Posts Tagged 'biogeochemistry'

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Monsoon-linked gut microbiome variation in a tropical sea urchin: a single-reef case study from the Arabian Sea

Published 29 October 2026 Science Leave a Comment
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Highlights

  • Stomopneustes variolaris gut microbiota shows clear seasonal structuring in response to monsoon-driven biogeochemical variability.
  • The gut microbiota of Stomopneustes variolaris is dominated by Bacteroidota, and most diverse during winter and spring inter-monsoon.
  • Gut microbial composition covaries with seasonal oxygen, pH, temperature, and nutrient gradients.
  • The sea urchin gut microbiome shows seasonal plasticity under monsoon-linked stress.

Abstract

Tropical coastal ecosystems of the Eastern Arabian Sea (EAS) experience pronounced monsoon-driven shifts in water-column biogeochemistry, yet their influence on the gut-associated microbiomes of benthic invertebrates remains poorly understood. Sea urchins are ecologically important grazers in reef-associated habitats, and their gut microbiota may reflect both environmental and dietary variability. Here, we provide an exploratory characterisation of the gut-associated microbiome of the sea urchin Stomopneustes variolaris from the Grande Island archipelago (Goa, India) across four monsoon-associated periods: Winter monsoon (WM), Spring-inter monsoon (SIM), Summer monsoon (SM), and Fall-inter monsoon (FIM). Using 16S rRNA gene sequencing, we analysed gut tissue and gut digesta from S. variolaris alongside environmental samples (seawater, sediment, and seaweed). Bacteroidota dominated the urchin gut microbiome, whereas Proteobacteria were more abundant in environmental samples. Gut microbial diversity was highest in WM and SIM and lowest during SM and FIM. Exploratory analyses indicated that gut microbial composition varied temporally, with taxa affiliated with sulphur-cycling lineages more prevalent during sampling points associated with low-oxygen monsoon phases (SM and FIM) and Firmicutes and organic matter-associated taxa more prominent during comparatively oxygenated periods (WM and SIM). Ordination analyses suggested an association between gut and environmental microbial assemblages and gradients in dissolved oxygen, temperature, pH, nutrients, and pCO2. This study provides baseline insights into the gut microbiome of a tropical echinoid from an understudied region and highlights monsoon-associated patterns in microbial community composition, forming a foundation for future functional and experimental investigations.

Graphical abstract

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Coastal benthic ecosystems in a changing ocean: a mini review

Published 29 October 2026 Science Leave a Comment
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Coastal benthic ecosystems are shaped by the combined effects of climate change and anthropogenic pressures. Ocean warming, acidification, and deoxygenation interact with eutrophication, pollution, bottom trawling, and coastal development, altering biodiversity, habitat structure, and functioning from shallow coasts to continental margins. Benthic communities, comprising infaunal and epifaunal taxa, regulate key processes including nutrient cycling, sediment dynamics, and carbon burial, thereby underpinning benthic-pelagic coupling and biogeochemical dynamics. Rising temperatures and marine heatwaves drive widespread losses of habitat-forming species, while ocean acidification constrains calcification. Warming, acidification, hypoxia, eutrophication, and pollution increasingly simplify assemblages, and alter trophic pathways and performance. Interactions among stressors can amplify organismal sensitivity and push ecosystems towards ecological thresholds. However, many knowledge gaps remain. Recent methodological advances, including environmental DNA metabarcoding, automated imaging, trait-based analyses, and multi-stressor experiments, are improving the detection of ecosystem changes at large and our mechanistic understanding of ecosystem responses to ongoing change. By summarizing the various stressors that benthic habitats and taxa face, their interactions and potential outlooks, this review provides a concise synthesis of current understanding and identifies key knowledge gaps for future research and management under accelerating climate change.

