Ocean Acidification

The impact of ocean acidification (OA) on seawater carbonate chemistry, particularly on the aragonite saturation state (Ω_arag), is inadequately understood in the Gulf of Mannar (GoM) and Palk Bay (PB). In this study, we analyzed the spatial and temporal variability of Ω_arag to pinpoint potential ocean acidification hotspots within these two ecologically critical coastal habitats along the southeastern coast of India. A pilot study was conducted over 24 locations during 2023–2024. The findings reveal that PB had a higher mean pH (8.33 ± 0.06) than GoM (8.08 ± 0.02). In GoM, Ω_arag values were lower, at 2.82 ± 0.20, indicating that calcification conditions were not adequate. In PB, it reached 3.22 ± 0.57, which means that conditions for calcifying organisms were favourable. Seasonal variability in Ω_arag was primarily controlled by pH, carbonate ion concentration, pCO₂, and monsoonal hydrographic processes Seasonal patterns corroborated that these conditions were modulated by both anthropogenic activities and natural processes. During the Northeast and Southwest monsoons, there were significant drops in Ω_arag that matched changes in important biogeochemical indices. Overall, these results reveal that PB experiences greater variability in Seawater carbonate chemistry, while GoM exhibits relatively stable but slightly lower carbonate saturation. Pearson correlation and Structural Equation Modelling further demonstrated that pH and carbonate ion concentration positively influenced Ω_arag, whereas pCO₂ and Revelle factor showed negative relationships. The findings enunciate that GoM is more vulnerable to ocean acidification than PB and emphasize the value of Ω_arag as a sensitive indicator for detecting emerging acidification hotspots in coastal waters. It also emphasizes the importance of regular monitoring of OA parameters in these regions to protect these critical ecosystems and their associated services.

Ocean warming and acidification pose significant threats to marine biodiversity and human nutrition by fundamentally altering the biochemical composition of marine organisms. A primary concern is the potential decline in omega-3 fatty acids (FAs) that are essential to human health and primarily obtained through seafood consumption. The influence of these climate stressors on FA content across marine food webs remains poorly understood. To address this critical knowledge gap, we conducted a global meta-analysis of 489 experiments across 143 publications and 132 marine species, quantifying the effects of warming and acidification on nutritionally important FAs in marine primary producers, invertebrates and fishes. Under warmer conditions, we detected reductions of up to 34% and 50% in omega-3 FA proportions and concentrations, respectively, and up to 53% in omega-3: omega-6 ratios, together with increases of up to 22% and 17% in saturated FA proportions and concentrations, respectively across taxa groups. Critically, these effects were intensified as warming increased. The most severe reductions in omega-3 FAs were observed in primary producers, suggesting that climate-driven changes at the base of the food web could impair trophic transfer of these micronutrients, limiting nutrient availability for higher trophic levels and ultimately humans. Ocean acidification, conversely, demonstrated a minor overall effect on FA levels, although this result was based on substantially fewer studies. Furthermore, we identified species from environments with broader temperature ranges, diatoms, herbivorous invertebrates and low-resilience fish to display larger reductions in omega-3 FA proportions under warming. Our meta-analysis further highlighted the need for long-term studies under ecologically realistic conditions to improve predictions of nutritional responses to climate change. These findings are essential for understanding how the nutritional value of species change under climate change, and can inform fisheries management, aquaculture and public health policy aimed at securing the future availability of these vital micronutrients.

There is increasing demand to compare long-term biological observations with derived or measured carbonate chemistry data to characterize the biological consequences of ocean acidification (OA) in natural environments. Meeting this need calls for a standardized methodology by which to quantify and directly compare biological and carbonate chemistry change. Widdicombe et al. (2023) proposed a conceptual framework based on biological traits and paired biological-chemical rate comparisons, but its practical implementation has remained unresolved.

Here we operationalize that conceptual framework through a standardized analytical workflow that integrates heterogeneous biological observations with established OA time-series methodology. The workflow addresses five analytical challenges that hinder comparison of long-term biological and OA records: 1) fundamentally different temporal structures of biological and OA time series, 2) heterogeneity of biological observations, 3) sparse and irregular biological sampling, 4) nonlinear biological trajectories, and 5) appearance–disappearance events that invalidate conventional relative change metrics. The workflow addresses these challenges through standardized biological grouping, annual aggregation, piecewise linear segmentation, normalization by ecological boundaries, and direct pairing of biological and chemical rates of change.

