Posts Tagged 'chemistry'

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The internal consistency between calculated and measured variables of the marine carbonate system in Arctic open and coastal waters, case study: Atlantic Arctic

Published 21 January 2026 Science Closed
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Highlights

  • Good consistency between calculated and measured variables of the marine carbonate system in Oceanic waters.
  • Only pH and pCO2 can be calculated with good accuracy in coastal waters.
  • The nutrient data are not required to calculate accurate marine carbonate system data in this region.
  • Total Alkalinity and pH (or pCO2) can be used to obtain good quality pCO2 (or pH) data.

Abstract

The Arctic Ocean plays a crucial role in anthropogenic carbon sequestration, while also being among the regions most susceptible to Ocean Acidification (OA). To understand, quantify, and monitor the rapid biogeochemical changes in the Arctic shelves and coastal waters, it is necessary to accurately determine the complete marine carbonate system. However, the uncertainty range in the calculated values is still unclear, fogging our ability to properly estimate carbon inventory and OA. In this study, we collected samples in the Arctic open and coastal waters to estimate the internal consistency of total alkalinity (TA), pH, partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) when only two of them are measured and the other two calculated. In open ocean waters, calculated values generally show good consistency with observations, whereas in coastal areas, it was only possible to accurately calculate two variables: 1) pH using as input parameters pCO2 together with either TA or DIC, and 2) pCO2 using DIC and pH. Furthermore, we found that, in this dataset, using the TA estimated from its correlation with salinity together with pCO2 also allowed obtaining accurate pH values in both coastal and ocean waters. This opens a new possibility of monitoring changes in the carbon cycle by measuring only salinity and pCO2 in areas where its consistency has been evaluated. Finally, in this study, we provide guidelines for obtaining and reporting good-quality carbonate system data in Arctic coastal areas.

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Nonlinear interactions of timing and amplitude biases in modeled Southern Ocean pCO2: the roles of dissolved inorganic carbon, total alkalinity, and sea surface temperature

Published 21 January 2026 Science Closed
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The Southern Ocean is a major sink for atmospheric carbon dioxide and critical to the current and future carbon cycle. This net annual CO2 flux reflects the balance between strong seasonal variability characterized by opposing periods of winter outgassing and summer uptake. Using a simple framework, we evaluate how model biases in both the amplitude and timing of dissolved inorganic carbon (DIC) and total alkalinity (TA) and in the amplitude of sea surface temperature (SST) impact simulated pCO2. We examine seasonal CO2 fluxes and pCO2 south of the Subantarctic Front in 42 Earth System Model and three state estimate simulations. Only 11 of the 45 simulations have a seasonal pCO2 cycle with a correlation of ≥0.7 to observed pCO2, while 26 have a correlation of <0. Four of the well-correlated models accurately represent the seasonality of SST, DIC, and TA, while TA biases compensate for DIC or SST biases in the other seven. DIC and SST amplitude biases are related to mixed layer (MLD) biases, with shallow MLDs, especially in the summer, correlated with larger amplitude DIC and SST cycles than observed. The amplitude of seasonal Net Primary Production is correlated to DIC and TA timing. We provide input on the main adjustments needed to correct the simulated pCO2 seasonality in each of the evaluated models. These findings highlight the difficulty and importance of capturing the seasonal processes influencing the carbonate system to correctly model and predict the Southern Ocean carbon sink and its response to a changing climate.

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Metrological concepts applied to Total Alkalinity measurements in seawater: reference materials, inter-laboratory comparison and uncertainty budget

Published 20 January 2026 Science Closed
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Total alkalinity (TA) measurements in seawater are crucial for characterizing and monitoring the oceanic carbonate system. While international best practices and guidelines exist, the field still lacks widely available traceable reference materials and a well-established uncertainty budget of the measurement method. In this study, we applied key metrological principles – development of reference materials, inter-laboratory comparison and uncertainty quantification – to TA measurements. We developed two reference materials, including an artificial material with a rigorously characterized reference value and an associated uncertainty budget, being traceable to the International System of units (SI). These materials were tested in an inter-laboratory comparison involving five laboratories and demonstrated the applicability of the reference materials developed for quality control. Additionally, we established an uncertainty budget for the TA measurement method using two metrological approaches. The resulting expanded uncertainty was 5 µmol kg−1 (with a coverage factor k=2) in TA, approaching the 4 µmol kg−1 target set by the Global Ocean Acidification Observing Network for climate monitoring. These findings mark a significant step toward improving the quality and comparability of TA measurements, thereby strengthening long-term ocean carbonate system monitoring.

