Ocean Acidification

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.

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.

The ocean’s capacity to absorb anthropogenic CO₂ is predicted to decrease with global warming, reinforcing a climate–carbon cycle feedback. However, the effects of specific mechanisms such as circulation and temperature on marine carbon components and atmospheric CO₂ under future emission scenarios remain poorly quantified, especially on multi-centennial timescales. Here, using a decomposition of dissolved inorganic carbon, we show that under high-emission scenarios, circulation changes dominate the climate–carbon cycle feedback by reducing anthropogenic carbon uptake and redistributing alkalinity, despite compensating increases in biological carbon storage and air–sea disequilibrium. By contrast, under low emissions, temperature changes, amplified by increased physical disequilibrium, dominate the climate–carbon cycle feedback. Previous estimates using the apparent oxygen utilization approximation may have considerably underestimated changes in biological carbon storage. These results improve mechanistic understanding of long-term global carbon cycle dynamics, with implications for efforts to achieving zero net emissions and proposed marine CO₂ removal strategies.

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$$ fluxes. Equipped with a suite of oceanographic sensors, including a Lab-on-Chip (LOC) pH sensor and a Sea-Bird SeaFET pH sensor, the mission generated nearly 50,000 high-resolution pH measurements, providing one of the most detailed continuous coastal carbonate data sets collected to date in the region. We evaluated the adjustment of the SeaFET reference potential (k₀), testing both the co-deployed LOC sensor and neural network estimates as reference pH. Before correction, the LOC and SeaFET sensors showed close agreement (Δ$$pHT$$ = 0.013 ±$$ 0.009), which improved to Δ$$pHT$$ = 0.00004 ±$$ 0.007 after LOC-based k₀ adjustment. Both sensors diverged from model estimates, indicating reduced ability of models to resolve fine-scale coastal variability and reinforcing the need for direct in situ observations. Total alkalinity (TA) was derived from salinity-based relationships and model predictions, and paired with pH (SeaFET, LOC, and modeled) to estimate the partial pressure of CO2$$ (pCO2$$), which ranged 263–598 ±$$ 27 μatm. Resulting air-sea CO2$$ fluxes ranged −17.0 to 7.1 ±$$ 1.09 mmol m−2 d−1$$, with the Celtic Margin acting as a net CO2$$ sink in May–June of 2022. pCO2$$ and CO2$$ 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$$ fluxes, capturing processes and variability that are largely unresolved by ship-based surveys or global models.

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.

Coralline algae, calcifying red algae of the orders Corallinales, Hapalidiales and Sporolithales, are important components of many marine ecosystems, providing stability and protection from high energy waves in coral reefs, and providing shelter, substrate and nutrients for many organisms across a range of marine environments. Corallines are thought to be amongst the most vulnerable marine organisms in the face of climate change, as their high-Mg calcite mineralogy makes them especially vulnerable to ocean acidification. Corallines also require light to perform photosynthesis and are therefore vulnerable to siliciclastic sedimentation which can reduce illumination and bury corallines. Lab based acidification experiments show most coralline species respond negatively to elevated CO2 conditions, but the nature and magnitude of effects vary between species, and the rate of experimental CO2 increase is much faster than natural rates, making it difficult to use experimental data to predict how corallines respond to acidification on time scales of thousands to millions of years. While most coralline groups are not directly affected by temperature changes, global warming can alter the hydrosphere, increasing precipitation intensity and therefore terrestrial runoff in some coastal areas. Increased runoff can directly lower illumination in the water column and can also cause phytoplankton blooms that significantly reduce illumination. The earth’s climate and ocean conditions have changed greatly throughout the Cenozoic, with significant effects on many marine organisms including corals, benthic foraminifera, bivalves, sponges, and many other groups. The response of coralline algae to Cenozoic climate change, however, has received less research attention despite their importance in marine ecosystems.