We evaluated workflow performance using 36 synthetic biological time series generated by applying predefined temperature-pH-driven biological performance functions to a 108-year hydrodynamic-biogeochemical model simulation under the SSP3-7.0 emissions scenario. Biological rates estimated by the workflow closely agreed with the corresponding reference rates. Rate estimates remained similar across the two annual representations, and their relative ordering was largely preserved across contrasting site-specific temperature-pH transitions. These results demonstrate the robustness of the workflow under the tested patterns of abiotic forcing.

The analytical workflow developed here operationalizes the conceptual approach of Widdicombe et al. (2023) by standardizing the estimation and comparison of rates of change in biological and carbonate chemistry time series. It supports comparisons among compatible biological indicators while retaining the ecological context of each time series. The workflow does not establish causality; instead, it assesses the temporal coherence between long-term changes in candidate biological indicators of OA and concurrent changes in carbonate chemistry. It therefore provides a practical foundation for regional and global syntheses of biological responses to OA and other long-term abiotic stressors.

Highlights

• Acidification compresses benthic community isotopic space via carbon homogenization.
• Functional groups show differential isotopic responses consistent with bottom-up trophic reorganization.
• Arbacia lixula shifts from a sponge-dominated diet to herbivory under low pH.
• Paracentrotus lividus maintains stable trophic niche despite acidification.
• Asymmetric grazer responses drive site-specific increases in isotopic niche overlap under acidification.

Abstract

Ocean acidification is expected to alter marine food webs, yet community-level evidence from long-term, naturally acidified systems remains limited. We used stable isotope ratios (δ¹³C and δ¹⁵N) to investigate how natural acidification restructures benthic trophic interactions, from basal resources to consumer niche dynamics, a naturally acidified system (pH gradient: 8.1 to ~7.4; pHT scale) spanning present-day to end-of-century projected conditions (SSP2–4.5 to SSP5–8.5). After accounting for local differences in the isotopic baseline, we found that: (1) community isotopic space contracted under acidification, with the carbon range decreasing as primary producers converged toward ¹³C-depleted values, while the nitrogen range expanded, indicating species-specific trophic adjustments; (2) functional groups responded asymmetrically, with primary producers showing the strongest δ¹³C shifts, filter-feeders remaining relatively stable, and omnivorous consumers showing reduced δ¹³C and δ¹⁵N values; (3) the two dominant sea urchin grazers responded in opposite directions — the macrophyte-based omnivore Paracentrotus lividus maintained stable isotopic values across the pH gradient, whereas the invertebrate-based omnivore Arbacia lixula exhibited pronounced isotopic shifts — resulting in markedly increased, though site-dependent, niche overlap at the most acidified sites (up to 27%); (4) Bayesian mixing models revealed that this convergence was driven by dietary shifts in A. lixula, which transitioned from a predominantly invertebrate-based diet (70–79% animal prey) toward algal-based feeding under acidification (19–24% animal prey), while P. lividus maintained relatively consistent dietary composition (50–70% algae). Together, these results indicate that acidification-driven homogenization of primary producers, combined with asymmetric consumer responses, compresses trophic niche space, with broad implications for the resilience and functioning of benthic ecosystems under future ocean change.

Graphical abstract

Along a natural CO₂ gradient in the Canary Islands (pH 8.1–7.4, pHT scale; spanning present-day to end-of-century projected conditions, SSP2-4.5–SSP5-8.5), ocean acidification compresses community isotopic space through homogenization of primary producers. The two dominant sea urchin grazers respond asymmetrically: Paracentrotus lividus maintains a stable trophic niche, while Arbacia lixula shifts toward algal-based feeding, increasing niche overlap up to 27% at the most acidified sites.