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Response of volatile halocarbons release to atmospheric deposition and ocean acidification in the tropical eastern Indian Ocean

Published 27 October 2026 Science Closed
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Highlights

  • Fe(III) inputs increased CH3I emission by photochemical and biogenic processes.
  • Nutrient inputs enhanced volatile halocarbons by boosting biogenic processes.
  • Ocean acidification enhanced CH2Br2/CHBr3 emissions by BrPO-mediated halogenation.

Abstract

Short-lived volatile halocarbons (SLVHCs) are important trace greenhouse gases and ozone-depleting substances influencing global climate. Anthropogenic activities have accelerated ocean acidification (OA) and enhanced atmospheric deposition, yet how these perturbations regulate SLVHCs production in open ocean remains unclear. Herein, we investigated the effects of OA and atmospheric deposition derived Fe(III) and nutrient input on SLVHCs production through ship-based incubation experiments at two stations in the tropical eastern Indian Ocean (10°N-10°S) from March to May 2022. Our findings revealed that nutrient addition promoted phytoplankton growth with the maximum Chlorophyll a (Chl a) increasing to 6.17 μg L−1, enhanced bromoperoxidase (BrPO) activity (by 108.3 % compared to the control group), and stimulated release of dissolved organic carbon (DOC), ultimately driving the production of CH3I, CH2Br2, and CHBr3 (by 152.0 %, 200.2 %, and 141.8 % increases, respectively). OA and Fe(III) additions promoted phytoplankton growth (Chl a up to 1.85 μg L⁻1), thereby enhancing SLVHCs production through biological and photochemical processes, whereas they exhibited no significant effects on SLVHCs production in relatively oligotrophic regions. Mechanistically, CH3I release was regulated by phytoplankton-mediated biogenic processes and photochemical processes. In contrast, CH2Br2 and CHBr3 production was predominantly associated with phytoplankton-mediated biogenic processes (BrPO-mediated halogenation). These results demonstrating that OA, Fe(III), and nutrient inputs regulate marine SLVHC production by altering phytoplankton-mediated biogenic processes and photochemical reactivity. Our study offers a novel perspective and direct evidence for quantifying the production of SLVHCs under climate change scenarios, thereby underscoring the need to reassess oceanic climate feedbacks in the context of anthropogenic forcing.

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Large-scale water-sediment regulation (Yellow River) substantially reshapes organic carbon mineralization pathways and benthic fluxes in sediment of a river-dominated sea (Bohai Sea)

Published 26 October 2026 Science Closed
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Large-scale hydrological engineering has fundamentally altered the rhythm of land-to-ocean material transport globally, yet the mechanistic response of coastal benthic biogeochemistry to these “pulse” events remains poorly quantified. Here, we investigated the transient impacts of the annual Yellow River Water-Sediment Regulation (WSR), a massive anthropogenic flood, on sediment organic carbon (OC) mineralization and benthic fluxes in the Bohai Sea (BHS) by combining geochemical analyses with sediment incubations. Results indicated that the WSR enhanced sediment OC mineralization rates by an average factor of 1.65, driven primarily by pulsed input of labile OC and probably also reflecting a priming effect. This stimulation reconfigured dominant diagenetic pathways: significantly elevating the role of dissimilatory iron reduction at the expense of both aerobic respiration and sulfate reduction. In addition, pulsed nitrate inputs promoted denitrification, establishing it as an important OC mineralization pathway. Concurrently, diffusive nutrient fluxes increased by a factor of 1.9–5.1 following the WSR, and the mean ratio of total alkalinity to dissolved inorganic carbon fluxes decreased from 1.34 to 0.96. This shift in the ratio signifies a reduced capacity for carbonate buffering in the bottom waters, which could exacerbate seasonal bottom-water acidification. Collectively, our findings demonstrate that short-term anthropogenic perturbations can rapidly reprogram sediment diagenesis and benthic fluxes, thereby perturbing carbon and nutrient cycling as well as benthic-pelagic coupling in river-dominated coastal systems.