This dissertation uses the fossil record to assess how coralline algae responded to different types of Cenozoic environmental change at three different scales. Chapter 1 looks at coralline response to rapid warming, acidification and sediment influxduring the Paleocene-Eocene Thermal Maximum (PETM) in northeastern India and Tibet. The PETM is the largest hyperthermal of the Cenozoic, where a massive release of CO2 caused widespread ocean acidification and warming of ~5-8 °C over ~10,000 years. At both study locations, coralline abundance is high in the pre-PETM and remains high into the PETM onset. Corallines then disappear abruptly as carbonate sedimentation rapidly ends under heavy siliciclastic input. The results of this chapter suggest that coralline algae may be more tolerant of acidification, but less tolerant of sedimentation, than previously thought.

Chapter 2 uses the early Eocene Sierra Blanca Limestone in Santa Barbara County, California, as a case study for rhodolith bed construction in the early Eocene greenhouse. Elevated temperatures and CO2 in the early Eocene prevented rhodolith beds from developing in most areas, so the Sierra Blanca Limestone provides rare insight into the environmental conditions allowing a robust coralline deposit to form in a seemingly hostile ocean. The lithology, taxonomic assemblages, and characteristics of Sierra Blanca rhodoliths suggest they developed on an offshore uplifted block along an upwelling coastline, allowing rhodoliths to thrive in relatively cool water beyond the reach of siliciclastic sedimentation.

Chapter 3 examines what controls the partitioning of coralline algae into reef and non-reef habitats on carbonate platforms during the Paleogene and Neogene. Coral reefs are significantly impacted by climate change during this interval, but coralline algae are more resilient. Coralline algae occupy non-reef settings through the studied interval and their abundance in non-reef environments shows little variation. As coral reefs develop, corallines persist in non-reef environments and expand into reefs where they play a crucial role in cementing the framework. When coral reefs collapse, corallines do not take over reef building, but persist and even expand in non-reef settings, often constructing rhodolith beds that serve some of the same ecological roles as coral reefs.

Together, these chapters highlight the ability of coralline algae to adapt to changing environmental conditions despite their assumed vulnerability. The discrepancies between lab-based experiments and paleontological observations suggest that the rate of change is vitally important: corallines can adapt to even significant change if it occurs slowly enough but are less able to adapt to rapid environmental change.

Coastal and marine systems are some of the most biologically productive and economically important systems on Earth, yet they are being compromised at an accelerating pace by climate change. Ocean warming, marine heatwaves, sea-level rise, ocean acidification and deoxygenation along with increasing extreme events are interacting to change the structure, function and resilience of ecosystems among coral reefs, mangroves seagrass meadows and salt marshes. Although effort has been devoted to understanding individual ecosystem effects of climatic change, we know less about comparative mechanistic synthesis and cross-ecosystem vulnerability trajectories. This synthesis explores recent understanding of the biophysical determinants of ecosystem decline, with particular reference to thresholds, regime shifts and feedbacks that limit capacity to adapt. We explore different sensitivities among these biogenic coastal systems, and explain why coral reefs are confronted with sharp thermal and carbonate chemistry thresholds, while mangroves and tidal wetlands have threshold-based resilience contingent on sediment supply and geomorphologic setting. Beyond ecological effects, we compile social–environmental responses including fishery reduction, coastal defense loss and blue carbon release, and rising human vulnerability especially in small island and developing coastal regions. The review also examines ecosystem-based adaptation and nature-based solutions, considering their co-benefits and implementation challenges in a high-emission world. By synthesizing climate drivers, ecosystem thresholds, and social–ecological tipping points within a unified conceptual framework, this review integrates mechanisms of change and alternative stable-state transitions to improve understanding of coastal ecosystem resilience under accelerating climate change.

Highlights

• Thermal stress was the most studied natural driver of seagrass decline.
• Study frequency reflects research effort, not ecological importance.
• Most multi-stressor studies assessed only two interacting drivers.
• Warming–acidification effects varied among species and exposure contexts.
• Indonesia remains underrepresented in long-term seagrass research.