Macroalgal-herbivore interactions are fundamental to the stability of coastal ecosystems and their vulnerability to climate-driven shifts will affect biodiversity and ecosystem functioning. However, the effects of climate change on species interactions may be context dependent and vary by seasonal oceanographic processes. We examined the effects of warming and acidification on an intertidal primary producer foundation species, Silvetia compressa, and its herbivore, Agathistoma eiseni, under differing upwelling regimes in early (low upwelling) and late spring (high upwelling). We measured metabolic rates including photosynthesis, respiration, and calcification after four weeks of exposure to a range of nine pH levels (7.2–8.0) at two temperature levels (16°C, 20°C). Rockweed responded more to acidification than to warming, but showed different seasonal responses, decreasing photosynthetic rates in early spring and increasing rates during late spring. Their snail consumer, however, responded most strongly to temperature, increasing both respiration and calcification rates under warm conditions in late spring. Our findings demonstrate that seasonal differences can modulate responses of macroalgae and herbivores to changing environmental conditions. Warming may increase top-down effects through increased snail metabolism, while acidification affects rockweed production to affect bottom-up forcing. By assessing physiological responses across different ecological regimes, our study advances understanding of how top-down and bottom-up processes interact in intertidal ecosystems under global change.

Highlights

• Impact of ocean acidification on coastal copepods is proportional to its intensity.
• Projected low pH significantly increases Pseudodiaptomus serricaudatus mortality.
• Lower pH reduces overall population density and alters life-stage composition.
• Predicted acidification delays larval development in P. serricaudatus.

Abstract

Anthropogenic ocean acidification, driven by increased atmospheric CO₂ absorption by seawater, poses significant threats to marine ecosystems, particularly in coastal environments, where pH reductions are expected to be more severe than in the open ocean. Copepods represent a critical link between primary producers and higher trophic levels in the classical marine food chain. However, their responses to the rapid decline in seawater pH expected by the century’s end remain poorly constrained—especially in dynamic coastal regions. The current study quantifies how near-future pH scenarios affect the survival and population dynamics of the Pseudodiaptomus serricaudatus, a perennial coastal and estuarine copepod of the West Coast of India. Adult copepods of this species were incubated at four target pH levels (8.10, 7.81, 7.51, 7.04) using an automated CO₂-bubbling system to assess mortality and population dynamics. Mortality was evaluated every three days up to day 10; population stage composition was recorded on day 15. Mortality remained 23% at pH 8.1 (representing open-ocean conditions) but rose to 30% at pH 7.81 (present-day coastal average), 47% at pH 7.51 (projected late-21st-century coastal value), and 77% at pH 7.04 (extreme 23rd-century scenario). Population data revealed declines across life stages and copepodite-to-nauplius (C_t/N) ratio at lower pH values, indicating impaired development. Any potential impact of ocean acidification on copepods could affect the fisheries sector, as these copepods are the chief food source for mackerel, an economically important fish in the region. The critical pH threshold for P. serricaudatus appeared to lie between 7.51 and 7.81, corresponding closely to current coastal conditions and near-future projections. These findings suggest that coastal copepod populations may already be experiencing sublethal stress, with potential demographic collapse occurring within decades rather than by the end of the century.

Highlights

• Hatching success of Artemia salina was impaired by acidified conditions.
• Development of Artemia salina pre-nauplii was negatively impacted by both species of harmful algae and low pH exposure.
• Only toxin-producing harmful algal species produced significant morphological abnormalities of pre-naupliar Artemia salina.
• No synergistic effects of harmful algae and low pH exposure was seen in this study.

Abstract

Harmful algal blooms (HABs) and ocean acidification are major stressors in coastal marine ecosystems, yet their combined effects on early zooplankton development remain poorly understood. This study tested whether HABs and low pH exposure exert synergistic effects on the embryonic development and hatching success of brine shrimp (Artemia salina) cysts. Cysts were exposed to controls, toxin-producing HAB species (i.e. Margalefidinium polykrikoides or Alexandrium catenella), or a non-toxin producing HAB control (i.e. Gymnodinium aureolum) and at either ambient (pH ∼ 8) or acidified (pH ∼ 7.2) conditions. We hypothesized that the combined exposure to a toxin-producing HAB at acidified pH conditions would reduce hatching success and alter embryonic morphology. Low pH exposure consistently reduced hatching success and slowed developmental progression across treatments, reducing nauplii production. HAB exposure alone did not affect hatching success, but exposure to toxin-producing HABs in ambient pH conditions did significantly increase the morphological abnormalities at the membrane stage. Overall, no synergistic effects of HABs and low pH exposure were found for any response variable. Instead, HABs and low pH exposure exerted distinct, stage-specific impacts that were additive rather than interactive. These findings suggest that while exposure to toxin-producing HABs primarily disrupts morphology and low pH exposure slows development and reduces hatching success, their combined effects do not exceed additive expectations. Future work should explore physiological mechanisms underlying these stress responses and assess whether similar patterns occur in other cyst-producing zooplankton.

