Posts Tagged 'biogeochemistry'

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

Published 8 January 2026 Science Leave a Comment
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Highlights

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

Abstract

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

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

Published 1 January 2026 Science Leave a Comment
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Rising atmospheric CO2 levels have significantly increased ocean acidification (OA), endangering coral reefs, and nutrient (nitrate (NO3−), and phosphate (PO43−)) pollution also weakens the coral reef resilience. Therefore, the study evaluates the prevailing OA level over the Sri Lankan coral reef areas using the aragonite saturation state (ΩAr) and assesses the nitrate (NO3−), and phosphate (PO43−) concentrations over the coral sites. The study was conducted on coral reefs on the eastern coast (EC), southern coast (SC), northern coast (NC), and west coast (WC) of Sri Lanka from April to June 2024. A total of 63 seawater samples were collected around each coastal site for analysis. The Ω Ar were supersaturated (ΩAr> 1) and ranged from 2.98±0.04 to 4.92±0.12. Throughout the study period, the study sites had ΩAr values exceeding 2.92±0.16, indicating that the nation’s corals were resilient to deterioration, and the comparative analysis demonstrates that these sites were not vulnerable to OA. The NO3− concentrations of 2–5 µmol L− 1, from human activities, may intensify coral bleaching during heat stress. Results showed that SC (2.19±1.28 µmol L− 1) and WC (3.52±1.48 µmol L− 1) had NO3− above the permissible range, which may be due to waste discharge and high runoff. The significantly higher PO43− concentrations were reported in EC (0.35±0.07 µmol L− 1). Coral bleaching hotspot (HS) identification emphasizes how spatially distributed HS are from January to June. The OA risk assessment confirmed that climate change brought high risk to the coral reef ecosystems, which impact on the ecology and economy of Sri Lanka.

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Water property variability into a semi-enclosed sea dominated by dynamics, modulated by properties

Published 15 December 2025 Science Closed
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The biogeochemistry of the Salish Sea is strongly connected to its Pacific Ocean inflow through Juan de Fuca Strait (JdF), which varies seasonally and interannually in both volume and property flux. Long-term trends in warming, acidification, and deoxygenation are a concern in the region, and inflow variability influences the flux of tracers potentially contributing to these threats in the Salish Sea. Using ten years (2014–2023, inclusive) of Lagrangian particle tracking from JdF, we quantified the contributions of distinct Pacific source waters to interannual variability in JdF inflow and its biogeochemical properties. We decompose variability in salinity, temperature, dissolved oxygen, nitrate, and carbonate system tracers into components arising from changes in water source transport (dynamical variability) and changes in source properties (property variability). Observations in the region provide insight into source water processes not resolvable in the Lagrangian simulations, including denitrification and trace metal supply. Deep source waters dominate total inflow volume and drive variability in nitrate flux through changes in transport. Shallow source waters, particularly south shelf water, exhibit greater interannual variability and disproportionately affect temperature, oxygen, and [TA–DIC], driving change through both dynamical and property variability. This study highlights the combined roles of circulation and source water properties in shaping biogeochemical variability in a semi-enclosed sea, and how these roles differ between biogeochemical tracers. It provides a framework for attributing flux changes to specific source waters and physical and biogeochemical drivers, with implications for forecasting coastal ocean change under future climate scenarios.

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Seasonal variations of physico-chemical variables interaction and their influence on phytoplankton and pCO2 dynamics in the Southwest Bay of Bengal

