Posts Tagged 'biogeochemistry'

Hydroclimatic controls on nearshore pollution and ecosystem stress along a tropical urban coastline

Published 19 October 2026 Science Leave a Comment
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Highlights

  • Integrated indices revealed nutrient, microbial, and organic pollution patterns.
  • Urbanization and monsoon hydroclimate jointly intensified coastal degradation.
  • Seasonal eutrophication and oxygen depletion were strongly temperature driven.
  • Microbial contamination increased during post-monsoon runoff and tourism.
  • Multi-index approach clarified pollution processes in tropical coastal waters.

Abstract

Urbanizing coasts face escalating pollution from anthropogenic sources and climate change. This study provides an integrated seasonal assessment of coastal water quality along the Visakhapatnam coast, India, using a multi-index framework combining the Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI), Trophic State Index (TSI), and Pollution Index (PI). Surface water samples were collected from eleven representative locations including fishing settlements, recreational beaches, leisure beaches, and industrial-port zones. Physicochemical factors, nutrients, chlorophyll-a, and microbial indicators were assessed using multivariate statistical analysis to determine significant environmental stressors. Results revealed clear spatial and seasonal differences driven by monsoon runoff interacting with localized urban pollution sources. Jalaripeta exhibited the lowest water quality, with low WQI scores (<40), eutrophic to hypereutrophic status (TSI >56), and elevated microbial contamination associated with untreated sewage and inadequate flushing. In comparison, RK Beach and the Harbour area showed moderate degradation, characterized by intermittent nutrient influx, spikes in microbes, localized hypoxia associated with tourism pressure and port activities. Principal Component Analysis also revealed that nutrient-microbe enrichment, thermal-biological productivity, and organic matter breakdown are major environmental gradients influencing water quality variability. The findings indicate that the interplay between urbanization and seasonal hydroclimate intensifies environmental stress along the Visakhapatnam coast. This integrated multi-index framework provides as a valuable decision-support tool for identification pollution hotspot and adaptive management of tropical urban coasts under increasing climate and anthropogenic pressures.

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Reducing nitrate loads to the St. Lawrence river could mitigate hypoxia and acidification in its deep estuary and gulf

Published 15 October 2026 Science Leave a Comment
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Hypoxia in marine systems is expanding worldwide due to anthropogenic activities. Deep waters of the St. Lawrence Estuary and Gulf are rapidly deoxygenating (∼1 mmol O2 m−3 y−1), driven by changes in Atlantic water-mass intrusion and nutrient‐fueled remineralization, and are simultaneously acidifying (∼0.004 pH unit y−1). These changes alter biogeochemical cycles and degrade habitats, causing biotic and economic consequences. Using a coupled physical-biogeochemical model, we tested whether watershed-scale nitrate reductions could mitigate deep‐water deoxygenation and carbonate-system degradation. Three nitrate‐reduction scenarios reduced riverine St. Lawrence River nitrate loads by about 11%, 30%, and 50% (−2.8, −7.5, and −11.7 mmol N m−3, respectively). Relative to the baseline simulation, these scenarios increased dissolved oxygen concentrations (DO) (+1.6, +4.3, and +6.7 mmol O2 m−3), pH (+0.004, +0.010, and +0.016), and aragonite saturation state (Ωarag) (+0.003, +0.009, and +0.015) in the deep Laurentian Channel (⩾175 m). The volume of deep waters with DO saturation <30% decreased by 3%, 8%, and 13% (−27, −75, −122 km3), while volumes with pH <7.65 declined by 1.0%, 2.8%, and 4.5% (−22, −60, −96 km3). Severely corrosive waters (Ωarag <0.6) dropped by 1.4%, 3.7%, and 5.9% (−26, −71, −112 km3). Over 2014–2024, modelled improvements exceeded small interannual variability but remained within seasonal amplitudes of deep DO concentrations (∼11 mmol O2 m−3) and pH (∼0.015 units), nevertheless pointing to a detectable management signal. Although nutrient‐driven improvements remained smaller than climate‐forced trends, modelled nitrate load reductions could offset ∼56% and 80% of recent observed oxygen and pH declines, respectively, while reducing hypoxic and acidified (pH and Ωarag) water volumes. These findings provide quantitative evidence that watershed‐scale nitrogen mitigation, through improved agricultural practices and wastewater nitrogen removal, which necessitates cooperation across jurisdictions, can alleviate hypoxia and acidification in deep coastal ecosystems, making nutrient management an essential complement to climate adaptation strategies.