Highlights

• Thermal stress modulates protein digestibility and bioaccessibility in I. badionotus.
• Induction at 28 °C maximized protein digestibility and intestinal bioaccessibility.
• Sublethal 34 °C for 12 h peaked digestibility; longer exposure caused decline.
• Intestinal bioaccessibility was the most sensitive indicator to pre-harvest stress.
• Pre-harvest thermal and pH stress can alter the macronutrient nutritional quality in I. badionotus.

Abstract

The impact of in vivo thermal and pH stress on the macronutrient nutritional quality of the sea cucumber Isostichopus badionotus remains poorly understood from a food-nutrition perspective. This study investigated how acute induction temperatures (16–36 °C) and sublethal thermal exposure (34 °C, 3–48 h), combined with two ambient pH levels (7 and 8), modulate in vitro protein, lipid and carbohydrate digestibility and protein bioaccessibility of the sea cucumber I. badionotus, a collagen-rich tropical food, using the standardized static INFOGEST gastrointestinal simulation. In the acute induction experiment, protein digestibility, total bioaccessibility and intestinal bioaccessibility were strongly affected by temperature, with a narrow optimum at 28 °C (digestibility ∼58–66%; intestinal bioaccessibility ∼64–68% at both pH levels). Lipid digestibility varied within a relatively narrow range across treatments, whereas carbohydrate digestibility showed a marked temperature × pH interaction, with higher values at 28 and 36 °C under alkaline conditions. In the sublethal-stress experiment, protein digestibility and bioaccessibility followed a time-dependent pattern, with a peak at 12 h followed by a decline at 24–48 h, consistent with patterns of progressive collagen oxidation reported in other holothuroids. Across both experiments, intestinal bioaccessibility was the most responsive nutritional indicator to pre-harvest stress. These results demonstrate that pre-harvest thermal and pH stress can qualitatively alter the macronutrient nutritional quality of I. badionotus, with implications for harvest timing, processing optimization and seafood quality management under ongoing ocean warming and acidification.

Graphical abstract

Live sea cucumbers (Isostichopus badionotus) were exposed in vivo to controlled thermal and pH stress (16–36 °C, pH 7–8; 6 h acute induction and 3–48 h sublethal exposure), inducing proteolysis, oxidative responses, heat-shock protein expression and structural remodeling of the collagen-rich body-wall matrix. Samples were then subjected to standardized INFOGEST in vitro digestion (oral, gastric and intestinal phases with pepsin, pancreatin and bile salts), followed by dialysis at 12–14 kDa to quantify intestinal bioaccessibility. A narrow optimum at 28 °C and a sublethal window around 12 h at 34 °C tended to maximize protein digestibility and intestinal bioaccessibility, whereas conditions outside this window were associated with increased oxidative crosslinking and reduced protein quality.

Ocean acidification and ocean warming are among the most pressing threats to marine ecosystems, with significant implications for calcifying organisms such as crustose coralline algae (CCA). These habitat-forming species play critical ecological roles in reef systems, yet they are particularly vulnerable to ocean acidification and ocean warming. While laboratory studies have documented declines in coralline algal growth and calcification under elevated CO₂ and temperature, less is known about their responses in natural settings where environmental variability and potential acclimatization may modulate stress impacts. Furthermore, the interactive effects of multiple stressors across generations remain poorly understood, limiting our ability to predict ecosystem-level consequences of climate change. This thesis explores the physiological responses of coralline algae to ocean acidification, ocean warming, and marine heatwaves through complementary field and laboratory approaches. Using natural climate analogues, I investigated how CCA communities respond to chronic exposure to low pH and elevated temperatures in situ (Chapters 2 and 3). These natural systems provide unique opportunities to assess long-term acclimatization and identify potentially resilient populations. To disentangle the mechanisms underlying stress responses and examine multi-generational effects, I conducted a multigenerational, multi-stressor laboratory experiment exposing coralline algae to factorial combinations of elevated pCO₂ and temperature over successive generations (Chapter 4). Together, these approaches enabled assessment of single-driver impacts, stressor interactions, and the capacity for phenotypic plasticity and acclimatization under both fluctuating natural conditions and controlled experimental regimes. Results from natural analogues revealed species-specific variation in stress tolerance, with some CCA populations maintaining growth and calcification despite chronic exposure to conditions projected for year 2100. The laboratory experiment revealed that growth significantly declined when exposed to ocean acidification and the combined effect of ocean acidification and ocean warming over multiple generations. This research advances our understanding of how marine calcifiers respond to climate change across ecological and temporal scales. The findings have important implications for predicting ecosystem trajectories and identifying potential climate refugia in an increasingly acidic and warming ocean.