Abstract

Seagrass ecosystems are exposed to natural environmental variability and climate-modulated extremes that can interact with anthropogenic pressures and accelerate ecological decline. This systematic review synthesized 60 peer-reviewed publications published between 2010 and 2025 to examine how temperature and marine heatwaves, ocean acidification and pH, salinity, drought and flooding, hydrodynamics, sea-level change, turbidity and light limitation, sediment conditions, and oxygen availability affect seagrass ecosystems. Thermal variables were the most frequently investigated category; however, publication frequency was treated as an indicator of research effort rather than a ranking of ecological importance. Experimental and field evidence showed that warming and marine heatwaves often reduce growth, photosynthetic performance, biomass, and recovery, although outcomes depend on species, exposure duration, light, nutrients, and prior stress history. Evidence for warming–acidification interactions was contrasting: acidification impaired plant performance during thermal stress in some studies, whereas enhanced inorganic carbon availability supported biomass or carbon metabolism in others. Most multi-stressor experiments evaluated only two drivers, and relatively few examined three or more simultaneous or successive stressors. The most commonly reported ecosystem consequences included habitat contraction, canopy and biomass loss, physiological impairment, altered carbon and nutrient cycling, sediment destabilization, community reorganization, and reduced resilience. Geographic evidence remained concentrated in Europe, the Mediterranean, Australia, China, and North America. Indonesia and much of Southeast Asia were comparatively underrepresented despite high seagrass diversity and documented declines associated with interacting environmental change. Future research should prioritize standardized long-term monitoring and multifactorial experiments that distinguish research frequency from ecological effect and support regionally calibrated risk assessment.

Brooding organisms can modify the chemical environment experienced by developing offspring, potentially altering their exposure to climate change. In marine systems, however, the carbonate chemistry of brood chambers, and the mechanisms governing it, remain poorly resolved. Here, we combine high-resolution pH and CO₂ measurements of the empty brood chamber (i.e., mantle cavity above gills) in the European flat oyster (Ostrea edulis) while also tracking valve gape to quantify how warming, ocean acidification, and ventilation interact to shape the internal environment experienced by larvae during incubation. Brood chamber pH was consistently lower than that of the surrounding seawater and responded to external conditions in a state-dependent manner governed by ventilation and respiration. When oysters were actively ventilating, brood chamber chemistry tracked ambient seawater with a persistent offset, whereas valve closure led to rapid CO₂ accumulation and pronounced declines in pH. These dynamics generated highly variable and behaviorally mediated exposure regimes, in which metabolic processes distorted ambient chemical signals over short timescales. Paired pH and CO₂ measurements provided preliminary constraints on aragonite saturation state (Ωar), revealing that brood chambers can remain undersaturated (Ωar < 1) for prolonged periods even in ventilating oysters and while overlying seawater remains favorable. Together, these results demonstrate that brood chambers function as dynamic microenvironments in which larval exposure to ocean acidification is governed not only by external conditions, but also by behavior. Because warming enhances metabolic CO₂ production and intensifies acidification within the chamber, future climate change is likely to amplify both the magnitude and variability of larval exposure. These findings help explain the apparent resilience of brooding species to ocean acidification, while also highlighting potential limits as environmental conditions move beyond historical bounds.

Highlights

• Ocean acidification disrupts calcium homeostasis in the mantle of mussels.
• Ocean acidification and N-NPs together intensify the inhibition of calcification.
• Surface charge determines distinct, mechanism-specific response pathways.
• Both omics reveal key roles for calcium transport, CA, and ECM synthesis.

