Although net negative emissions of carbon dioxide (CO2) are essential to meet climate targets, little is known about how declining atmospheric CO2 levels will affect ocean acidification. Here, by analysing the acidity ([H+]) and corrosivity to aragonite (ΩArag) in eight Earth system models that made simulations under rising then falling CO2 levels, we identify the Arctic as a hotspot for delayed reversibility of ocean acidification. Under falling CO2, Arctic surface waters remain comparatively more acidic, and aragonite-corrosive conditions (ΩArag < 1) persist until atmospheric CO2 drops ~120 ppm below the threshold at which they first appeared under rising CO2. This hysteresis arises from the erosion of the natural surface-layer deficit in dissolved inorganic carbon, initially maintained by sea ice limiting air–sea gas exchange and not fully restored as sea ice recovers during CO2 decline. Thus, the Arctic Ocean experiences not only the greatest acidification but also the most delayed benefits from negative emissions.
Continue reading ‘Persistence of Arctic Ocean acidification under negative emissions’Posts Tagged 'chemistry'
Persistence of Arctic Ocean acidification under negative emissions
Published 14 August 2026 Science Leave a CommentTags: Arctic, biogeochemistry, chemistry, globalmodeling, modeling
Method for correcting the temperature dependence of field-type glass electrode pH sensors
Published 14 August 2026 Science Leave a CommentTags: chemistry, field, methods, modeling, North Pacific, sensor
Seawater pH measurement is essential for monitoring ocean acidification and its biogeochemical impacts. Glass electrode pH sensors are widely used for in situ applications due to their ease of operation, low power consumption, and no reagent requirements. However, their accuracy can be affected by temperature-dependent effects, particularly those associated with the internal reference solution. Conventional calibration methods based on the Nernst response do not explicitly account for temperature-induced variations in the internal solution, which may lead to systematic bias under conditions with large temperature gradients. In this study, a correction method is proposed to address temperature-dependent errors by modeling the internal pH (pHin). The model incorporates two primary temperature-dependent contributions: the intrinsic temperature dependence of the phosphate buffer and potential shifts caused by activity changes in saturated KCl. These effects are combined through the electrode response factor (fR), defined as the ratio of the observed electrode slope to the theoretical Nernst slope, to estimate pHin as a function of temperature and correct measured pH values to a reference temperature of 25 °C. Application of the method to vertical seawater profiles demonstrated a reduction in deviation from colorimetric reference measurements. The corrected results showed reduced temperature-dependent bias across depth. This approach provides a practical framework for improving the reliability of in situ glass electrode pH measurements under varying thermal conditions. The proposed method addresses equilibrium temperature-dependent effects of the internal solution but does not explicitly account for transient hysteresis, pressure effects, or long-term sensor drift.
Deep dive: evaluating students’ use of Toulmin’s argumentation pattern in chemistry classrooms for constructing scientific arguments about ocean acidification
Published 14 August 2026 Science Leave a CommentTags: chemistry, education
Science education plays a key role in helping students to become responsible citizens, capable of making appropriate decisions based on scientific information, participating in discussions on socio-scientific issues (SSIs), and taking action in the context of climate change. Thus, achieving scientific literacy in general and practising scientific argumentation skills – as part of scientific literacy – in particular, are essential. Therefore, the pilot study presented here focusses on improving upper secondary students’ (ISCED 3) scientific argumentation skills by implementing Toulmin’s Argumentation Pattern (TAP) in the chemistry classroom. We focus on chemistry education to better understand subject-specific argumentation in chemistry classrooms. This pilot study is part of an interventional study that aims to implement TAP as a construction mechanism to improve students’ scientific argumentation skills. Our study is embedded in the SSI of human-induced ocean acidification. In order to gain insight into students’ scientific argumentation patterns and develop adequate teaching materials, we asked 29 upper secondary students to write well-founded arguments to confirm a claim about calcification of coral skeletons made in a newspaper headline. Students’ arguments were constructed prior to and following the provision of support material based on TAP. These arguments were analysed with regards to their formal quality using qualitative content analysis. Our findings indicate that TAP aligned well with our participants’ intuitive arguments. We discuss our findings and propose how using TAP can contribute to teaching and learning chemistry-specific argumentation skills.