Reef-building corals are under pressure to acclimatize and/or adapt to multiple co-occurring stressors, including climate change and declines in water quality. However, their capacity to do so is largely unknown. Yet, some corals naturally persist in habitats with multi-stressor conditions which can provide crucial insight into coral adaptive capacity under future ocean conditions. To assess the role of phenotypic plasticity and local adaptation in coral acclimatory responses to novel conditions, we reciprocally transplanted two coral species (Siderastrea siderea, branching Porites) between an inland bay characterized by multiple, co-varying stressors, including warmer seawater temperatures, higher pollution and more variable temperatures, pH and dissolved oxygen concentrations, and an environmentally less extreme, more stable fringing reef. We measured key phenotypic traits (e.g., calcification, tissue biomass, metabolic rates, chlorophyll concentrations) of all transplant groups after 0, 4 and 12 months. Reef-origin corals demonstrated high plasticity when transplanted to the bay, as both species maintained high survival and calcification, and matched bay natives’ other phenotypic traits. Yet, reef-to-bay transplants of both species had reduced photosynthesis-to-respiration ratios in the bay, highlighting the risk of metabolic trade-offs unless compensated for through increased heterotrophy. Conversely, bay-origin corals had enhanced photosynthetic performance but lower calcification on the reef. Reaction norms provided stronger evidence for environmental specialization in bay-origin than reef-origin corals, which could limit their use as stress-tolerant source corals for reef restoration. Overall, our findings suggest multi-stressor variability may promote specialized genotypes, rather than increased phenotypic plasticity, though this may also depend on predictability of environmental variability.

We characterize surficial seawater physico-chemical conditions and dissolved CO2 dynamics at the active volcanic system of Deception Island (Antarctica), with emphasis on identifying hydrochemical plumes associated with shallow sea hydrothermal vents (SHV). During the austral summers of 2024 and 2025, spatial surveys of pH, temperature, salinity, and total alkalinity were conducted along the inner coast of Port Foster. Acidification plumes (down to pH ~ 6.1), carbonate undersaturation, and elevated total alkalinity (2,400–7,000 μmol kg⁻¹) were identified in areas affected by volcanic–hydrothermal emissions. Dissolved CO₂ reached up to 90,000 μatm, with extreme values approaching 160,000 μatm, far exceeding atmospheric equilibrium. The results reveal a strong tidal control on CO₂ dynamics in Port Foster, with low tide conditions enhancing nearshore accumulation and potential outgassing. Fluid compositions suggest mixed contributions from hydrothermal inputs, seawater, and meteoric components. These findings highlight the value of carbonate system parameters for detecting and monitoring SHV activity, and position Deception Island as a natural laboratory to investigate volcanic impacts on coastal ocean acidification under polar conditions.