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Spatial dynamics of aragonite saturation state and blue carbon stocks in seagrass meadows of the Palk Bay, Southeast Coast of India

Published 19 January 2026 Science Closed
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Seagrass meadows are increasingly recognized for their role in mitigating climate change through blue carbon sequestration and their influence on local carbonate chemistry. This study investigates the spatial variability of aragonite saturation state (Ωarag) and assesses the blue carbon storage potential of seagrass meadows along the Palk Bay, Southeast Coast of India. Subsurface water samples were collected across multiple seagrass-dominated stations between May and June 2024. Key seawater carbonate system parameters, including pH, temperature, total alkalinity (TA), and salinity, were measured to calculate Ωarag using CO2SYS software. Sediment cores were analyzed for organic carbon content and bulk density to estimate carbon stock. Results revealed significant spatial variation in Ωarag, influenced by seagrass density, species composition (Cymodocea serrulata and C. rotundata), and hydrodynamic conditions. Stations with dense C. serrulata beds showed elevated Ωarag values, suggesting local amelioration of acidification stress. The mean carbon stock was estimated at 1.97 Mg C/ha−1, with higher values in more mature (> 60% cover) and dense seagrass patches. These findings highlight the dual ecological function of seagrass meadows in enhancing local carbonate saturation and functioning as effective carbon storage systems, underlining their significance in coastal ecosystem-based climate mitigation strategies.

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Explainable machine learning models for coastal pH forecasting at aquaculture-relevant thresholds in Eastern Canada

Published 19 January 2026 Science Closed
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Highlights

  • Benchmark of ML models for coastal pHSWS forecasting.
  • Models trained on rare high-frequency data from Eastern Canada.
  • XGBoost balances sensitivity and precision at pHSWS < 7.75
  • SHAP shows Julian day dominance as composite environmental driver.
  • Promising low-cost framework for aquaculture acidification early warning.

Abstract

Ocean acidification poses a growing threat to marine ecosystems and aquaculture productivity, particularly in under-monitored coastal regions such as Eastern Canada. Existing pH prediction frameworks typically rely on multi-year records combining extensive carbonate chemistry, physical, and biological parameters. While these models can achieve high accuracy, their data requirements make them costly, complex, and challenging to implement for local, site-specific acidification forecasting in aquaculture contexts. To address this limitation, this study benchmarks several machine learning models for coastal pHSWS prediction using only three routinely measured environmental variables (temperature, salinity, sea level), from which we derived moving-average descriptors, local gradients, and two temporal indicators, resulting in a compact set of 11 input features. Six different models and a multivariate linear regression baseline were trained on one of the most complete and extended high-frequency datasets available (BSSS2018) and evaluated across four independent datasets: one from the same site but six months earlier (BSSS2017), and three from nearby bays in northeastern New Brunswick collected between 2017 and 2019. Among all tested models, XGBoost emerged as the most reliable and interpretable, achieving the best trade-off between sensitivity and precision at the operational acidification threshold (pHSWS < 7.75). Its performance remained acceptable within-site but declined across bays due to environmental and seasonal discrepancies, underscoring the importance of training data representativeness. SHAP-based explainability confirmed that Julian day was the dominant predictor, integrating the composite effects of seasonal environmental variability. Overall, this study demonstrates that using only low-cost, routinely measured features provides a promising foundation for short-term coastal pH forecasting, particularly for aquaculture monitoring needs. Despite limited inter-bay generalization, the proposed framework shows that interpretable machine learning models can deliver actionable early-warning insights under realistic data constraints. It constitutes one of the first data-driven benchmarks explicitly tested at aquaculture-relevant thresholds, highlighting a scalable and transparent approach toward operational acidification forecasting.

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Interannual variability of oceanic pH in a highly human-perturbed bay in China

Published 15 January 2026 Science Closed
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Due to anthropogenic carbon dioxide (CO2) emissions, open oceans are acidifying, and the acidification rate is relatively stable. While coastal waters experience even greater pH fluctuations from terrestrial inputs, upwelling, and biological activity, this variability necessitates detailed long-term research in these regions. Based on field observations and historical literature data from 1980 to 2016, the interannual variability of seawater pH and its driving mechanisms were analyzed in Jiaozhou Bay, a highly human-perturbed bay in China. The results revealed an overall decreasing trend in pH over the three-decade period, with a decline rate of 0.0062 years−1, which is 3.6 times faster than that observed in the open ocean, indicating significant anthropogenic impacts on pH variations in Jiaozhou Bay. The long-term pH changes showed strong correlations with coastal environmental pollution status, characterized by three distinct phases: a decline from 1980 to 1986, followed by an increase during 1991 to 2004, and subsequently another decreasing trend from 2004 to 2016. Terrestrial (including wastewater) inputs were identified as predominant anthropogenic factors influencing these pH variations. Furthermore, this study highlights that while future management should focus on reducing organic matter and nutrient inputs, particular attention should be paid to the direct impacts of dissolved inorganic carbon (DIC) from treated wastewater discharge on pH reduction.