Published 12 December 2025 Science Closed
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The carbonate system and nutrient dynamics play a crucial role in regulating phytoplankton productivity and carbon cycling in tropical coastal ecosystems, which are highly sensitive to climate change and anthropogenic activities. The present study investigates the spatio-temporal variability of physico-chemical parameters, nutrient dynamics and their influence on phytoplankton community structure along the southwest coast of Bay of Bengal (SWBoB), with particular focus on their relationship with partial pressure of carbon di-oxide (pCO₂). Seasonal sampling was carried out entirely with onboard cruise programs, with each cruise representing different season such as pre-monsoon, monsoon, post-monsoon and summer. The study covered SWBoB among six stations namely Tuticorin, Nagapattinam, Poombuhar, Pondicherry, Mahabalipuram and Chennai during 2022–2023. A total of 77 phytoplankton species representing five taxonomic classes were identified and quantified, where minimum and maximum phytoplankton density were observed during summer (7.498 × 103 cells. L-1) and pre-monsoon (7.0014 × 104 cells. L-1) respectively. A pronounced spatio-temporal variations were observed in physico-chemical parameters and nutrients with peak phytoplankton density and pCO₂ value (487.47 µatm) during pre-monsoon period were attributed to enhanced microbial respiration, riverine input and upwelling of CO₂-rich subsurface waters. In contrast, reduced pCO₂ level (274.27 µatm) observed during summer coincided with water column stratification, nutrient limitation and elevated photosynthetic uptake by phytoplankton. Canonical Correspondence Analysis (CCA) indicated a strong association were attributed nutrient availability and phytoplankton assemblages, with diatoms prevailing under nutrient-rich and moderate pCO₂ conditions, simultaneously dinoflagellate dominated at high pCO₂ conditions. A significant positive relationship between pCO₂ and phytoplankton species with canonical score (0.91) of Noctiluca scintillans highlights the sensitivity of SwBoB productivity to carbon system variability. During pre-monsoon, high pCO₂ (487.47 µatm), chlorophyll-a (3.10 µg L-1) and phytoplankton density (7.0014 × 104 cells. L-1) at station T2, co-dominated by both diatom (46 %) and dinoflagellates (40 %), specifically Noctiluca scintillans (6.32 %). This indicated that nutrient enrichment and CO₂-rich upwelling enhanced phytoplankton productivity and carbon dynamics. These findings imply that pCO₂ variations, determined by temperature, salinity and nutrient inputs which influence the phytoplankton structure and productivity, impacts carbon cycling and ecosystem dynamics in the SWBoB region. This study provides valuable insights into carbon cycling and ecosystem functioning, crucial for sustaining regional fisheries and anticipating monsoon-driven changes in coastal productivity.

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Ocean acidification alters phytoplankton diversity and community structure in the coastal water of the East China Sea

Published 5 December 2025 Science Closed
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Anthropogenic CO2 emissions and their continuous dissolution into seawater lead to seawater pCO2 rise and ocean acidification (OA). Phytoplankton groups are known to be differentially affected by carbonate chemistry changes associated with OA in different regions of contrasting physical and chemical features. To explore responses of phytoplankton to OA in the Chinese coastal waters, we conducted a mesocosm experiment in a eutrophic bay of the southern East China Sea under ambient (410 μatm, AC) and elevated (1000 μatm, HC) pCO2 levels. The HC stimulated phytoplankton growth and primary production during the initial nutrient-replete stage, while the community diversity and evenness were reduced during this stage due to the rapid nutrient consumption and diatom blooms, and the subsequent shift from diatoms to hetero-dinoflagellates led to a decline in primary production during the mid and later phases under nutrient depletion. Such suppression of diatom-to-dinoflagellate succession occurred with enhanced remineralization of organic matter under the HC conditions, with smaller phytoplankton becoming dominant for the sustained primary production. Our findings indicate that, the impacts of OA on phytoplankton diversity in the coastal water of the southern East China Sea depend on availability of nutrients, with primary productivity and biodiversity of phytoplankton reduced in the eutrophicated coastal water.

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Resilience of pH to seasonal change in a large subtropical lagoonal estuary

Published 5 December 2025 Science Closed
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Highlights

  • The lower Patos Lagoon Estuary displays a broad range of alkalinity and pH values, with riverine inputs marked by low buffering capacity.
  • A critical period of corrosive conditions occurs from winter to mid-spring, likely driven by enhanced respiration and/or external CO₂ inputs.
  • The estuary operates as a moderate to weakly buffered system, exhibiting aragonite undersaturation even under medium to high salinity conditions.
  • pH sensitivity to environmental drivers is highest in summer and winter, whereas autumn presents the most uniform seasonal response.

Abstract

Coastal ecosystems exhibit a wide range of pH trends, from −0.023 to 0.023 pH units yr−1, making them particularly susceptible to acidification or basification. These variations are primarily driven by ecosystem metabolism and the influence of oceanic and riverine endmembers, as observed in the subtropical system of the Patos Lagoon Estuary (PLE, southern Brazil), where biogeochemical variability is largely governed by mixing of water masses with different properties. This study provides the first quantification of the seasonal variability of pH buffering capacity in the inner and outer zones of PLE. From May 2017 to September 2023, we assessed temporal variability using multiple approaches: (i) carbonate system parameters, (ii) sensitivity factors, (iii) buffering capacity of pH to fractional change of dissolved inorganic carbon (βDIC), (iv) metabolic effects on pH, and (v) environmental drivers of pH. The results revealed a distinct seasonal pH pattern, especially between summer with winter and spring, with consistently higher values at the outer station compared to the inner station, though spatial differences were not statistically significant. In winter and particularly in early spring, calcium carbonate (CaCO3) dissolution prevailed due to riverine input characterized by low buffering capacity. Along the salinity gradient, pH exhibited a pronounced difference, particularly between low and high salinity conditions. However, the persistent negative deviation of the metabolic effect on pH throughout the year and in salinity ranges, even under seawater conditions, supports the characterization of this coastal ecosystem as a net CO2 source, with especially high variability at mid-salinity conditions. Although the salinity gradient was comparable between stations, they exhibited differences in the magnitude of pH sensitivity to seasonal biogeochemical changes. These findings indicate that PLE functions as a system with moderate to low buffering capacity, with the outer zone showing greater resilience to pH fluctuations.