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Variability in nitrogen uptake rates related to anthropogenic nutrient sources and carbonate system dynamics in the tropical coastal ocean

Published 12 October 2026 Science Leave a Comment
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Highlights

  • Elevated CO2 and diverse nitrogen inputs interactively modulate nitrogen uptake rates.
  • Ammonium-induced suppression of nitrate and subsequent co-dominance maximize nitrogen uptake.
  • Efficient biological pump as the regenerated and new production coexist.
  • West coast of India serves as a model system for rapidly developing tropical basins.

Abstract

The interaction between CO2 enrichment and nitrogen uptake under anthropogenic nitrogen inputs in the tropical coastal seas remains poorly understood. In this experimental study, we investigate the effects of elevated CO2 and impacts of various nitrogen sources such as river runoff, submarine groundwater discharge, industrial effluents, and atmospheric deposition, adding on NH4+ and NO3−uptake in tropical coastal waters. Using microcosm experiments along the west coast of India, a representative model system for tropical coastal ecosystems experiencing rapid anthropogenic development, we show that CO2 enrichment enhances nitrogen uptake, under intense anthropogenic nutrient enrichment. Our observations revealed a distinct uptake pattern of initial preference for NH4+ actively suppressed NO3− assimilation, subsequently leading to the co-dominance of NO3− and NH4+ uptake rates. The correlation between regenerated and new production acts as an efficient coastal biological pump in the coastal waters. These findings suggest that rising anthropogenic CO2, alongside nutrient inputs, may intensify nitrogen cycling in tropical coastal oceans, with implications for primary production and ecosystem dynamics under changing environmental conditions.

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Coupling and decoupling of DIC, TA and O2 benthic fluxes along the Rhône River-dominated ocean margin

Published 9 October 2026 Science Leave a Comment
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River-dominated ocean margins (RiOMar) and their benthic compartments play a key role in coastal carbon cycling due to the combined effect of high inputs of particulate organic carbon (POC), benthic mineralization and burial processes. This study quantified benthic fluxes of DIC, TA, and oxygen along a transect influenced by riverine POC inputs, providing new insights in diagenetic processes when switching from anaerobic to aerobic dominated sediment form the river mouth to the adjacent shelf. Fluxes at the sediment-water interface were measured ex-situ with sediment cores incubation during two cruises at the end of spring 2018 and 2022. Stations closest to the river mouth exhibit high benthic fluxes averaging 54 ± 20, 30 ± 21 and −27 ± 14 mmol m−2 d−1 for DICflux, TAflux and TOU, respectively, reflecting intense anaerobic remineralization and a strong decoupling between oxygen consumption and DIC production (DICflux:TOU ratio of 1.5 to 3.5). These large TA fluxes further support that FeS burial limits the reoxidation of reduced compounds in these sediments, allowing the upward diffusion of TA produced at depth towards bottom waters. Offshore, In contrast, shelf stations show lower flux intensities, 8 ± 3 mmol m−2 d−1 for DICflux, −1 ± 2 mmol m−2 d−1 for TAflux and −9 ± 2 mmol m−2 d−1 for TOU, suggesting a greater influence of aerobic mineralization, reoxidation processes and nitrification. Stations located in the Rhône prodelta show intermediate benthic fluxes of 24 ± 11, 6 ± 4 and −17 ± 7 mmol m−2 d−1 for DICflux, TAflux and TOU, respectively. By substantially expanding the spatial and temporal resolution of benthic DICflux, TAflux and TOU in this system, this study provides a robust quantitative framework for assessing the contribution of RiOMar sediments to coastal alkalinity budgets and emphasizes the important role of TA fluxes released from the sediment in increasing the buffering capacity bottom waters.