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Ocean acidification and anthropogenic carbon in the Eastern Mediterranean Sea and the effects of acidification on marine organisms

Published 15 January 2026 Science Closed
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Ocean acidification (OA), driven by rising atmospheric carbon dioxide (CO2) levels, is a critical issue affecting our oceans. The Eastern Mediterranean Sea (EMS) remains poorly understood in terms of the carbonate system and the impact of OA, despite its key role in Levantine Intermediate Water (LIW) formation and its peculiar characteristics in buffering capacity and ongoing OA. This study provides the first comprehensive spatial and temporal assessment of carbonate system in the North-Eastern Levantine Basin, in EMS, providing essential reference data for Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), and Anthropogenic Carbon (CANT). The mean TA of the measurements was 2622.11 μmol/kg, with higher surface values in summer, reflected also in the surface salinity (S) maximum caused by strong evaporation. A clear vertical gradient was observed, with TA decreasing with depth. Hot and dry meteorological conditions contribute to increased S and TA, resulting in seasonal and vertical variations in the water column. The mean DIC of the measurements was 2291.23 μmol/kg. In contrast to the observations for TA, surface DIC values were higher in winter than in summer. The higher DIC values in winter are attributed to thermodynamic equilibrium and vertical mixing in the surface waters. This study has also investigates the presence of CANT, has infiltrated deep layers, with a mean concentration of 52.07 μmol/kg, decreasing significantly throughout the water column. These findings confirms the ongoing influence of human activities on intermediate and deep layers in EMS. To reconstruct past carbonate system dynamics, the relationships of TA and DIC were determined with salinity (S) and temperature (T) data. Long-term data from METU-IMS Erdemli Time Series (ETS) stations, collected monthly for a decade, provided valuable findings into seasonal patterns and temporal shifts in TA, DIC, and pH. The coastal station displayed clear trends in the carbonate system over time, reflecting its sensitivity to local environmental changes. In contrast, the offshore station exhibited minimal variability, indicating greater stability against seasonal and long-term fluctuations. These results highlight the heightened vulnerability of coastal waters to carbonate system changes, while offshore waters remain more stable. Understanding carbonate chemistry and acidification levels is crucial for assessing impacts on marine life. In addition to the characterization of carbonate chemistry, this study also explores OA’s biological impacts on two key organisms of the Mediterranean ecosystem: phytoplankton and mussels. Firstly, effects of elevated CO₂ on phytoplankton, an essential primary producer in aquatic food webs and global biogeochemical cycles are explored. Specifically, the study explores the impacts on phytoplankton physiology, focusing on growth rates, respiration, and photopigment content in selected species from the coccolithophores, dinoflagellates, and diatoms groups. While growth rates and respiration remained relatively stable under reduced pH conditions, photopigment content was significantly influenced by changes in seawater pH, highlighting the importance of considering environmental influences on photopigment composition. The study further investigated the effects of acidification on calcifying organisms through a global program aimed at understanding the long-term effects of acidification on key seafood species and exploring adaptation strategies with a collaborative approach. This study focused on the long-term (6 months long experiment) physiological impacts of OA on marine calcifiers, specifically Mediterranean mussel, Mytilus galloprovincialis, an abundant species and one of the most consumed non-fish marine species in Türkiye. Results indicate that OA poses a substantial threat to mussel health and survival. Reduced pH levels negatively impacted survival rates, while other physiological parameters like clearance rate, condition index, respiration, and the distribution of a radionuclide, 210Po, did not significantly change. However, lipid content and immune response were affected. Oxygen consumption decreased over time, especially at lower pH. This study underscores the potential risks of OA to the fitness of the commercially important mussel species, indicating that future OA may impact both this key seafood species and its associated ecosystems. The established baseline data are crucial for future monitoring and provide valuable insights into the vulnerability of marine organisms and ecosystems to ongoing OA. By integrating chemical, biological, and ecological perspectives, this dissertation offers a comprehensive assessment of OA in EMS. It establishes baseline data for carbonate system variables, revealing distinct spatial and temporal variations influenced by S, T, and mixing processes. By linking changes in carbonate chemistry to physiological responses in primary producers and a commercially vital shellfish species, this study highlights the ecological and economic impacts of OA in EMS. The findings emphasize the need for continued research and mitigation efforts to protect marine ecosystems and commercially important species. This integrated approach provides valuable insights into the vulnerability of marine organisms and ecosystems to ongoing OA, underscoring the significance of this research for the Mediterranean Sea.