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A global perspective on river alkalinity: drivers and implications for coastal ocean carbonate chemistry

Published 4 December 2025 Science Closed
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Abstract

The chemical nature of river water significantly influences the coastal carbonate system, contributing to coastal acidification and creating suboptimal conditions for marine calcifiers. While several regional efforts have assessed observationally based riverine concentrations and fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC), these values in global ocean biogeochemical models have generally been simplified, often set to zero or balanced against global sediment calcium carbonate burial. To enhance our understanding of rivers’ role in the coastal carbonate system, we applied multiple linear regression (MLR) to develop global empirical relationships for estimating river TA and DIC from watershed properties. We find that river TA values are primarily controlled by forest, carbonate rock coverage, and annual mean precipitation, explaining 74% of the spatial variability in TA. The variability explained improves to 77% with the inclusion of permafrost and glacial coverage, especially in high latitude and altitude regions. Additionally, nearly 30% of the spatial variability in the river DIC-to-TA ratio can be explained by terrestrial gross primary production and carbonate rock coverage. Applying these MLR-derived TA and DIC concentrations to a 1/4° resolution global ocean model reduces the high bias in model estimates of global coastal CO2 uptake by 69% (equivalent to 0.11 Pg C yr−1 less CO2 uptake) compared to the case with zero river TA and DIC. This study elucidates key drivers of the river carbonate system and underscores the importance of accurately representing riverine inputs to improve predictions of global coastal carbon dynamics and ecosystem responses to environmental changes.

Plain Language Summary

Rivers play a critical role in shaping the chemistry of coastal waters, influencing how much carbon dioxide (CO2) the ocean absorbs and creating conditions that affect marine life, such as shellfish and corals. Global models are essential for predicting carbon dynamics at large scales, offering insights into the interactions between rivers, coastal systems, and the global ocean. However, global models often simplify or partially overlook key chemical contributions from rivers, leading to biases in predictions. In this study, we analyzed how river chemistry, particularly river carbon inputs, is influenced by factors such as forest cover, carbonate rocks, rainfall, permafrost, and glaciers on land. We developed statistical models to estimate two key properties: total alkalinity and dissolved inorganic carbon. Incorporating these improved river chemistry estimates into a global ocean model markedly reduced the overestimation of coastal CO2 absorption. This research underscores the importance of accurately including riverine inputs in global models to enhance predictions of coastal carbon dynamics and ecosystem responses to climate change.

Key Points

  • Global empirical relationships are developed using multiple linear regression (MLR) to estimate river TA and DIC concentrations from watershed properties
  • Forest and carbonate rock coverage, and annual mean precipitation explain 74% of the spatial variability in global river TA values
  • Applying MLR-derived river TA and DIC concentrations to a global ocean model substantially reduces biases in coastal CO2 uptake estimates
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Typhoon-induced cascade effects on hydrological and biogeochemical dynamics in estuary-coast continuum: insights from multidisciplinary observations and model forecasting in Zhanjiang Bay, China

Published 28 November 2025 Science Closed
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Highlights

  • Typhoon-induced nutrient surge triggered cascade effects in Zhanjiang Bay.
  • Skeletonema costatum proliferated and decayed rapidly in eutrophic waters.
  • Bloom collapse caused water acidification and oxygen depletion.
  • A CNN-LSTM model achieved 73% relative accuracy in 6 h Chl-a rolling forecast.

Abstract

Typhoons can trigger biogeochemical cascade effects, including eutrophication, algal blooms, acidification, and dissolved oxygen (DO) depletion in the estuary-coast continuum, yet the underlying mechanisms remain poorly understood. This study employed a multidisciplinary observation approach, combining high-frequency in situ monitoring with field surveys, to capture the dynamics of the cascade during Typhoon “Yagi” (Sep 2024) in Zhanjiang Bay (ZJB). The analysis incorporated multivariate hydrological, meteorological, and physicochemical parameters, stable isotopes (δ15N-NO3−, δ18O-NO3−, δ18O-H2O, δD-H2O), and phytoplankton community characterization. Results showed that during the pre-algal bloom period, freshwater discharge, fluxes of dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP) increased by factors of 8.7, 43.4, and 3.0, respectively, relative to the pre-typhoon stage, while salinity decreased by 9.7%. This nutrient surge exacerbated eutrophication, leading to a serious algal bloom eight days after the typhoon. Subsequent bloom decay triggered acidification and DO decline. Field investigations confirmed typhoon-driven freshwater input and algal blooms. Moreover, a deep learning model was developed for Chlorophyll-a forecasts, achieving 73% relative accuracy (RA) in rolling 6-hour forecasting. Typhoon-driven nutrient surge triggered the cascade: Skeletonema costatum bloomed under extreme thermohaline perturbation and decayed rapidly, leading to acidification and DO depletion. This study advances mechanistic understanding of typhoon-driven cascading effects in tropical coastal ecosystems, providing a scientific basis for the assessment of their ecological consequences and predictive coastal management strategies.