Potentially toxic elements pollution in marine environments is intensifying under anthropogenic activities and climate-driven changes, yet the interactive effects of multiple stressors on metal bioaccumulation, ecosystem health, and human risk remain inadequately synthesized. This review aims to quantitatively integrate multi-stressor interactions involving ocean warming, acidification, and eutrophication with potentially toxic elements dynamics across coastal, estuarine, and open ocean systems. Following PRISMA guidelines, 154 peer-reviewed studies from 652 screened records were synthesized to establish a comprehensive, evidence-based framework for predictive risk assessment. This review indicates that multi-stressor conditions fundamentally alter speciation and bioavailability of mercury (Hg), cadmium (Cd), lead (Pb), arsenic (As), and chromium (Cr), producing synergistic and other non-additive effects that amplify biomagnification. Methylmercury (MeHg) exhibits the strongest trophic magnification, with factors reaching 11.1 in polar shelf ecosystems, while cadmium and lead show biodilution yet elevated tissue burdens in benthic organisms under combined warming and acidification. Arsenic speciation shifts toward more toxic inorganic forms under acidification, and chromium bioavailability increases with pH-dependent redox changes. Sublethal effects, including reproductive impairment, oxidative stress, and metabolic disruption, cascade to population declines and impaired ecosystem functions, with early life stages exhibiting up to 40% greater sensitivity under combined exposures. Human health risk assessments reveal that seafood-consuming populations face elevated risks, with hazard quotients for mercury exceeding one in high-consumption groups, lifetime cancer risks from arsenic and cadmium surpassing safe thresholds, and lead and chromium contributing to cardiovascular and renal toxicities. These findings suggest that current regulatory thresholds, which overlook interactive effects and additive toxicities, systematically underestimate cumulative threats in a changing ocean, necessitating urgent adoption of integrated, climate-aware frameworks aligned with international instruments such as the Minamata Convention on Mercury to protect marine ecosystems and human health.

Highlights

• Day-15 shell closure ≠ functional repair: pH 7.7 disrupts microstructure, delays crystallization, and causes ∼87% strength loss.
• Hemocytes: early divergence (pH 7.9 inflammatory vs. pH 7.7 stress), late convergence with weak effectors; mantle upregulates immunity, suppresses OXPHOS/ECM.
• TNF-NF-κB (TNFSF14/TRAF6/IκBα) mediates immune-mineralization crosstalk, linking immunity to repair impairment.
• Multi-tissue dysregulation causes repair failure – not a single bottleneck, but decoupling of recruitment from deposition.

Abstract

Ocean acidification (OA) impairs biomineralization in bivalves, but its effects on immune-biomineralization crosstalk during shell repair remain unknown. Here, we exposed adult Mytilus edulis bearing standardized shell perforations to three pH levels (8.1, 7.9, and 7.7) for up to 40 days. OA slowed early repair and caused microstructural disorganization and an approximately 87% reduction of compressive strength at pH 7.7, yet the damaged area appeared largely closed by day 15, suggesting a decoupling between morphological closure and functional recovery. In addition, transcriptomic profiling of hemocytes and mantle tissue, based on an average of 6.5 Gb of clean reads per sample mapped to the M. edulis reference genome (NCBI Assembly GCF_000511035.1), revealed that these shell-level defects were accompanied by coordinated immune and metabolic reprogramming. Hemocytes, the primary immune effector cells of bivalves, exhibited pH- and time-dependent shifts with moderate acidification (pH 7.9) promoting inflammatory transcripts, whereas severe acidification (pH 7.7) suppressed these signals while upregulating stress-associated pathways; both treatments consistently downregulated lysosomal proteases and NF-κB negative regulators. The mantle, a primarily mineralizing organ, paradoxically upregulated immune-related genes while suppressing oxidative phosphorylation and extracellular matrix pathways. This tissue-level imbalance, with hemocytes recruited but functionally constrained and mantle metabolically suppressed yet immunologically activated, points to TNF-NF-κB pathway modulation as a key mediator of shell repair under acidification. Our findings demonstrate that visible shell closure masks underlying structural and mechanical failure, and that immune regulation, rather than simple suppression or activation, critically shapes the repair outcome. These results advocate for multifunctional indicators beyond closure area to assess shell integrity in acidified marine environments.