Continue reading ‘Deep dive: evaluating students’ use of Toulmin’s argumentation pattern in chemistry classrooms for constructing scientific arguments about ocean acidification’Giant kelp-associated variation in coastal seawater chemistry across contrasting sites in Chile and Tasmania
Published 13 August 2026 Science Leave a CommentTags: algae, chemistry, field, Indian, South Pacific
Background and Aims
Widespread shifts in seawater chemistry are occurring across spatial and temporal scales, with important consequences for coastal ecosystems. Giant kelp (Macrocystis pyrifera) forests elevate seawater pH and dissolved oxygen (DO) through photosynthesis, potentially providing short-term refugia from ocean acidification and deoxygenation. However, whether these effects persist across contrasting environmental settings remains unclear. Here, we assess how biological and oceanographic conditions regulate giant kelp-mediated modification of seawater chemistry across multiple sites.
Methodology
Hourly measurements of seawater pH, DO and temperature were collected during spring–summer 2022–23 using paired deployments inside and outside giant kelp forests at one site in central Chile and four sites in Tasmania, Australia. The influence of giant kelp density was also evaluated at two sites in southern Chile and three sites in Tasmania. An upwelling index was calculated for the central Chile site to assess the influence of regional oceanographic forcing on kelp-associated seawater chemistry patterns.
Key Results
Hourly pH and DO were higher inside giant kelp forests than outside at the central Chile site and at one Tasmanian site. At these locations, stronger daytime pH–DO relationships indicated that photosynthetic carbon uptake exceeded night-time respiration, generating a net positive metabolic signal. In Tasmania, giant kelp density was positively associated with hourly pH and DO, whereas no such relationship was detected in southern Chile. At the central Chile site, kelp-associated effects intensified during a strong upwelling event, reducing the severity of low pH and DO conditions.
Conclusions
Giant kelp forests can locally buffer short-term fluctuations in seawater pH and DO, but this capacity is highly site-dependent and influenced by giant kelp density and environmental conditions. Overall, our findings suggest that continued loss of giant kelp forests in Tasmania may reduce their potential to provide short-term refugia, while in Chile the strength of kelp-mediated seawater chemistry modification is likely to remain strongly influenced by variability in upwelling and freshwater inputs.
Continue reading ‘Giant kelp-associated variation in coastal seawater chemistry across contrasting sites in Chile and Tasmania’CARIMED (CARbon, tracers, and ancillary data In the MEDiterranean Sea): a ship-based data synthesis product – overview and quality control procedures
Published 12 August 2026 Science Leave a CommentTags: chemistry, field, Mediterranean
The Mediterranean Sea (MedSea) is highly sensitive to climate-driven changes in temperature, oxygen, and pH, among other variables. To better assess these long-term trends, we developed CARIMED (CARbon, tracers, and ancillary data In the MEDiterranean Sea), the first comprehensive, harmonised data synthesis product for the MedSea. CARIMED integrates hydrographic, inorganic carbon, transient tracer, and ancillary measurements from 46 research cruises spanning the period from 1976 to 2018, containing observations for the entire water column across all MedSea sub-basins. A substantial component of the data was retrieved from fragmented or locally archived historical records, thus consolidating previously inaccessible measurements. Following global synthesis approaches, CARIMED applies a quality-controlled, and bias-adjusted framework. A key adaptation was the secondary quality control (2QC) procedure, specifically tailored to the MedSea’s unique hydrography, utilising sub-basin divisions and supplementary checks (including statistical consistency assessments) to resolve complex, often contradictory, inter-cruise offsets. This rigorous process minimised systematic biases, yielding a dataset with improved consistency, and highlights the urgent need for adapted standard operating procedures and reference materials to address the MedSea biogeochemical particularities. CARIMED delivers two complementary, freely available products: the aggregated original cruise data product (https://doi.org/10.20350/digitalCSIC/17785, García-Ibáñez et al., 2025) and the final bias-adjusted data synthesis product (https://doi.org/10.25921/cp5b-zq67, Álvarez et al., 2025; hosted at https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/oceans/CARIMED/, last access: 26 June 2026). This essential resource establishes a new benchmark for assessing long-term biogeochemical trends, validating regional ocean models, and supporting climate-change mitigation and adaptation strategies in this rapidly changing semi-enclosed basin.