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Time series in the Mediterranean Sea

Published 15 January 2026 Web sites and blogs Closed
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13 January 2026/Kiel/Limassol. Today, Expedition M216 set sail for the Mediterranean Sea with the research vessel METEOR. An international research team led by GEOMAR Helmholtz Centre for Ocean Research will assess the state of the Mediterranean over the coming weeks. The research is conducted as part of a time series that was last carried out in 2018. The data collected now are therefore central to assessing current and future changes in the Mediterranean Sea. Among other things, temperature, salinity, nutrients and trace gases are being investigated, as well as the stratification and circulation of the water masses.

Like the Baltic Sea and the Black Sea, the Mediterranean Sea is an inland sea connected to the global ocean only by the Strait of Gibraltar. As a result, it responds more quickly to changes. It warms more rapidly, absorbs more carbon dioxide and acidifies more strongly than the open ocean. At the same time, through its connection with the Atlantic, the Mediterranean Sea also influences the global ocean and additionally plays an important role for the regional climate, fisheries and tourism.

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Several factors come together in the Mediterranean Sea: it has a very high salinity and pronounced water circulation. The high salt concentration and temperature promotes the uptake of anthropogenic CO2. At the same time, surface water is transported relatively quickly to the depth, where it remains at a temperature of around 12 to 13°C. In addition, the circulation ensures that deep water reaches the surface, which can again absorb CO2. This creates a dynamic cycle that enables the Mediterranean Sea to bind a comparatively large amount of CO2 from the atmosphere,” explains Dr Toste Tanhua, expedition leader and chemical oceanographer at GEOMAR.

Measurements from the surface to the sea floor

The measurements cover the entire water column, i. e. all layers of the Mediterranean Sea from the surface to the sea floor. At its deepest point, the Mediterranean Sea is over five thousand metres deep. “The unique topography of the Mediterranean Sea is challenging for us. There are several basins with different conditions and water depths,” says Toste Tanhua. The expedition focuses on measuring nutrients, salinity, oxygen, alkalinity, dissolved inorganic carbon, organic carbon, CO2 and the degree of ocean acidification. This enables the researchers to deduce how the Mediterranean Sea is faring under the influence of climate change and to estimate further changes. They are also investigating the distribution of water masses, currents and the exchange between surface water and the deep ocean.

“The Med-SHIP programme gives us the opportunity to take a very close look at the individual components of the entire water column. Among other things, we will be investigating inorganic carbon. This refers to carbon that is not found in living organisms, but in rocks, water or in the atmosphere as CO2. Among other things, it is responsible for transporting CO2 between the atmosphere, the surface and deep water. As the water masses in the Mediterranean are in contact with the atmosphere relatively frequent, this is particularly interesting for us,” explains Toste Tanhua.

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Seasonal and regional dynamics of carbonate chemistry and buffering capacity in contrasting marine systems: the Northeastern Mediterranean and the Sea of Marmara

Published 12 January 2026 Science Closed
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This thesis investigates seasonal and spatial variability in carbonate system dynamics and buffering capacity across two contrasting semi enclosed Turkish seas: the oligotrophic Northeastern Mediterranean and the eutrophic Sea of Marmara. Data were collected in winter and summer on research cruises conducted in 2022–2023. High quality multi-index measurements included total alkalinity (TA), nutrients, and spectrophotometric pH. Derived carbonate system parameters were calculated with PyCO2SYS, and a standardized, layer-based approach was applied to reflect regional hydrography and vertical structure. The Sea of Marmara showed high TA, often above 2700 μmol kg⁻¹, together with elevated dissolved inorganic carbon (DIC) below the halocline due to respiration and weak ventilation. Revelle factors were high, vertical gradients in pH and aragonite saturation state were strong, and hypoxia was present in sub halocline and deep waters. In the Northeastern Mediterranean, TA and DIC were lower and more stable. Revelle factors were lower, and buffering was stronger in surface and intermediate layers, with only modest sensitivity increases at depth. Advanced indices added diagnostic value beyond the Revelle factor. In the Sea of Marmara, βDIC and γDIC were highest below the halocline, identifying where small DIC additions most strongly raise the partial pressure of carbon dioxide (pCO₂) and lower pH. γAlk and ωAlk indicated greater benefit of marginal TA gains at depth. ωDIC isolated saturation state sensitivity to carbon loading in respiration dominated layers. In the Northeastern Mediterranean, the Revelle factor captured first order seasonal shifts in well mixed winter layers, while the advanced indices flagged narrow coastal and subsurface windows of rising sensitivity. These results provide the first multi seasonal, multi-index baseline for Turkish seas. They show that eutrophication, stratification, and ventilation control local acidification risk and that advanced buffer indices sharpen process attribution. The study supports monitoring that couples carbonate chemistry with oxygen and nutrients and guides management toward nutrient reduction and protection of sub halocline habitats where buffering is weakest.