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Carbon in the ocean

Published 5 October 2026 Science Closed
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This chapter examines the role of the oceans as the largest active surface reservoir of carbon in the Earth system and the chemical and physical processes that regulate the storage and redistribution of carbon in seawater. The oceans contain roughly 38,700 Gt of carbon, about fifty times more than the atmosphere, mostly in the form of dissolved inorganic carbon distributed among carbonic acid, bicarbonate, and carbonate species, whose proportions are largely controlled by seawater pH and alkalinity. The chapter introduces the fundamental equilibria of the marine carbonate system, including gas exchange with the atmosphere governed by Henry’s law, and explains how dissolved inorganic carbon and alkalinity determine the ocean’s capacity to absorb atmospheric CO2. It also reviews the origin and composition of seawater and the geochemical processes responsible for ocean salinity. Finally, the chapter describes the major mechanisms transferring carbon from the surface ocean to deeper waters, by the solubility, biological, and carbonate pumps, and examines how the depth of the saturation state of calcium carbonate, the lysocline, and the carbonate compensation depth regulate carbonate sedimentation and provide long-term negative feedback on atmospheric CO2.

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The divergence between chronic offshore acidification and volatile estuarine pH dynamics: a 60-year East China sea record

Published 2 October 2026 Science Closed
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Coastal ocean acidification in river-dominated marginal seas exhibits distinct evolutionary trajectories from the open ocean. However, quantitatively decoupling its multi-stressor drivers remains challenging due to historical data scarcity and the confounding effects of shifting hydrology and localized human activities. We established a robust calibration framework using sedimentary 3‑hydroxy fatty acids to reconstruct a 60-year pH history in the Changjiang Estuary and the adjacent East China Sea shelf. Our reconstruction reveals that the shelf water pH declined at a rate of −0.00093 units yr-1 over the past six decades, closely tracking the invasion of atmospheric CO2, whereas the estuarine zone was governed by regional biogeochemical processes. Variation partitioning analysis and generalized additive models quantify that hydrological forcing, anthropogenic forcing and hydro-biological fluctuations accounted for independent variance contributions of 41.3%, 24.8% and 16.3% of the pH variability in the estuary. These findings suggest that the sensitivity of coastal carbonate chemistry to climate-related environmental change depends not merely on atmospheric CO2 forcing, but also on the balance between regional hydrological regimes and ecosystem state. Scenario-based perturbation analyses further suggest that variations in freshwater discharge, warming, and oxygen dynamics will either amplify or partially buffer coastal acidification signals, depending on the dynamic competition between physical dilution and biologically mediated carbonate processes. This underscores the critical necessity of conducting integrated, multi-stressor assessments to guide adaptive watershed coastal management, rather than treating river-dominated marginal seas as a single homogeneous domain.

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Environmental impact on calcifying organisms: focus on the effects of river runoff and ocean acidification on health status and pollutant bioaccumulation

Published 29 September 2026 Science Closed
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This doctoral thesis presents a multi-scale and interdisciplinary investigation into the impacts of climate change and anthropogenic pressures on coastal marine ecosystems, integrating the regional dynamics of the Adriatic Sea and the broader context of the Mediterranean Sea. This research combines long-term environmental data analysis with physiological and biochemical studies on key indicator species to elucidate the complex, and often synergistic, nature of the major threats facing these vulnerable environments. The first part of the work highlights a shift in the Adriatic Sea’s physical and biogeochemical regime, from historical eutrophication toward contemporary oligotrophic conditions. The combined effects of reduced Po River runoff and climate-driven warming, particularly during summer, modify the hydrography of the entire basin. These changes have cascading effects, clearly visible in the clam Chamelea gallina. Northern populations, while showing short-term resilience (e.g., higher survival in air), exhibit a lower condition index and higher bioaccumulation of trace metals, which activate costly metabolic defense mechanisms. This energetic stress makes the populations more vulnerable to additional pressures, possibly aligning with the recurrent mass mortality events. The second part of this work was conducted at the Panarea CO2 vents, which constitute a natural laboratory for assessing future global change impacts. Through the study of the coral Balanophyllia europaea, it is found that ocean acidification (OA), especially when combined with ocean warming, significantly impairs coral tissue regeneration, with species-specific responses. A crucial finding was that OA appears to stimulate a response mediated by the coral’s microbiome, enhancing its bacterial degradation capacity and reducing the presence of PAHs in coral tissues. Collectively, this work demonstrates that synergistic stressor interactions pose the greatest threat to marine ecosystems. While evidence of resilience emerges through physiological and microbial adaptations, the findings emphasize the urgent need for integrated management strategies that address cumulative impacts across scales.