Continue reading ‘CARIMED (CARbon, tracers, and ancillary data In the MEDiterranean Sea): a ship-based data synthesis product – overview and quality control procedures’Crustose coralline algae buffer shallow reef environments from dissolution
Published 11 August 2026 Science Leave a CommentTags: algae, biological response, chemistry, dissolution, field
Ocean acidification threatens coral reefs by reducing seawater pH and carbonate saturation state. Crustose coralline algae are particularly vulnerable because their high-magnesium calcite skeletons dissolve more readily than coral aragonite skeletons. However, this dissolution may increase alkalinity and buffer reef-water chemistry. Here we show, using repeated low-tide observations and in situ incubations in a shallow reef system in the southern Great Barrier Reef, that reef-water pH varies by more than one unit over the day (7.47 – 8.61), reaching levels comparable to those projected for the end of this century. Nighttime respiration promotes dissolution of high-magnesium calcite produced by crustose coralline algae, increasing alkalinity and helping maintain seawater supersaturated with respect to aragonite. At the same time, isolated coral incubations experience a greater decline in pH and aragonite saturation state in the absence of this buffering effect. These findings suggest that high-magnesium calcite-producing communities may help partially buffer reefs against future acidification.
Continue reading ‘Crustose coralline algae buffer shallow reef environments from dissolution’Annual variations of seawater carbonate system in the surface layer of the South-Eastern Mediterranean Sea
Published 7 August 2026 Science Leave a CommentTags: chemistry, Mediterranean
Seawater carbonate system measurements in the Mediterranean Sea are relatively scarce, especially in its eastern basin. This study describes the monthly variations of carbonate system parameters in the open sea and coastal water column of the eastern Mediterranean Sea during the period February 2018 to January 2019. Water samples were collected for analysis of Total Alkalinity and pHT at the shallow coastal THEMO1 and the deeper open-water THEMO2 (1,500 m bottom depth) stations off the Mediterranean coast of Israel. In the surface layer (<25 m), TA increased with salinity at a rate of +72 μmol·kg−1 and the pCO2(SW) was strongly and positively correlated with temperature. During summertime, surface waters were highly super-saturated with respect to atmospheric pCO2 (pCO2(SW) ∼ 600 μatm), while during winter they were only slightly super-saturated. The estimated net annual CO2 flux from the surface of the eastern Mediterranean Sea to the atmosphere during the study period was +2.8 Tg C·y−1, which is three times greater than a previous estimate made for this region (0.9 Tg C·y−1), and is attributed to the smaller volume of Atlantic water input in the present study compared to the previous one as well as the substantially shallower total dissolved inorganic carbon enriched core of the Levantine Intermediate Water in this study. Based on these results, it is still expected that future warming and salinization, reduced Atlantic Water influx to the eastern Mediterranean Sea, and increased stratification will increase the flux of CO2 to the atmosphere in this region.
Plain Language Summary
This study investigated annual variations in atmospheric CO2 flux from the surface of the eastern Mediterranean Sea based on monthly seawater sampling and carbon chemistry analyses conducted at two locations off the Mediterranean coast of Israel from February 2018 to January 2019. The results indicate that the southeastern Mediterranean Sea was a net source of CO2 to the atmosphere. This behavior was largely driven by seasonal changes in seawater temperature, which controls the saturation concentration of dissolved CO2 in seawater. During summer months, warm surface waters were a strong source of CO2 to the atmosphere, while during winter they were a weak sink. The study estimates that approximately 2.8 million tons of carbon were released annually from the sea to the atmosphere during the study period. This revised and more comprehensive estimate is ∼3 times higher than previous estimates reported for the same region. The increase is suggested to result from recent changes in regional circulation, particularly a reduced inflow of Atlantic Ocean waters into the eastern Mediterranean Sea. Based on these findings, continued warming, increasing salinity, and enhanced stratification of the eastern Mediterranean Sea are expected to further intensify CO2 release to the atmosphere in the future.