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A trend-based ecological indicator framework for spatially classifying ocean acidification risk to global coral reefs

Published 8 January 2026 Science Closed
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Highlights

  • Global aragonite saturation state declined at −0.0068 ± 0.00013 yr−1 from 1985 to 2023.
  • Equatorial Pacific shows fastest acidification with Ωₐᵣ declines of −0.012 yr−1.
  • Novel trend-based K-means clustering identifies emerging coral reef risk zones.
  • Strong pCO₂-Ωₐᵣ correlations (ρ < −0.9) in tropical upwelling regions monthly.
  • Framework supports SDG 14.3 and Global Biodiversity Framework implementation.

Abstract

Ocean acidification driven by anthropogenic CO₂ uptake poses a critical threat to coral reef ecosystems. Using global surface ocean carbonate data from 1985 to 2023, we provide a high-resolution, observation-based assessment of long-term trends in pCO₂, pH, and aragonite saturation state (Ωₐᵣ). Our results show a robust global decline in Ωₐᵣ (−0.0068 ± 0.00013 yr−1), with the most pronounced losses in the equatorial Pacific and Southern Hemisphere. Monthly correlations reveal strong inverse pCO₂–Ωₐᵣ relationships (ρ < −0.9) and positive pH–Ωₐᵣ correlations (ρ > 0.9) in tropical upwelling zones, highlighting spatially persistent acidification stress. A key innovation of this study is the use of a trend-based K-means clustering framework that classifies ocean regions into high, moderate, and low impact categories based on Ωₐᵣ decline rates. Unlike conventional assessments relying on absolute Ωₐᵣ thresholds, this approach identifies regions that remain suitable today but are deteriorating rapidly, thus at risk of crossing biological thresholds in the near future. High-impact zones, including the Coral Triangle and eastern tropical Pacific, show Ωₐᵣ declines of −0.010 to −0.012 yr−1 and have already reached levels near 2.4–3.0, suggesting growing stress on reef calcification. By leveraging high-resolution observational data rather than model projections, this approach reduces uncertainty and offers a scalable tool for anticipating ecological vulnerability under ongoing acidification. The findings underscore the urgent need for conservation in tropical high-impact zones and for sustained monitoring in lower-risk regions. This work provides a science-based framework to support spatially targeted reef management and informs global policy priorities including SDG 14.3 and the Global Biodiversity Framework.

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Ocean acidification in Massachusetts bay and Boston harbor: insights from a 1-D modeling approach

Published 8 January 2026 Science Closed
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Highlights

  • NeBEM, an ERSEM-based biogeochemical and ecosystem model, is established for the U.S. Northeast.
  • NeBEM provides process-based insights into carbonate system variability beyond the capability of empirical data-fitting methods.
  • Biological processes strongly influence TA and DIC variability in outer Massachusetts Bay.

Abstract

Massachusetts Bay (MB)/Boston Harbor (BH) in the northeastern United States has reduced buffering capability, making it highly vulnerable to ocean acidification (OA). We applied the U.S. Northeast Biogeochemistry and Ecosystem Model (NeBEM), integrating the unstructured grid, Finite Volume Community Ocean Model with a modified European Regional Seas Ecosystem Model (ERSEM), to investigate seasonal and interannual OA variability through one-dimensional (1-D) experiments. Objectives were to (a) evaluate model skill in reproducing observed seasonal cycles of OA-related variables, particularly pCO2 and pH, in shallow and deep regions, and (b) assess sensitivity to parameterizations and algorithms for calculating dissolved inorganic carbon (DIC), total alkalinity (TA), pCO2, and pH. The 1-D NeBEM reproduced variability of nutrients, dissolved oxygen, chlorophyll-a, pCO2, and pH at the deep outer bay site, where air-sea interactions dominate, but failed at the shallow inner bay site due to the absence of river discharge-driven advection. Of TA algorithms tested, the semi-diagnostic method best captured observed seasonal pCO2 variation, achieving the highest correlation and lowest root mean square error, although all methods performed similarly for pH. Comparisons with multi-linear regression methods showed that empirical models are highly sensitive to calibration set. Mechanistic analysis indicated that TA variability is mainly regulated by nitrification and net community production (NCP), while DIC variability is driven primarily by NCP. Atmospheric CO₂ loading was the first-order contributor to DIC change in magnitude. However, it has decreased in MB over the past two decades, in contrast to regional and global trends. Therefore, it is not a major driver of OA progression in this system.