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Submarine groundwater discharge drives massive carbon outwelling and impacts ocean acidification buffering in a mangrove-dominated bay of the Beibu Gulf, China

Published 28 September 2026 Science Closed
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Mangrove ecosystems are critical blue carbon sinks, yet the role of submarine groundwater discharge (SGD) in carbon outwelling and coastal acidification buffering remains a major knowledge gap. This study integrates radium isotopes (224Ra and 228Ra) and carbonate system parameters to quantify SGD-driven carbon fluxes and their impacts on ocean acidification buffering in Tieshan Bay, a mangrove-fringed semi-enclosed bay in Beibu Gulf, China. Field measurements revealed SGD fluxes of (3.93 ± 1.52) × 107 m3/d (11.6 ± 4.5 cm/d), delivering 31-fold more dissolved inorganic carbon [DIC, (1.06 ± 0.13) × 108 mol/d] and 21-fold more total alkalinity [TA, (7.25 ± 0.89) × 107 mol/d] than local rivers. Notably, SGD exhibited a low TA/DIC ratio (0.68 ± 0.14), which mechanistically weakens the bay’s buffering capacity against acidification by reducing the efficiency of the carbonate system to neutralize atmospheric CO2. Furthermore, bicarbonate dominated SGD-derived DIC (84.7% ± 14.3%), enhancing long-term carbon sequestration potential but concurrently suppressing carbonate saturation states via dilution of carbonate ions [SGD: (27.4 ± 28.1) µmol/L vs. bay: (80.3 ± 22.4) µmol/L]. These findings demonstrate SGD’s dual role as a major carbon outwelling vector and a driver of coastal acidification, redefining blue carbon budgets in mangrove ecosystems.

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Aragonite saturation state indicates emerging ocean acidification hotspots in the Gulf of Mannar and Palk Bay along the southeast coast of India

Published 23 September 2026 Science Closed
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The impact of ocean acidification (OA) on seawater carbonate chemistry, particularly on the aragonite saturation state (Ωarag), is inadequately understood in the Gulf of Mannar (GoM) and Palk Bay (PB). In this study, we analyzed the spatial and temporal variability of Ωarag to pinpoint potential ocean acidification hotspots within these two ecologically critical coastal habitats along the southeastern coast of India. A pilot study was conducted over 24 locations during 2023–2024. The findings reveal that PB had a higher mean pH (8.33 ± 0.06) than GoM (8.08 ± 0.02). In GoM, Ωarag values were lower, at 2.82 ± 0.20, indicating that calcification conditions were not adequate. In PB, it reached 3.22 ± 0.57, which means that conditions for calcifying organisms were favourable. Seasonal variability in Ωarag was primarily controlled by pH, carbonate ion concentration, pCO2, and monsoonal hydrographic processes Seasonal patterns corroborated that these conditions were modulated by both anthropogenic activities and natural processes. During the Northeast and Southwest monsoons, there were significant drops in Ωarag that matched changes in important biogeochemical indices. Overall, these results reveal that PB experiences greater variability in Seawater carbonate chemistry, while GoM exhibits relatively stable but slightly lower carbonate saturation. Pearson correlation and Structural Equation Modelling further demonstrated that pH and carbonate ion concentration positively influenced Ωarag, whereas pCO2 and Revelle factor showed negative relationships. The findings enunciate that GoM is more vulnerable to ocean acidification than PB and emphasize the value of Ωarag as a sensitive indicator for detecting emerging acidification hotspots in coastal waters. It also emphasizes the importance of regular monitoring of OA parameters in these regions to protect these critical ecosystems and their associated services.

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Multi-centennial response of marine carbon pumps to global warming

Published 16 September 2026 Science Closed
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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.