Continue reading ‘Annual variations of seawater carbonate system in the surface layer of the South-Eastern Mediterranean Sea’Trends and persistence in ocean acidification as measured by station ALOHA
Published 6 August 2026 Science Leave a CommentTags: chemistry, modeling, review
Ocean acidification, largely driven by the uptake of anthropogenic carbon dioxide, is reflected in a sustained decline in seawater pH. This paper examines the dynamics of surface-ocean pH at Station ALOHA over the period 1985–2024 using fractional integration methods, which allow for a flexible characterisation of persistence and trend behaviour. The differencing parameter is estimated under alternative assumptions concerning the error term, namely white-noise and autocorrelated Bloomfield disturbances, and recursive estimation is used to assess the evolution of the relevant parameters over time. The results show a negative and statistically significant time trend in both the original and logged pH series. The estimates of the differencing parameter are positive and significantly different from zero in all cases, providing evidence of long-memory behaviour. Under white-noise errors, the estimate of the differencing parameter is 0.89 and the unit-root hypothesis cannot be rejected, whereas allowing for autocorrelation yields an estimate of approximately 0.55, implying mean reversion with long-lasting but transitory effects of shocks. Recursive estimates further indicate that both persistence and the magnitude of the negative pH trend have increased over time. These findings suggest that pH dynamics at Station ALOHA are characterised by a persistent decline and slow adjustment following disturbances, highlighting the importance of long-term ocean monitoring and modelling approaches that account for fractional dependence. Nevertheless, the results should be interpreted with caution because the analysis is based on a single long-term record combining observational and reconstructed data.
Continue reading ‘Trends and persistence in ocean acidification as measured by station ALOHA’Ocean acidification as a planetary signal linking Earth system memory to deep-time lithosphere–ocean geochemical interactions
Published 5 August 2026 Science Leave a CommentTags: chemistry, globalmodeling, modeling
Highlights
- First ever reinterpretation of ocean acidification as a planetary-scale signal encoding Earth system disequilibrium.
- Links rapid surface pH decline to deep-ocean, sedimentary, and lithospheric memory.
- Reveals a rate mismatch that overwhelms geological buffering despite modest pH magnitude.
- Identifies CCD shoaling as the key integrator of short-term forcing and deep-time response.
- Synthesis elevates ocean acidification to a diagnostic of long-term carbon cycle stability.
Abstract
Ocean acidification (OA) is widely recognized as a defining chemical signature of the Anthropocene ocean, yet it is still predominantly framed as a near-surface ecological stressor. Here, we advance a planetary-scale reinterpretation of OA as an information-bearing signal that links rapid anthropogenic carbon forcing to deep-time lithosphere–ocean geochemical interactions and long-lived Earth system memory. Observations since the late 20th century document a persistent, spatially heterogeneous decline in global surface-ocean pH, corresponding to a ∼30–35% increase in hydrogen ion concentration, driven by sustained air–sea CO2 uptake. Projected pH declines of 0.3–0.4 units by 2100 under high-emission scenarios imply rates of change at least an order of magnitude faster than most natural variations recorded in the geological archive. We synthesize carbonate system thermodynamics, carbonate compensation depth dynamics, sedimentary proxy evidence, and Earth system modeling to demonstrate how small, rapid surface perturbations propagate vertically, trigger widespread carbonate undersaturation, and imprint durable stratigraphic and geochemical signatures through sediment dissolution, altered burial fluxes, and delayed alkalinity restoration. Comparison with hyperthermal events, particularly the Paleocene–Eocene Thermal Maximum, reveals that the uniqueness of modern OA lies not in magnitude alone but in an unprecedented rate mismatch that overwhelms geological buffering mechanisms. Furthermore, we identify critical knowledge gaps regarding irreversibility, sedimentary signal emergence, and threshold behavior under rapid forcing. Overall, this study reframes OA as a planetary-scale diagnostic of whole-Earth system disequilibrium, revealing its fundamental significance for evaluating long-term carbon cycle stability, Earth system resilience, and the enduring geological legacy of anthropogenic emissions beyond human timescales.