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Assessing the influence of ocean acidification on the deterioration of coral reefs in Sri Lanka

Published 1 January 2026 Science Closed
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Rising atmospheric CO2 levels have significantly increased ocean acidification (OA), endangering coral reefs, and nutrient (nitrate (NO3−), and phosphate (PO43−)) pollution also weakens the coral 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 Ω Ar 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. The NO3− concentrations of 2–5 µmol L− 1, from human activities, may intensify coral bleaching during heat stress. Results showed that SC (2.19±1.28 µmol L− 1) and WC (3.52±1.48 µmol L− 1) had NO3− above the permissible range, which may be due to waste discharge and high runoff. The significantly higher PO43− concentrations were reported in EC (0.35±0.07 µmol L− 1). Coral bleaching hotspot (HS) identification emphasizes how spatially distributed HS are from January to June. The OA risk assessment confirmed that climate change brought high risk to the coral reef ecosystems, which impact on the ecology and economy of Sri Lanka.

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Assessing the impact of riverine water on the Northwest Pacific using normalized total alkalinity

Published 30 December 2025 Science Closed
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The impact of riverine water was assessed using salinity-normalized Total Alkalinity observations of the Northwest Pacific, including the coastal areas of Japan (20–50° N, 120–160° E). The observational data included surface carbonate parameters obtained from decades of surveys conducted by volunteer cargo ships and research vessels in this area. This study uses data and statistical methods (e.g., re-gridding and Fourier regression) like those in a previous study that analysed air-sea CO2 flux but focuses instead on the diffusion of normalized Total Alkalinity from land. First, the seawater area affected by riverine water was identified using an Empirical Orthogonal Function analysis of normalized Total Alkalinity. The differences in normalized Total Alkalinity and Dissolved Inorganic Carbon from the surrounding area were then analysed to evaluate the potential drivers, such as riverine water supply, advection effects, and biological activities. In addition, the impact of riverine water on oceanic CO2 uptake and acidification in the study area was assessed. The analysis showed that riverine water was the main cause of the higher total Alkalinity compared to the surrounding area, whereas its contribution to the increase in Dissolved Inorganic Carbon was relatively minor. The supply of riverine water had little effect on oceanic CO2 uptake throughout the year. The supply of riverine water had a minor effect on pH but contributed to coastal acidification, as indicated by a decrease in the calcification index (Ωarg, the aragonite saturation state) by 0.09±0.01 over the past 20 years, even after accounting for the buffering effect of riverine Total Alkalinity, which reduced the overall decrease by approximately 71 %. The results of this study are expected to be further improved by enhancing observations, such as the vertical profiles of carbonate parameters, and are expected to expand to other sea areas and be applied to global budgets.

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Ventilation and buffering capacity effects on ocean acidification in low oxygen environments

Published 30 December 2025 Science Closed
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Ocean acidification results from oceanic uptake of anthropogenic CO2 (ΔCant). Weak carbonate buffering capacity (high Revelle factor, RF) amplifies acidification, but its role in persistently low-oxygen, poorly ventilated regions is unclear. Here we compare preindustrial to present changes in partial pressure of CO2 (pCO2), hydrogen ion concentration ([H+]), pH, aragonite saturation state (Ωara), and RF within permanent oxygen minimum zones (OMZs) versus well-ventilated regions. We find that acidification is negligible in the least-ventilated, poorly buffered lower OMZs, but detectable in moderately ventilated upper OMZs. In upper OMZs, pCO2 and [H+] increase faster while Ωara, pH, and RF change more slowly than in adjacent well-ventilated regions. Our analysis reveals that limited ΔCant delivery by ventilation ultimately constrain acidification in low-oxygen regions. Accordingly, low-oxygen regions with poor ventilation will experience less acidification than well-ventilated regions, and different metrics (notably [H+] versus Ωara) respond distinctly due to their different definitions and sensitivities.

