic fluxes of CO2, CH4, and N2O in estuarine sediments, alongside concurrent changes in benthic oxygen exchange. While warming and pCO2 exerted strong independent effects, their influence was shaped by diel and tidal fluctuations in redox conditions and oxygen availability, reflecting shifts in metabolic balance between primary production and respiration. Light consistently limited CO2, CH4, and N2O emissions through enhanced autotrophic uptake and oxygenation, while dark promoted anaerobic production pathways. N2O showed the greatest sensitivity to the combined effects of climate forcing and redox dynamics. Despite warming-driven stimulation of benthic heterotrophy and the production of all GHGs, CO2 remained the dominant greenhouse gas, with minimal CH4 and N2O fluxes due to the limited organic matter availability within the sediment. This reflects the strong redox controls on CH4 and N2O production, which relies on both oxygen depletion and organic substrate supply. Our findings emphasize that fine-scale temporal variability can significantly shape both the magnitude and climate sensitivity of benthic GHG emissions. Capturing these fine-scale controls is essential for accurately modeling the contributions of estuarine sediments to global GHG budgets and their feedbacks.

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Quantifying the role of land-based inputs on coastal ocean acidification from a tropical semi-arid region

Published 15 April 2026 Science Closed
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The land-based inputs in the form of river discharge, wastewater runoff, and submarine groundwater discharge (SGD) are among the major land-based natural pathways for the Coastal Ocean Acidification (COA). This study evaluates the direct influence of these land-based drivers, along with the aerosol deposition, and in-situ biogeochemical processes on COA along a highly populated tropical coastal area. The results suggest that spatially, aerosol deposition and in-situ biogeochemical processes in Kutch region are the major (72%) contributors to COA. In contrast, cumulative land runoff significantly (70%) contributes to COA in South Gujarat. Among these drivers, river water mixing causes the most significant pH decrease (0.093), while wastewater input results in the minimum pH drop (0.016) along the Gujarat coast. The seasonal nature of river water discharge, compared to continuous seepage of both fresh and recirculated (saline) SGD, highlights the role of SGD in COA. These findings align with the global studies represented SGD as one of the prominent land-based drivers for COA. Additionally, the low annual average pH (~ 7.954) along the Gujarat coast is attributed to the region’s macrotidal characteristics, which facilitate the release of sediment bound CO2, leading to a reduction in pH levels. The findings from the current study emphasis the need for comprehensive data collection on physicochemical and biogeochemical parameters to accurately assess COA dynamics and quantification of spatial and seasonal impacts of each driver along the India’s west coast.

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Effects of pH on phytoplankton growth and diversity in a tropical coastal ay: an experimental study

Published 14 April 2026 Science Closed
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This research was intended to investigate the effects of reduced pH on the growth rates and diversity of phytoplankton in the coastal waters of Visakhapatnam in the Bay of Bengal. A short-term (six days) microcosm experiment was conducted with different pH conditions such as ambient (control-in situ pH), pH 8.0 (0.2 pH units drop from in situ pH) and pH 7.8 (0.4 pH units drop from in situ pH) corresponding to low, medium, and high future pH decline scenarios, respectively, to study the direct acidification impact on phytoplankton. The results revealed that the phytoplankton communities exhibit a wide range of responses including changes in growth rate during incubation. From the two treatments, a more pronounced response was observed in pH 7.8 conditions compared to the present pH scenario. Some phytoplankton communities exhibited positive growth responses to acidification, while others showed negative reactions in terms of biodiversity. Notably, Pseudo-nitzschia sp. became dominant during acidification, whereas larger centric diatoms such as Skeletonema spp., Chaetoceros spp., Rhizosolenia sp., Dactyliosolen fragilissimus, and Ditylum brightwellii showed no significant growth response to upcoming acidified conditions. This indicates a diverse array of physiological tolerance among the plankton species to environmental shifts. This study recommends further research to explore the impact of ocean acidification on other planktonic species in the coastal waters of Bay of Bengal.