Continue reading ‘Ocean acidification as a planetary signal linking Earth system memory to deep-time lithosphere–ocean geochemical interactions’Localized biogeochemistry and seasonality govern carbonate chemistry in estuarine mangrove ecosystems
Published 3 August 2026 Science Leave a CommentTags: biogeochemistry, chemistry, field, Indian
Sundarbans, the world’s largest contiguous mangrove ecosystem and representing shallow coastal Bay of Bengal of the Northern Ocean, faces dynamic climate variations, including ocean acidification. To delineate ocean acidification from natural pH variations, it is crucial to perform long-term measurements of multiple carbonate chemistry parameters such as pH, total alkalinity (TA), and dissolved nutrients, among others. In the present study, surface water carbonate chemistry parameters, including TA, pH, and dissolved nutrients (o-phosphate and silicate), were analysed monthly between 2014 and 2022 in three pre-defined stations, namely Stn1, Stn2, and Stn3, part of Sundarbans Biological Observatory Time Series (SBOTS) located in Sagar Island, the largest island of the Indian Sundarbans. The observed deviation from the linear TA-Salinity curve in the studied sites of SBOTS showed the influence of freshwater in modulating TA. Generalized Additive Model (GAM) revealed substantial seasonal variability in the controls on TA. During monsoon, salinity was a dominant driver of carbonate chemistry, consistent with enhanced freshwater discharge. In contrast, during the post-monsoon season, primary productivity as indicated by the relationship with Chla, dissolved silicate, was found to exert a stronger influence on TA variability. Multilinear regression (MLR) analysis of calculated pCO2 further supported these seasonal trends. Overall, the findings highlight the importance of season-specific assessments, highlighting the critical role of freshwater discharge in shaping estuarine carbonate dynamics. These insights are vital for predicting the vulnerability and response of mangrove estuaries under future climate change scenarios.
Continue reading ‘Localized biogeochemistry and seasonality govern carbonate chemistry in estuarine mangrove ecosystems’Seasonal variations in the bulk density of planktic foraminiferal tests in response to oceanographic changes in the western North Pacific
Published 3 August 2026 Science Leave a CommentTags: biogeochemistry, biological response, chemistry, field, laboratory, North Pacific, zooplankton
Ocean acidification (OA) is a major component of ongoing global environmental change, yet its biological impacts on open-ocean calcifiers remain insufficiently quantified. Here, we investigate seasonal variability in the individual test density of the planktic foraminifer Globigerina bulloides in the western North Pacific. Test density was determined using high-resolution microfocus X-ray computed tomography, enabling micron-scale structural assessment. Time-series samples collected by sediment traps moored at 150 m and 540 m at station K2 (2008–2009) reveal pronounced seasonal variability, with test density reduced by ~ 20% during winter relative to other seasons. Seasonal reductions were associated with enhanced vertical mixing and positively correlated with mixed-layer pH, carbonate-ion concentration, and temperature. Additional plankton-tow samples collected between 2010 and 2016 further support a strong linkage between carbonate chemistry and calcification intensity. Multiple regression analysis shows that carbonate-ion concentration independently explains 46.6% of the variance in test density, whereas temperature accounts for only 0.25%, indicating that carbonate-ion availability exerts a dominant control on test density. Given the ongoing decline in carbonate-ion concentration in the North Pacific (~ 0.77 µmol kg−1 yr−1), our results imply an annual decrease of ~ 2 µg mm−3 in foraminiferal test density. Continued OA may therefore reduce biogenic CaCO3 shell density, potentially weakening the efficiency of the carbonate-based biological carbon pump. This study provides quantitative field-based evidence linking seasonal carbonate chemistry to shell density variability in open-ocean calcifiers.
Continue reading ‘Seasonal variations in the bulk density of planktic foraminiferal tests in response to oceanographic changes in the western North Pacific’Variation and influencing factors of water alkalinity in estuary-bay waters of Zhanjiang Bay, China
Published 27 July 2026 Science ClosedTags: chemistry, field, modeling, North Pacific, regionalmodeling
This study investigated the spatial distribution, seasonal variation, and drivers of surface seawater alkalinity (Alk) in Zhanjiang Bay (ZJB) using high-frequency seasonal sampling in the summers and winters of 2023. Surface Alk ranged from 525.3 to 2213.3 μmol·L−1, with mean values of 1373.1 ± 420.9 μmol·L−1 (summer, n = 28) and 1612.3 ± 343.7 μmol·L−1 (winter, n = 20). Spatially, Alk increased progressively from the estuary to the inner bay and further to the bay mouth, reflecting a typical dilution gradient. Correlation analyses showed that summer Alk was positively correlated with salinity (ρ = 0.706, p < 0.001), indicating that salinity changes associated with conservative mixing were a dominant control, whereas the weaker winter correlation (ρ = 0.473, p < 0.001) suggested that biological processes may play a more important role. Tidal forcing was significantly associated with diurnal Alk variations, particularly in the estuary and inner bay. In the estuary, high Alk occurred during high tide, consistent with tidal mixing; in the inner bay, elevated Alk was observed during low tide, suggesting a possible tidal pumping effect. These findings provide baseline data on Alk dynamics in a subtropical estuarine bay and contribute to understanding the carbonate system and buffering capacity in similar coastal systems. However, because measurements of dissolved inorganic carbon and pCO2 were unavailable, a quantitative assessment of carbon sink capacity requires further investigation.