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Seasonal forecasts of pH and aragonite saturation for the Bering Sea shelf

Published 28 August 2026 Science Closed
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The number of numerical model forecasts of ocean environmental conditions has greatly expanded in recent years, including biogeochemical variables. Forecasts can provide marine resource managers with advance warning of extreme events such as heatwaves and hypoxia, though forecast products are scarcer in high-latitude environments. The Bering Sea shelf is a large marine ecosystem that supports critical commercial, cultural, and subsistence ecosystem services that are vulnerable to extreme events and anthropogenic stressors such as ocean acidification (OA). Here, we use a regional oceanographic model of the Bering Sea to assess model forecast skill in predicting bottom water pH and aragonite saturation state (Ωarag) on lead times from 1 to 9 months. We simulate 28 years (1982–2010) of 3-member ensemble retrospective forecasts, initialized both in April and May following the retreat of winter sea ice and several months in advance of early fall when the most acidic bottom water conditions occur. The results suggest that the model is skillful (anomaly correlation coefficients > 0.5) in forecasting shelf-wide anomalies in bottom water pH and Ωarag, on lead times up to 9 months owing primarily to strong persistence. However, predictive skill for finer scale spatial anomalies is more limited and variable. Model forecast skill is also strong in Bristol Bay, home to the red king crab fishery that has faced recent closures and is threatened by OA. The model forecasts presented here can provide several months advanced notice for acidified water conditions and bolster a suite of products used to support evidence-based fisheries management.

Plain Language Summary

Similar to weather forecasts for the atmosphere, numerical models can also be used to forecast ocean conditions. The skill of these forecasts can be high up to several months in advance because the ocean has a relatively long memory and changes occur more slowly, particularly below the surface. The Bering Sea sustains substantial marine fisheries, which are threatened by changing ocean conditions such as increasing water acidity. Here we used an ocean model to test how well the model can forecast acidified bottom water conditions several months in advance. We did this by running a suite of reforecasts (i.e., we retroactively forecast years that have already occurred) and comparing to the model simulation of the conditions that did occur. Our results suggest that the model can generate skillful forecasts with lead times of up to 9 months, mainly because the conditions that occur at the start of our forecast tend to persist through the summer and into the fall. These forecasts can provide Bering Sea resource managers with advance warnings of water conditions that are harmful to marine species, such as Bristol Bay red king crab.

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Aragonite saturation horizon variability along North Pacific seamounts and implications for deep-sea coral reefs

Published 27 August 2026 Science Closed
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The 2014 discovery of living deep-sea coral reefs along the Northwest Hawaiian Islands (NWHI) and lower Emperor Seamount Chain (ESC), despite the North Pacific’s shallow aragonite saturation horizon (ASH) and high CaCO3 dissolution rates, underscores the need to understand the local seawater chemistry where these reefs persist. We investigated seawater carbonate chemistry using discrete samples along NWHI and ESC from two cruises ∼1 year apart (08/26/21–09/26/21, 09/09/22–10/24/22). Across the two cruises, ASH depth difference ranged from 15 to 77 m. Since the Pacific ASH shoals by 1–2 m yr−1, this long-term trend cannot explain the magnitude of ASH change observed. Potential contributions from anthropogenic CO2 and examining intermediate water mass changes from temperature-salinity plots did not provide an explanation for the observed changes. Instead, ASH depth variability was primarily governed by localized biogeochemical processes, namely changes in intermediate water respiration and CaCO3 dissolution. Indicators for dissolution (TA*) and respiration (AOU) suggest changes in ASH depth were driven by changes in dissolution at the northern- and southern-most sites, whereas respiration exerted stronger control at central sites. Combining 2021 and 2022 data with data from 2014 to 2019 revealed high interannual ASH variability, by as much as >200 m. Deep-sea coral reefs across the NWHI and ESC currently reside close to the ASH depth and likely experience interannual shifts between under- and supersaturation. As ocean acidification induced shoaling occurs alongside these interannual fluctuations, the frequency of undersaturation will be an important consideration for deep-sea coral reef longevity.

Plain Language Summary

In 2014, thriving deep-sea coral reefs were found in the Pacific near Hawaii and the Emperor Seamounts, where conditions were thought too acidic for reef development. To understand how these reefs persist, we studied the carbonate chemistry of this region during two research cruises (2021 and 2022). We found that the depth at which seawater becomes corrosive for coral skeletons (aragonite saturation horizon, ASH) changed far more from year to year than the gradual Pacific trend of 1–2 m yr−1. Human sourced carbon dioxide and natural shifts in ocean water masses do not explain the observed large changes. Instead, local biological and chemical processes, respiration and the dissolution of calcium carbonate, played a major role. When comparing seawater chemistry from 2014 to 2022, the ASH fluctuated even more dramatically than the predicted trend of 1–2 m yr−1. While climate change continues to drive corrosive deep waters closer to the surface, processes that occur on shorter timescales, local respiration and dissolution, can similarly expose deep sea coral reefs to corrosive conditions that can impact their longevity. Therefore, short term and local scale processes can affect long term acidification trends, making gradual shoaling harder to observe in short term data.