Plain Language Summary

The annual Water-Sediment Regulation of the Yellow River is one of the world’s largest engineered flow manipulations, rapidly delivering huge amounts of freshwater and sediment into the Bohai Sea (BHS). While much is known about its effects on the water column ecochemistry, how such a large-scale human intervention alters organic carbon degradation and chemical exchange in seafloor sediments has remained unclear. By combining field measurements and laboratory experiments, we show that this event strongly enhances sediment carbon degradation rates and shifts the dominant pathways, favoring iron reduction and nitrate reduction processes. These changes drive substantial increases in benthic fluxes of dissolved inorganic carbon and nutrients (ammonium, silicate, and phosphate) into overlying waters. Notably, the regulation reduces the ratio of alkalinity to inorganic carbon flux, implying a temporary decline in the capacity of the sea to absorb atmospheric CO2. Our findings provide critical insights by revealing that large-scale human-regulated river discharges can rapidly reshape coastal sediment biogeochemistry. These results have important implications for carbon and nutrient cycling, offering valuable references for understanding not only the BHS but also other river-dominated coastal systems worldwide.

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Hypoxia and acidification antagonistically suppress nitrification but enhance its N2O production in coastal waters

Published 26 October 2026 Science Closed
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The concurrent intensification of hypoxia and acidification poses a growing threat to estuarine-coastal ecosystems globally, yet their interactive impacts on nitrification, a key process in the nitrogen (N) cycle and a substantial source of nitrous oxide (N2O), remain poorly understood. This knowledge gap hinders reliable projections of future N cycling and climate feedbacks in these critical ecosystems. Here, we demonstrate that along the estuarine-coastal continuum, the co-occurrence of hypoxia and acidification suppresses nitrification rates but stimulates nitrification-derived N2O emissions—although their combined effect is lesser than the sum of their individual effects. Metatranscriptomic analysis provided molecular evidence for these isotopically traced responses and uncovered critical microbial coping strategies through transcriptional adjustments in metabolic pathways for energy conservation and stress tolerance. This previously overlooked interaction disrupts microbial N cycling with potential impacts on N2O emissions and climate feedbacks, highlighting the necessity to revise predictive models by incorporating such multistressor dynamics.

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Assessing the skill of a biogeochemical model tailored to a global coupled forecasting system: a case study for the Bay of Bengal carbon cycle

Published 22 October 2026 Science Closed
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To achieve real-time prediction of surface ocean carbonaceous variables in the Indian Ocean, we integrate an extended-range coupled forecast system with a global ocean biogeochemistry model incorporating an explicit carbon cycle. The physical ocean state is provided by the Indian Institute of Tropical Meteorology Extended Range Prediction for Applications to Society (IITM-ERPAS) coupled model, which drives the Ocean Tracer Transport Model (OTTM) with phosphate-dependent biology and OCMIP-II-type carbonate chemistry (hereafter referred to as OTTM-ERPAS-BGC). Daily mean outputs from IITM-ERPAS are used to drive the biogeochemical simulations. Model skill is evaluated against in situ observations of surface partial pressure of CO₂ (pCO₂) and pH from the Bay of Bengal Ocean Acidification (BOBOA) mooring (90°E, 15°N) over the period 2013-2018. The model successfully reproduces observed variability, with strong and statistically significant correlations (r = 0.65 for pCO₂ and r = 0.80 for pH; p < 0.001). Based on this performance, a coherent region of influence (87.5°E-92.5°E, 12.5°N-17.5°N) is identified to further assess model skill against BOBOA time-series observations, climatological products and other derived datasets of pCO₂ and pH in terms of seasonal cycle, subseasonal variability and long-term trends. Seasonal cycle analysis reveals that the updated Takahashi climatological pCO₂ product is systematically underestimated by ∼18 ± 3 μatm compared to BOBOA observations, OTTM-ERPAS-BGC simulations and other derived products. Subseasonal variability exhibits pronounced intraseasonal signals, with significant spectral peaks at 13-41 days for pCO₂ and 17-41 days for pH, reflecting dominant intraseasonal modulation of the Bay of Bengal carbon cycle. Long-term trends indicate a robust increase in pCO₂ (2.02 μatm yr⁻¹) and a concomitant decline in pH (-0.00253 yr⁻¹), consistent with progressive acidification of the basin. The decomposition analysis demonstrates that dissolved inorganic carbon (DIC) and total alkalinity (ALK) exert dominant and complementary controls on pCO₂ and pH variability, respectively, while sea surface temperature (SST) plays a secondary role. Overall, this study demonstrates that coupling extended-range physical forecasts with ocean biogeochemical models provides a viable pathway for the operational prediction of carbon variables. Such capability is critical for the early detection of ocean acidification and hypercapnia hotspots, with significant implications for marine ecosystem management and climate services in the Indian Ocean.