Continue reading ‘Variation and influencing factors of water alkalinity in estuary-bay waters of Zhanjiang Bay, China’A satellite-derived re-analysis of surface-ocean pH variability in the coastal waters of India
Published 22 July 2026 Science ClosedTags: chemistry, Indian
Monitoring pH variations in coastal waters is essential for maintaining marine life and developing strategies to combat ocean acidification caused by climate change. The present study examines pH changes on interannual and seasonal scales along the northwest (NW), southwest (SW), southeast (SE), and northeast (NE) coasts of India over 30 years (1993–2022) using satellite data. Results reveal a significant decline in pH levels (R 2 = 0.727) across Indian coastal waters, with notable regional differences. The northeastern coast showed the greatest pH stability and the smallest decrease (R 2 = 0.798), while the southwestern coast experienced the highest variability (R 2 = 0.490). These regional pH fluctuations suggest varying resilience to ocean acidification. The NE and SW coasts, with higher variability, may be more susceptible to environmental changes, underscoring the importance of targeted monitoring and mitigation measures. In contrast, the more stable trends along the SE and NW coasts present opportunities to explore long-term resilience mechanisms. Seasonal fluctuations were evident everywhere, with winter consistently showing higher pH values and monsoon seasons the lowest. Principal component analysis indicated that the first two components accounted for 85.7% of the variance, highlighting factors such as seasonal river inflows, biological activity, and monsoonal freshwater input. Overall, these findings emphasise the importance of region-specific coastal management strategies to address climate change impacts and human pressures, while also providing a baseline for tracking future ocean acidification trends and assessing their effects on marine biodiversity and ecosystem services.
Continue reading ‘A satellite-derived re-analysis of surface-ocean pH variability in the coastal waters of India’Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ13CDIC and 224Ra–228Th disequilibria
Published 21 July 2026 Science ClosedTags: algae, chemistry, field, North Pacific, sediment
Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO2 accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO3) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO3 dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the 224Ra–228Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ13CDIC) were employed to trace DIC sources in porewater. The results showed that CaCO3 dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO3 dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO2 accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO2 content, the TA supplied by the CaCO3 dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO3 dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO2 elevation.
Continue reading ‘Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ13CDIC and 224Ra–228Th disequilibria’Carbonate chemistry in groundwater and rivers draining to the Baltic Sea: implications for coastal ocean acidification
Published 21 July 2026 Science ClosedTags: Baltic, chemistry, field
Terrestrial inputs can alter total alkalinity (TA) and dissolved inorganic carbon (DIC) of the coastal ocean and modify seawater pH. Here, we first characterize the carbonate system in river and groundwater draining to the Baltic Sea using observations of TA, DIC, δ13C-DIC, and major ions across 6 countries and 17 beaches. We then assess whether submarine groundwater discharge (SGD) may impact coastal acidification. TA and DIC concentrations were about 2 times greater in groundwater than river water. 84% of the groundwater and 72% of river samples showed potential to acidify receiving Baltic Sea waters and degas CO2 due to low TA/DIC ratios. Mixing plots revealed non-conservative production of TA and DIC in subterranean estuaries. δ13C-DIC values imply that organic matter respiration was a main source of DIC to northern catchments, while calcium carbonate (CaCO3) dissolution was more important along the southeastern coast. Fresh SGD contributed only < 2% of TA and DIC, and 5–7% of Ca, Mg, and SO4 fluxes compared to river discharge when extrapolated to the entire Baltic Sea. However, unquantified total SGD (fresh groundwater plus recirculated seawater) water and chemical fluxes are likely higher. Overall, SGD can locally acidify the Baltic Sea and should be considered in regional carbon budgets.