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Hydrogeochemistry of submarine groundwater discharge along a Bruneian coastline: iron and aluminum enrichment along with coastal acidification

Published 30 December 2025 Science Closed
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Submarine groundwater discharge (SGD) and its influence on coastal acidification and trace-metal enrichment have not been studied in Borneo. This study characterizes SGD from northwest Borneo into the South China Sea, focusing on iron (Fe) and aluminum (Al) inputs, hydrogeochemical controls on their mobility, and SGD’s role in coastal acidification. Samples were collected along transects at Tungku and Empire beaches, spanning the peritidal to subtidal zones, as well as from streams, pools, and beach sand. SGD contained elevated Fe and Al (Tungku: 4.07 mg/L Fe, 1.31 mg/L Al; Empire: 2.12 mg/L Fe, 0.38 mg/L Al), identifying these as key SGD-derived trace metals. pH was near-neutral in many samples (minimum 6.6), rising from 7.72 (Tungku) and 7.48 (Empire) in SGD to 8.11 and 8.01 in adjacent seawater, creating steep pH gradients favoring Al and Fe precipitation. Acid sulfate soils and high dissolved organic matter enhance groundwater acidity and trace-metal mobility. Major-ion chemistry indicates dominance of non-carbonate alkalis (SO₄²⁻ + Cl⁻ >CO₃²⁻ + HCO₃⁻; Na⁺ + K⁺ >Ca²⁺ + Mg²⁺) and low phosphate and nitrate, with mixed freshwater–saline contributions. The combination of low pH, elevated Fe and Al, and anthropogenic disturbance may degrade coastal and groundwater quality, affecting marine biogeochemical cycles, biodiversity, and ecosystem functioning. Overall, SGDs in Brunei deliver acidic, Fe- and Al-enriched water, contributing to coastal acidification and contamination, with implications for regional climate resilience.

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Aquaculture of seaweeds (Saccharina latissima, Ulva spp., Gracilaria spp.) significantly improves the growth of co-cultivated bivalves in mesotrophic, but not eutrophic, estuaries

Published 29 December 2025 Science Closed
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The co-cultivation of seaweeds with bivalve shellfish is a potential strategy for protecting bivalve crops against anthropogenic coastal acidification and hypoxia. We co-cultivated seaweeds and bivalves using a succession of seaweed species according to season (winter, Saccharina latissima → spring, Ulva spp. → summer, Gracilaria spp.) together with eastern oysters (Crassostrea virginica) and blue mussels (Mytilus edulis). Bivalves and seaweeds were deployed in two estuaries that contrasted in trophic state, one mesotrophic and one eutrophic. In all five experiments in the mesotrophic system, cocultivation with seaweeds significantly increased weight- and/or shell-based growth of bivalves (p < 0.05). Growth rate increases for C. virginica were modest, with weight-based growth improving by 17–21% and shell-based growth improving by 3–27% with seaweed co-culture of all macroalgal species. For M. edulis, the effect was large; co-culture with S. latissima caused 47% and 114% increases in shell- and weight-based growth rates, respectively. In the four experiments in the eutrophic estuary, co-culture with seaweeds did not significantly improve bivalve growth. Seaweed cultivation significantly improved water quality metrics (increased pH and dissolved oxygen (DO); p < 0.05 in all cases) in and around the seaweed sites at both locations, although increases in pH and DO were modest, and even in control treatments, there were no prolonged periods of harmful pH or DO levels. An abundance of macroalgal detritus may have bolstered the diets of co-cultivated bivalves in the mesotrophic estuary, a hypothesis supported by lower chlorophyll a concentration, and therefore lower planktonic food levels, at that site. Given that seaweeds display species-specific allelopathic effects against phytoplankton, it is also possible that the presence of seaweeds altered the phytoplankton community to the benefit of the bivalves. Regardless, the findings here demonstrate that co-cultivation with seaweeds can accelerate the growth of bivalves.

Continue reading ‘Aquaculture of seaweeds (Saccharina latissima, Ulva spp., Gracilaria spp.) significantly improves the growth of co-cultivated bivalves in mesotrophic, but not eutrophic, estuaries’

An autonomous pH sensor for real-time high-frequency monitoring of ocean acidification in estuarine and coastal areas