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Plastic pollution as a pathway to ocean acidification and disruption of the seawater carbonate system

Published 22 October 2026 Science Closed
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Ocean acidification threatens calcifying organisms by reducing seawater pH and carbonate ion availability, while plastic pollution may intensify this effect through chemical and microbial processes, exacerbating impacts on calcification. Here, we investigated, for the first time, the biogeochemical mechanisms of plastic-pollution-induced ocean acidification and the role of plastic as a modifier of the seawater carbonate system. Microcosm experiments were conducted using naturally weathered plastic particles incubated in seawater under irradiated and dark conditions. The experiment comprised two seven-day phases: (1) an abiotic phase evaluating the physicochemical effects of plastic degradation and (2) a biotic phase assessing microbial contributions to plastic-induced acidification. Variations in pH, dissolved CO₂, Ωarag, and Ωcalc were monitored as acidification indicators. During the abiotic phase, plastic increased dissolved CO₂ concentrations threefold under irradiation (27.08 µmol kg⁻¹) compared with the dark treatment (9.65 µmol kg⁻¹), reducing pH, Ωarag, and Ωcalc. By the end of this phase, irradiated plastic reached pH = 7.6, Ωarag = 2.16, and Ωcalc = 3.25, corresponding to values associated with impacts on calcifying organisms and considered terminal for coral reef development. During the biotic phase, no significant changes were observed, likely due to limited dissolved organic carbon release, indicating that UV-driven abiotic processes, rather than microbial activity, predominated in CO₂ release. The observed chemical changes matched end-of-century projections under high-emission scenarios, highlighting that plastic pollution may intensify reductions in carbonate ion concentration and calcium carbonate saturation states, accelerating calcifier mortality and biodiversity loss.

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Seasonal prediction of ocean biogeochemistry in the northeast U.S. large marine ecosystem using a regional ocean-biogeochemistry model

Published 20 October 2026 Science Closed
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We couple an ocean biogeochemical model to a regional ocean model of the Northwest Atlantic and assess the potential skill of bottom dissolved oxygen, aragonite saturation state, and pH forecasts along the Northeast U.S. Continental Shelf using a reanalysis-forced model run as the true biogeochemical state. Across 30 years of retrospective forecasts initialized four times per year, the model has substantial potential skill for all three variables, with skill generally highest in the deep Gulf of Maine and lowest in the shallower Mid-Atlantic Bight and Georges Bank. Forecasts initialized in January and April benefit from the insulation of the bottom from the unpredictable atmosphere by stratification and tend to have greater skill at longer lead times. Using two case studies, we show how predictable variations in ocean temperature, salinity, and currents are associated with predictable variability in bottom biogeochemistry in both the Mid-Atlantic Bight and Gulf of Maine. The forecast model evaluated in this study has been developed as an operational prototype for NOAA’s Changing Ecosystems and Fisheries Initiative, with the goal of applying the forecasts to improve the yield and sustainability of living marine resources.

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

Published 19 October 2026 Science Closed
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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 Closed
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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 Closed
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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 Closed
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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.

Continue reading ‘Seasonal forecasts of pH and aragonite saturation for the Bering Sea shelf’

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.

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

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