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A hidden driver of the greenhouse effect: production mechanisms and climate implications of N2O derived from marine planktonic ammonia-oxidizing archaea

Published 18 August 2026 Science Closed
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Nitrous oxide (N2O) is a potent and long-lived greenhouse gas, and the ocean represents its primary natural source. An accurate understanding of the emission intensity and driving mechanisms of oceanic N2O directly affects the estimation of atmospheric N2O budget and the projection of climate change. Recent studies have demonstrated that ammonia-oxidizing archaea (AOA) dominate nitrification in marine environments and are thus recognized as an important biological source of marine N2O production. However, their specific metabolic pathways, regulatory mechanisms, and environmental response patterns remain poorly understood. This paper systematically reviews the spatial distribution characteristics of marine N2O and its major biogeochemical sources, with a particular focus on the potential metabolic pathways and key intermediates involved in AOA-derived N2O production. We further synthesize the current understanding of how environmental factors—including dissolved oxygen, pH, temperature, substrate availability, and organic carbon—regulate AOA nitrogen metabolism and N2O generation. In addition, we discuss the potential mechanisms underlying the shift in AOA metabolic strategies and variations in N2O yield under multifactorial coupling scenarios the scenario of multifactorial coupling. On this basis, we identify the major knowledge gaps in existing research, particularly regarding culture systems, mechanistic elucidation, in situ validation, and global-scale quantification. Finally, we proposed future research directions for AOA-derived N2O emissions under ongoing oceanic changes. This review aims to advance the understanding of the biogeochemical mechanisms governing marine N2O production and to provide a theoretical basis for improving the parameterization of marine nitrogen cycle and climate models.

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Persistence of Arctic Ocean acidification under negative emissions

Published 14 August 2026 Science Closed
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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.

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Localized biogeochemistry and seasonality govern carbonate chemistry in estuarine mangrove ecosystems

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

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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 Closed
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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.

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Thermal regulation of benthic fluxes in temperate estuaries

Published 2 July 2026 Science Closed
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The effects of short-term heatwave extremes on biogeochemical cycling and fluxes in a temperate estuary of a semi-dry climate were studied using an experimental setup of temperature-controlled benthic incubations. The results demonstrated a strong thermal effect, notably under extreme warming events, for shifts in exchanges across the sediment-water interface. Extreme heatwave conditions (+5 °C of the seasonal mean) boosted acidification, hypoxia, and ammonification, due to accelerated remineralization rates, resulting in strong effluxes of NH4, Si(OH)4, and PO4 to the overlying water. These excessive nutrient loads may increase eutrophication risk via runoff or tidal action, specifically in adjacent oligotrophic coastal waters. CO2 production rates reached ~4000 µatm under extreme hypoxia and acidification, 2.3-fold higher than the ambient rate, with a maximal flux of ~27.0 mmol m-2 d-1. Hence, our experiments show that marine heatwaves amplify CO2 emissions while reducing the CO2 buffering capacity of temperate estuaries. It emphasizes temperate estuaries as highly sensitive ecosystems to climate change.

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Skeletal porosity of a cold-water coral increases with decreasing aragonite saturation state along a depth gradient in the Mediterranean Sea

Published 29 June 2026 Science Closed
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Background

Cold-water corals (CWCs) are key ecosystem engineers that create complex three-dimensional habitats much like tropical reefs, but in deep, cold seas. However, like other reef-building systems, they are increasingly threatened by climate change and ocean acidification. CWC communities in the Mediterranean Sea may be especially vulnerable because these waters absorb more atmospheric CO2 than the global ocean, making it a mesocosm that mirrors broader global trends affecting marine life. Since calcification is energetically costly and likely becomes even more demanding as pH and carbonate ion availability decline, understanding how the decrease in aragonite saturation state (Ωarag) affects biomineralization is essential for predicting the future of these corals.