Published 26 December 2025 Science Closed
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In situ pH sensing is crucial for the real-time monitoring of ocean acidification and investigations into the marine carbon cycle. Although ion sensitive field-effect transistor (ISFET) has been proven suitable for marine pH monitoring, its supply and implementation remain challenging. An underwater pH sensor for environmental analysis (uSEA-pH) based on ISFET was developed herein, incorporating a modified commercial laboratory pH probe through engineering design. Laboratory characterization demonstrated that uSEA-pH exhibited a Nernstian response (slope −57.60 ± 1.05 mV/pH, R2 > 0.999), rapid response time (∼7 s), and low measurement uncertainty (<0.01 pH). The sensor supports a sampling frequency of 1 Hz with an average power consumption of only 0.72 W. Its compact design (self-contained with battery: Φ15 × 45 cm; miniaturized version: Φ6.4 × 21 cm) facilitates deployment on various observational platforms. During high-frequency underway monitoring in the Pearl River Estuary and Dongshan Bay, uSEA-pH successfully detected subtle pH variations (<0.05 pH). In extended in situ deployments, buoy-mounted uSEA-pH reliably recorded tidal-driven pH fluctuations in Dapeng Bay (27 days) and Xiamen Bay (7 days), generating over 2.3 million field measurements. This study presents a viable, robust, and high-resolution approach for continuous pH monitoring in estuarine and coastal areas.

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Chapter 4 – Insights into the role of micronanoplastics in accelerating ocean acidification

Published 18 December 2025 Science Closed
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While micronanoplastic pollution is among the most severe anthropogenic threats, ocean acidification is an equally concerning ecological issue. Several studies have highlighted the toxicological effects, interactions, and behavior of micronanoplastics across different environmental domains. However, their role in accelerating ocean acidification remains poorly understood. Ocean acidification refers to the decrease in seawater pH due to increased dissolution of atmospheric carbon dioxide into the ocean. This phenomenon poses a serious threat to marine life and ecosystem functioning. Based on current evidence regarding the behavior and interactions of micronanoplastics in marine systems, it can be proposed that these polymer particles may play a significant role in accelerating seawater acidification. This chapter focuses on exploring the evidence linking micronanoplastic pollution to ocean acidification and aims to comprehensively examine its causes, mechanisms, and consequences.

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Symmetrical pH electrochemical cell coupled to constant potential coulometry for improved sensitivity and precision: part 2. Submersible probe for in situ measurements

Published 17 December 2025 Science Closed
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Seawater pH is a critical parameter influencing many processes in the ocean. Today it is mainly measured by indicator- based spectrophotometry to allow for high precision. This, however, is at the expense of traceability and systematic errors originating from changes in temperature, salinity and other matrix effects. Moreover, in routine practice this approach is not performed in situ and requires sampling and manual manipulations, which is prone to introduce additional errors including gas exchange with the atmosphere. Unfortunately, in the last few decades the electrochemical sensing community has failed to make efforts to improve the performance of the gold standard method, which is potentiometric detection with pH glass electrodes. To address this, we aim here to improve the sensitivity and precision of submersible pH probes on the basis of pH glass electrodes by minimizing systematic errors from temperature changes and by implementing a recently described coulometric method. The electrodes are mounted in a symmet- rical cell reported in part 1 of this work to reduce sensor drift and minimize inaccuracies due to liquid junction potential variations and pH changes of the inner solution from temperature fluctuations. The development and construction of the probe is explained. The circuit is evaluated and the sensors are calibrated over a range of temperatures, approaching ideal behavior. The submersible probe was deployed in situ in April 2025 in the vertically stratified Krka River Estuary in Croatia. The precision of the probe were evaluated in situ by stability experiments in the seawater layer. The determined precision is 0.001 pH unit, which is significantly better than reported earlier for routine pH probes. A recalibration procedure with synthetic seawater is also evaluated for minimizing drift. A depth profile with changing salinity was performed and compared with CTD probes.

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Aragonite lithium/magnesium as an indicator of calcification media saturation state in marine calcifiers

Published 16 December 2025 Science Closed
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Marine calcifiers support ecosystem services, including shell fisheries and coral reefs. Constraining the saturation state of the calcification media of these organisms is essential to understand the response of biomineralisation to environmental change. Here we synthesise aragonite over variable pH, saturation state, temperature, and in the presence of simple biomolecules. We show that the lithium/magnesium distribution coefficient, relating aragonite and precipitation fluid compositions, is significantly affected by precipitation rate but not by temperature or pH. Precipitation rate reflects saturation state and temperature, so lithium/magnesium of biogenic aragonite can be used to calculate mineral precipitation rate and, if the precipitation temperature is known, to reconstruct calcification medium saturation state. Applying the distribution coefficients to a published calcifier dataset indicates that calcification media saturation state is ca. 9 to 13 at 18–30 °C and ca. 6 to 10 at 10–18 °C. Coral calcification media saturation state varies between ocean sites, species, and reef zones.

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