Continue reading ‘Carbonate chemistry in groundwater and rivers draining to the Baltic Sea: implications for coastal ocean acidification’Coral carbonate pH records show localized stability within Philippine waters
Published 20 July 2026 Science ClosedTags: biological response, chemistry, corals, laboratory, North Pacific
The process of decreasing seawater pH due to increasing atmospheric carbon dioxide known as ocean acidification is a global phenomenon with very strong regional effects. We report the first seasonal and interannual carbonate chemistry variabilities in the Philippines determined using carbonate boron systematics. Coral δ11B-pHsw from two contrasting site conditions showed more positive trends than basin-wide averages, suggesting that reef waters around the Philippines represented by these sites may be offsetting the general pH decline in global seawater. This pattern is consistent with δ11B-pHsw studies in neighboring Palau and Taiwan and can be explained by decreases in net ecosystem calcification and/or increases in net ecosystem productivity. Other potential site-specific local drivers of δ11B-pHsw changes include vent-driven input of nutrient- and CO2-rich groundwater that alters seawater dissolved inorganic carbon (DIC). The El Niño Southern Oscillation that influences water mass movement along the Philippine Pacific seaboard, and seasonal zonal current reversal in the Verde Island Passage (VIP) that switches source waters within the VIP, may also contribute to the recorded changes in δ11B-pHsw. Differences between the coral internal fluid chemistry and modelled regional seawater pH trends highlight the importance of localized monitoring of acidification for accurate management strategies of reef habitats.
Continue reading ‘Coral carbonate pH records show localized stability within Philippine waters’Distinct polar carbon regimes reveal hemispheric asymmetry in surface ocean pCO₂ regulation
Published 17 July 2026 Science ClosedTags: chemistry, field, modeling, regionalmodeling
Polar oceans play a major role in the global carbon cycle, absorbing a substantial fraction of human-emitted carbon dioxide and helping regulate Earth’s climate. Extreme conditions and seasonal sea-ice limit in situ observations, leaving major uncertainties in how carbon exchange varies across these regions. Consequently, the processes controlling surface ocean carbon at high latitudes remain poorly understood. Here we demonstrate that polar oceans exhibit a pronounced hemispheric asymmetry in the drivers of surface carbon variability. By combining machine learning with a data-driven regionalization of biogeochemical provinces, we reconstruct surface carbon patterns across both polar oceans over the period 1998-2022 and identify their dominant controls. Variability in the Southern Ocean is primarily governed by non-thermal processes linked to biological activity and wind-driven mixing, whereas in the Arctic Ocean thermodynamic forcing dominates in open waters and freshwater-driven stratification shapes the central basin. Polar oceans therefore do not operate as a single carbon regime. Instead, distinct mechanisms governing carbon cycling in each hemisphere are associated with opposing long-term pCO₂ trajectories, with weak or negative trends across much of the Southern Ocean but widespread increases throughout the Arctic.
Continue reading ‘Distinct polar carbon regimes reveal hemispheric asymmetry in surface ocean pCO₂ regulation’Ocean acidification, a silent threat to life on earth
Published 16 July 2026 Science ClosedTags: chemistry, education
Did you know that the ocean plays a vital role in controlling the planet’s temperature, as well as providing food, minerals, and half of the oxygen we breathe? Another important role is balancing the amount of carbon dioxide gas in the atmosphere. Unfortunately, due to large emissions of carbon dioxide over the past few centuries, the ocean is becoming acidic. This process is called ocean acidification. Ocean acidification affects many sea organisms, especially those with shells, like mussels and corals, but it can also affect other animals’ sense of orientation, like fish. Scientists are discovering more about the potential impacts of ocean acidification on the marine system and, consequently, on humans and other life forms who depend on or explore the ocean. In this article, we explain why ocean acidification started, where it has been detected, its present and future impacts on marine life, and how we can help correct it.