Results

Here, we investigated skeletal structural and compositional changes of the scleractinian CWC Desmophyllum dianthus along an Ωarag gradient in the Mediterranean Sea using specimens collected between 400 and 1200 m depth. Our findings indicate that skeletal porosity increases at the macro-scale with decreasing Ωarag, while micro- and nano-scale structural and compositional features remained unaffected.

Conclusions

The persistence of micro- and nano-scale skeletal features across an 800 m depth gradient suggests that D. dianthus maintains tight biological control over mineralization at these scales, even as Ωarag declines. This control does not extend to the macro-scale, where increasing porosity alters the skeleton’s overall architecture under lower Ωarag. D. dianthus thus appears to preserve the fundamental “building blocks” of its skeleton while changing its larger-scale structure, a decoupling that may make macro-scale porosity an early marker of acidification stress in CWCs.

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Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO2

Published 26 June 2026 Science Closed
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Ocean acidification, driven by rising atmospheric CO₂, threatens the ability of corals to build their skeletons by reducing their capacity to maintain an elevated pH at the calcification site (pHcf), a process essential for calcium carbonate precipitation. Boron isotopes have commonly been used to show that the response of pHcf to ocean acidification is highly species-specific. However, the physiological mechanisms underlying this variability remain poorly understood. Recently, lithium (Li) isotopes have been used to trace the activity of ionic transport involved in cellular pH regulation and calcification (e.g. H+, Na+ and Ca2+), and may therefore help resolve these mechanisms. Here, we investigate multiple coral species from Tutum Bay (Papua New Guinea), a natural CO₂ seep system creating pH gradients (mean pHT = 7.66 at seeps vs. 8.01 at control sites) analogous to future ocean acidification scenarios. Our results show a relationship between seawater pH, calcifying fluid chemistry, and lithium isotopic composition. Corals exposed to low seawater pH exhibit significantly altered δ⁷Li values relative to colonies from the control site, with some species becoming enriched in ⁷Li (up to 2‰) as pHcf declines. This isotopic shift is consistent with reduced efficiency of Na⁺/H⁺ exchangers (NHEs), active transporters that preferentially incorporate the lighter ⁶Li isotope under optimal conditions but may become less effective under elevated proton concentrations. By linking Li isotopes to calcifying-fluid chemistry, these results provide geochemical evidence that ocean acidification may disrupt ionic regulation in corals and that Li isotopes can help to resolve biogeochemical controls of carbonate-systems.  

Continue reading ‘Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO2’

A basin-wide assessment of pH changes in the Mediterranean Sea based on reanalysis products

Published 19 June 2026 Science Closed
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Ocean acidification, driven by increasing atmospheric CO2 concentrations, poses a growing threat to marine ecosystems and biogeochemical processes. The Mediterranean Sea, characterized by complex circulation patterns and distinct hydrographic sub-basins, represents a sensitive region for assessing basin-scale pH variability. However, long-term in situ pH observations remain spatially sparse and unevenly distributed, limiting the assessment of coherent spatiotemporal trends across the basin. Here, we present a basin-wide spatiotemporal assessment of pH trends in the Mediterranean using an 18-year biogeochemical reanalysis dataset from the Copernicus Marine Environment Monitoring Service. Our analysis reveals a consistent vertical structuring of pH trends, with negative trends in surface waters and contrasting, often neutral to weakly positive tendencies at depth. The magnitude and vertical extent of these trends vary regionally and are closely linked to local circulation regimes, water-mass formation processes, and remineralization dynamics. In deep-water formation regions such as the Adriatic, Ionian, and Aegean Seas, negative pH trends extend throughout much of the water column, whereas in the Levantine Basin, mesoscale circulation structures confine pH changes primarily to a relatively thin surface layer. These results demonstrate that basin-scale analyses based on high-quality, publicly accessible biogeochemical reanalysis products, such as CMEMS, can provide a spatially integrated perspective on long-term pH variability, complementing existing observational records by bridging spatial and temporal gaps. The framework presented here offers a reproducible approach for systematically assessing depth and region resolved pH trends.

Continue reading ‘A basin-wide assessment of pH changes in the Mediterranean Sea based on reanalysis products’

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