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Ocean acidification: the silent threat to marine biodiversity

Published 8 April 2026 Science Closed
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Ocean acidification (OA) is one of the quietest yet most profound changes unfolding in our seas. Caused mainly by the ocean’s absorption of excess carbon dioxide from the atmosphere, it steadily lowers seawater pH and depletes carbonate ions — the essential building blocks for shells, skeletons, and coral reefs. These chemical shifts ripple through marine ecosystems, weakening coral structures, slowing the growth of shellfish, disrupting plankton communities, and ultimately destabilising the food webs that sustain biodiversity and human livelihoods. Although OA is recognised as a global problem, its effects are not uniform. Some regions, particularly the Indian Ocean and other tropical waters, remain poorly studied despite being home to rich biodiversity and millions of people whose lives depend on healthy coastal ecosystems. This paper focuses on OA as a “silent” driver of biodiversity loss and addresses two major gaps: the lack of strong policy and governance integration, and the scarcity of regional data for Indian and tropical waters. To explore how OA is framed in global agreements such as the United Nations Convention on the Law of the Sea (UNCLOS) and the Paris Agreement, and examine its treatment in India’s environmental laws, including the Environment (Protection) Act, Coastal Regulation Zone rules, and the Biological Diversity Act. While these frameworks provide important protections, none directly target OA or mandate systematic monitoring. The shortage of long-term, high-resolution data on pH and carbonate chemistry in Indian waters makes it difficult to gauge the scale of the threat or design locally relevant solutions. The lack of species-specific studies in this region adds further uncertainty to impact predictions. This is mainly upon doctrinal studies. This study calls for integrating OA into national marine policies, creating dedicated monitoring networks in the Indian Ocean, and fostering interdisciplinary research that links chemical changes to ecological shifts and community livelihoods. Closing these gaps is vital not only for protecting marine biodiversity but also for ensuring food security and economic stability for coastal populations.

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Effects of upwelling-driven acidification and deoxygenation on the dissolved inorganic carbon system over the southeastern Arabian Sea shelf

Published 8 April 2026 Science Closed
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Highlights

  • Summer monsoon upwelling drives strong acidification and deoxygenation over the EAS shelf.
  • Non-upwelling DIC and TAlk variability is largely governed by conservative water-mass mixing.
  • Elevated nDIC35 during upwelling confirms DIC enrichment beyond salinity stratification alone.
  • AOU-nDIC35 coupling indicates respiratory amplification of upwelled CO2-rich source waters.
  • Reduced buffering and lower ΩCa–ΩAr increase seasonal chemical stress on shelf ecosystems.

Abstract

Repeated measurements of inorganic carbon system parameters over one year along two coastal transects (Kochi in the southern EAS and Mangalore in the central EAS) in the eastern Arabian Sea (EAS) reveal strong seasonal coupling between upwelling, deoxygenation, acidification, and inorganic carbon accumulation on the shelf. During the non-upwelling (oxic) period, the variability of dissolved inorganic carbon (DIC) concentrations and total alkalinity (TAlk) was governed predominantly by conservative water-mass mixing, particularly between low-salinity Bay of Bengal-derived waters and more saline Arabian Sea shelf waters, as demonstrated by the marked reduction in salinity normalised DIC (nDIC35) and TAlk (nTAlk35). In contrast, during the summer monsoon (June–September), coastal upwelling transported oxygen-poor, DIC-rich subsurface waters onto the shelf, leading to pronounced subsurface inorganic carbon enrichment, hypoxia, and acidification. Vertical profiles of nDIC35 showed that elevated inorganic carbon concentrations persisted even after removing salinity effects, increasing from ∼1950–2000 μmol kg−1 at the surface to >2100–2200 μmol kg−1 below ∼40 m. Nearshore surface waters during peak upwelling exhibited a strong offset between measured DIC and nDIC35, indicating localized freshwater dilution, but salinity-normalised values confirmed that the underlying carbon inventory remained high. Apparent oxygen utilisation (AOU) and nDIC35 were positively correlated, indicating that a substantial fraction of the residual DIC enrichment was associated with oxygen consumption, although this relationship reflects the combined imprint of DIC-rich upwelled source waters and subsequent microbial remineralisation within the stratified shelf system. Thus, carbon accumulation during the summer monsoon is best explained by a two-stage mechanism: (i) physical advection of CO2-rich, oxygen-deficient upwelled waters, followed by (ii) secondary amplification through local respiration. In contrast, TAlk exhibited much weaker non-conservative modification, and the elevated alkalinity generated under low-oxygen conditions was insufficient to counteract the strong DIC-driven reduction in carbonate-system buffering capacity, thereby increasing the system’s vulnerability to pCO2 build-up and acidification. Consequently, calcite and aragonite saturation states declined sharply during upwelling, with ΩCa and ΩAr falling to ∼2.5 and ∼ 1.5, respectively, when pCO2 exceeded 1000 μatm under severe oxygen depletion. The co-occurrence of hypoxia, acidification, and weakened carbonate buffering characterises the eastern Arabian Sea shelf as a highly dynamic natural laboratory for understanding multi-stressor impacts on coastal biogeochemistry and ecosystem vulnerability.

Continue reading ‘Effects of upwelling-driven acidification and deoxygenation on the dissolved inorganic carbon system over the southeastern Arabian Sea shelf’

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