Abstract

Ocean acidification (OA) and nanoplastics (NPs) increasingly co-occur in coastal ecosystems, yet their combined mechanistic impacts on calcifying invertebrates remain poorly resolved. Here, the mussel Mytilus coruscus was exposed for 30 days to factorial combinations of OA (pH 7.7 vs 8.1), surface-modified polystyrene NPs at a concentration of 100 μg/L (positively and negatively charged), and experimental shell damage to assess effects on shell repair, ion homeostasis, and energy metabolism. OA reduced shell repair quality by thinning repair layers, increasing porosity, and lowering calcium content, effects that were further enhanced by co-exposure to NPs, particularly negatively charged particles. OA and NPs jointly altered Ca²⁺ and Mg²⁺ levels, alkaline phosphatase activity, Ca²⁺Mg²⁺-ATPase activity, and key indicators of energy metabolism, including ATP content and cellular energy allocation. Transcriptomic and proteomic analyses revealed enrichment of ion transport, extracellular matrix, TGF-β signaling and other pathways, with divergent patterns linked to NPs surface charge. Together, these results suggest that nanoplastic surface charge under OA may impair mussel shell repair through associated alterations in ion homeostasis and energy metabolism. These findings highlight that particle surface properties shape how organisms respond to combined environmental stressors in acidifying marine environments.

This study applies the Forgotten Effects Theory (FET), a fuzzy-logic-based modelling framework, to analyse how construction life cycle processes generate direct and indirect pressures on multiple planetary boundaries. The approach captures multistage and nonlinear causal relationships derived from expert-based incidence matrices. Results show that mineral extraction, material distribution, and waste disposal strongly influence ocean acidification; construction operations and building use affect biosphere integrity; material production drives land-use change; and demolition contributes to climate change. The analysis revealed substantial hidden effects: the influence of mineral extraction on ocean acidification increased from 0.4 to 0.9 when material production was included as a mediator; the relationship between building use and biosphere integrity increased from 0 to 0.8 through indirect pathways; and the influence of demolition on climate change increased from 0 to 0.8 through waste-disposal mediation. A ±10% sensitivity analysis confirmed the stability of the main incidence hierarchy, with changes below 0.05 for the most relevant coefficients. These findings demonstrate that indirect (“forgotten”) effects can be as influential as direct impacts, significantly intensifying pressures across planetary boundaries. The construction sector is therefore characterised as a cumulative and interconnected system in which impacts propagate and reinforce each other across life cycle stages. These results highlight the limitations of silo-based environmental assessments and underscore the need for integrated, life cycle-oriented sustainability strategies. Policies prioritising upstream interventions, material efficiency and reduced demolition can generate broader systemic benefits. The study demonstrates the potential of FET as a transparent and reproducible modelling approach for linking local construction decisions with global environmental processes, supporting more integrated circular economy strategies and contributing to the achievement of Sustainable Development Goals 11, 12 and 13.

Coral reefs are especially sensitive to ocean warming and acidification caused by climate change and can have a major impact on reef-associated fish species and reef stability. This study analyzes the diversity of coral reef fishes over time using data from the National Coral Reef Monitoring Program (NCRMP) of NOAA from the year 2014 to 2024 from the Hawaiian Archipelago. The impacts of environmental factors such as sea surface temperature, ocean pH, and coral cover were evaluated on fish population structure. Shannon Diversity Index as the main statistical method to quantify the changes in biodiversity between monitoring years. The mean sea surface temperature (MST) ranged from 26.8°C in 2014 to 29.1°C in 2024, with a steady increase over the years, whereas the mean pH decreased from 8.12 to 7.95 over the years. During the same time frame, there was significant loss of coral cover, from 62% to 36%. The Shannon Diversity Index was then found to have decreased from 3.45 to 2.51, representing a loss of some 27.2% of biodiversity. The results indicate that the diversity of coral reef fishes is negatively correlated with environmental stress. Fish functional groups were further simplified, with habitat specialist fishes being more affected than generalist fishes under climate stress. The rising temperature and acidification of the oceans are important factors in the decline of coral reef fish. The continued monitoring and adaptive conservation strategies will be critical to ensure that reef biodiversity will persist into the future under different climate change scenarios.