Continue reading ‘Ocean acidification, a silent threat to life on earth’Development of hypoxia and acidification during harmful algal blooms: dynamic multi-stressor conditions in NY, USA, estuaries
Published 14 July 2026 Science ClosedTags: algae, biological response, chemistry, field, multiple factors, North Atlantic, sensor, toxicants
Highlights
- Alexandrium blooms occurred with normoxic conditions and moderate pH (7.5-8.0).
- Alexandrium blooms occurred with moderate pCO2 (400 – 1,000 µatm) and saturating conditions for aragonite.
- Dinophysis blooms co-occurred nocturnal acidification (8.71 to 13.6 hr d−1) and hypoxia (0.52 to 3.88 hr d−1).
- Dinophysis blooms co-occurred with high pCO2 (1,000 – 3,500 µatm) and undersaturated aragonite (Ωar < 1).
- The co-occurrence of nocturnal hypoxia, acidification, Ωar undersaturation, and HABs coupled is a significant threat for marine life.
Abstract
While harmful algal blooms (HABs), hypoxia, and ocean acidification are common occurrences in coastal zones, research investigating the co-occurrence and interactions between these processes has been lacking. Here, we documented the initiation, peak, and demise of eight distinct HAB events caused by Alexandrium catenella and Dinophysis acuminata over a two-year period in two estuaries (Northport Harbor and Cold Spring Harbor, NY, USA). We concurrently characterized the dynamics of carbonate chemistry, including pCO2 and the saturation state of aragonite (Ωar), pH, dissolved oxygen (DO), and general environmental conditions in space and time. HABs occurred in succession and reached high densities with A. catenella blooms exceeding 104 cells L−1 being succeeded by D. acuminata blooms exceeding 106 cells L−1. A. catenella blooms occurred during spring months under generally normoxic conditions with moderate levels of pCO2 (400 – 1,000 µatm), only brief periods of acidification (pH < 7.5), mostly saturating conditions for aragonite, and an absence of hypoxia. In contrast, D. acuminata blooms, which occurred in summer, consistently co-occurred with bouts of extended nocturnal acidification (8.71 to 13.6 hr d−1) and hypoxia (0.52 to 3.88 hr d−1) coupled with higher levels of pCO2 (1,000 – 3,500 µatm), and undersaturating conditions for aragonite (Ωar < 1). During both blooms, nearshore locations hosted higher cell densities, lower pH and DO, and higher pCO2 compared to open water regions. The co-occurrence of multiple stressors including nocturnal hypoxia, acidification, Ωar undersaturation, and HABs, coupled with strong diel cycling of DO, pH, and pCO2, especially during D. acuminata blooms, represents a significant and previously unrecognized threat for marine life.
Continue reading ‘Development of hypoxia and acidification during harmful algal blooms: dynamic multi-stressor conditions in NY, USA, estuaries’Coastal Ocean Data Analysis Product in North America (CODAP-NA, Version 2026) from 1981-08-23 to 2024-11-23 (NCEI Accession 0315529)
Published 10 July 2026 Science ClosedTags: chemistry
The coastal Ocean Data Analysis Product in North America (CODAP-NA) Version 2026 represents a major expansion of coastal ocean carbonate chemistry synthesis for North American continental margins. Compared to CODAP-NA Version 2021 (61 cruises, 3,391 profiles, and 28,206 data rows), the updated product integrates newly available cruise observations spanning more than four decades, substantially increasing both the spatial and temporal coverage of coastal biogeochemical measurements across all North American continental shelves. This version of the CODAP-NA is composed of 32,250 oceanographic profiles from 446 research cruises covering all continental shelves in North America (U.S. west coast, U.S. east coast, Gulf of Mexico, and Alaska coast). Data for 14 variables (temperature; salinity; dissolved oxygen concentration; dissolved inorganic carbon concentration; total alkalinity; pH on the Total Scale; carbonate ion concentration; fugacity of carbon dioxide; and concentrations of silicate, phosphate, nitrate, nitrite, nitrate plus nitrite, and ammonium) have been subjected to extensive quality control. Funding for this work comes from the National Oceanic and Atmospheric Administration (NOAA) Ocean Acidification Program.
Continue reading ‘Coastal Ocean Data Analysis Product in North America (CODAP-NA, Version 2026) from 1981-08-23 to 2024-11-23 (NCEI Accession 0315529)’