For the first time, the Autosub Long Range (ALR) completed a fully autonomous, long-distance (2,000 km) scientific mission, delivering new insight into coastal carbonate dynamics and air-sea CO2${\text{CO}}_2$ fluxes. Equipped with a suite of oceanographic sensors, including a Lab-on-Chip (LOC) pH sensor and a Sea-Bird SeaFET pH sensor, the mission generated nearly 50,000 high-resolution pH measurements, providing one of the most detailed continuous coastal carbonate data sets collected to date in the region. We evaluated the adjustment of the SeaFET reference potential (k₀), testing both the co-deployed LOC sensor and neural network estimates as reference pH. Before correction, the LOC and SeaFET sensors showed close agreement (Δ$\mathrm{\Delta }$pHT${\text{pH}}_{\mathrm{T}}$ = 0.013 ±$\pm$ 0.009), which improved to Δ$\mathrm{\Delta }$pHT${\text{pH}}_{\mathrm{T}}$ = 0.00004 ±$\pm$ 0.007 after LOC-based k₀ adjustment. Both sensors diverged from model estimates, indicating reduced ability of models to resolve fine-scale coastal variability and reinforcing the need for direct in situ observations. Total alkalinity (TA) was derived from salinity-based relationships and model predictions, and paired with pH (SeaFET, LOC, and modeled) to estimate the partial pressure of CO2${\text{CO}}_2$ (pCO2${\text{CO}}_2$), which ranged 263–598 ±$\pm$ 27 μatm. Resulting air-sea CO2${\text{CO}}_2$ fluxes ranged −17.0 to 7.1 ±$\pm$ 1.09 mmol m−2 d−1${\mathrm{m}}^{-2}{\mathrm{d}}^{-1}$, with the Celtic Margin acting as a net CO2${\text{CO}}_2$ sink in May–June of 2022. pCO2${\text{CO}}_2$ and CO2${\text{CO}}_2$ flux proved sensitive to subtle pH differences, but less so to TA estimates. Our findings demonstrate the critical role of high-resolution autonomous observations in quantifying coastal carbonate dynamics and CO2${\text{CO}}_2$ fluxes, capturing processes and variability that are largely unresolved by ship-based surveys or global models.

Plain Language Summary

For the first time, the Autosub Long Range (ALR), a robotic underwater vehicle, completed a long-distance (2,000 km) scientific mission without ship support. During the mission, the ALR collected ocean data using sensors that measured water temperature, oxygen, and seawater acidity (pH). The mission generated nearly 50,000 high-resolution pH measurements, providing one of the most detailed continuous coastal carbonate data sets collected in this region. We evaluated how well two onboard pH sensors agreed with each other and with model-based estimates, and tested an applied correction method. Before and after correction, the sensors closely agreed, showing that autonomous platforms can gather high-quality chemical measurements that resolve coastal marine carbon dynamics better than model estimates alone. We used the pH data, along with estimates of TA, to calculate carbon dioxide (CO2${\text{CO}}_2$) exchange between the ocean and atmosphere, finding that the region operates primarily as a carbon sink in May–June. Our calculations were strongly influenced by pH input but much less by alkalinity, highlighting the importance of accurate pH observations. This study shows that autonomous sensors can improve monitoring of carbon cycling in dynamic, understudied coastal waters that play a key role in the global carbon system.

Ocean acidification (OA) is expected to alter marine phytoplankton communities by modifying seawater carbonate chemistry, with important implications for marine food webs and biogeochemical cycling. In this study, we tested the hypothesis that comparable pH reductions achieved through different acidification methods generate distinct carbonate chemistry conditions and consequently different biological responses. A 5-day microcosm experiment was conducted using natural phytoplankton assemblages collected from the coastal Eastern Arabian Sea. OA was simulated by HCl addition (acid), CO₂ enrichment (CO₂), and acid–base manipulation. Changes in carbonate chemistry, nutrients, phytoplankton community composition, pigments, bacteria, microzooplankton, and organic carbon pools were monitored. As expected, the three manipulation methods produced significantly different carbonate chemistry conditions despite comparable pH reductions. Repeated-measures ANOVA and Tukey’s post hoc analyses confirmed significant treatment effects for pH, pCO₂, DIC, HCO₃⁻, CO₃²⁻, and total alkalinity. By Day 5, CO₂ enrichment significantly increased total phytoplankton abundance and promoted microplankton dominance, driven by bloom-like proliferation of Thalassiosira sp., whereas picoplankton declined significantly. Conversely, pennate diatoms, cyanobacteria, dinoflagellates, and coccolithophores exhibited weak or negative responses. Concurrent changes in phosphate, nitrite, autotrophic bacteria, and microzooplankton indicated that nutrient availability, microbial recycling, and grazing interacted with carbonate chemistry to regulate phytoplankton succession. Our findings demonstrate that phytoplankton responses in the oligotrophic Eastern Arabian Sea are governed by the combined effects of carbonate chemistry, nutrient dynamics, and ecological interactions rather than pH alone.