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Monsoon-driven biogeochemical shifts and acidification risk in tropical estuarine ecosystems: a case study from the Indian coast

Published 17 November 2025 Science Closed
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Tropical estuaries serve as biogeochemical hotspots where the interactions between monsoon hydrology and human activities significantly impact ecosystem health. However, limited information exists on their carbonate chemistry, which is crucial for assessing climate vulnerability. This study provides the first seasonal assessment of hydrography, nutrients, and carbonate system dynamics in the Haripur estuary, Bay of Bengal. Seasonal evaluation revealed significant variations in pH, carbonate system indicators, and nutrients (p < 0.001). During the monsoon, pH declined to 7.12 ± 0.17, dissolved oxygen dropped to near-hypoxic levels (2.95 ± 0.35 mg L−1), and nutrient enrichment was observed with elevated dissolved inorganic nitrogen (6.07 ± 0.74 μM) and phosphate (1.61 ± 0.39 μM). Carbonate saturation states remained persistently corrosive, reaching minima of ΩAr (0.03 ± 0.01) and ΩCa = 0.04 ± 0.01) among the lowest reported for Indian estuaries. Multivariate analysis identified nutrient enrichment and carbonate imbalance as the dominant stressors, explaining 32.4 % of the total variance. These findings clearly indicate that the Haripur estuary functions as a regional hotspot of monsoon-driven acidification and a global outlier exhibiting year-round carbonate undersaturation. Urgent management interventions are recommended to mitigate hypoxia and acidification risks in this vulnerable tropical estuary through nutrient load reduction, enhanced tidal flushing, and ecosystem-based adaptation. The results further provide a valuable basis for developing best management practices in the context of regional and global climate change, thereby supporting the objectives of Sustainable Development Goal 14 (Life Below Water).

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Biogeochemical controls on the co-occurrence of mid-depth pH and DO minima in the inner shelf of the East China Sea

Published 12 November 2025 Science Closed
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Highlights

  • Mid-depth pH minima co-occur with DO minima and nitrate maxima near the thermocline.
  • Mid-depth pH minima are driven by both organic matter respiration and upwelling.
  • Low carbonate buffering capacity amplifies mid-depth pH and pCO2 signals.

Abstract

While ocean acidification in coastal oceans is well documented, mid-depth pH dynamics remains largely understudied. In August 2017, we conducted high-resolution vertical profiling of temperature, salinity, pH, dissolved oxygen (DO), and nitrate using in situ biogeochemical sensors in the inner East China Sea shelf. Additionally, vertical distributions of dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO2 (pCO2), and aragonite saturation state (Ωa) were also calculated. Our observations revealed that mid-depth pH minima (<7.85) co-occurred with DO minima (<60 μmol L−1) and nitrate maxima within or just below the seasonal thermocline. The DO–pH relationships at these stations followed Redfield stoichiometry, indicating organic matter respiration as a primary driver of mid-depth pH minima. High chlorophyll a concentrations (>5.0 μg L−1) at these sites suggested recent phytoplankton blooms fueling the mid-depth oxygen and pH decrease. Although temperature-salinity relationships indicated upwelled water masses contribute to mid-depth pH minima at some stations, their low-pH signature is fundamentally caused by aerobic respiration. A synthesis of five years of cruise data showed that mid-depth pH and DO minima, as well as nitrate maxima, were consistently located along the margins of upwelling zones or salinity fronts—regions of high biological productivity. These patterns underscore the coupled effects of physical transport and biogeochemical processes on mid-depth pH dynamics. Additionally, waters at mid-depth exhibited the lowest carbonate buffer capacity and highest DIC/TA ratios in vertical profiles, amplifying pH declines and pCO2 elevations. Such mid-depth pH minima may negatively affect upper-layer coastal ecosystems, including shellfish aquaculture.

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From global emissions to local impacts: spatially explicit modeling of ocean acidification in life cycle assessment

Published 22 October 2025 Science Closed
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Ocean acidification poses a critical threat to marine ecosystems. While life cycle assessment frameworks provide a method for assessing and combatting many anthropogenic impacts, marine impact models remain underdeveloped compared to their terrestrial counterparts. This study presents the first spatially explicit characterization model for quantifying the impacts of ocean acidification that includes both midpoint and endpoint characterization factors (CFs). Midpoint CFs were spatially delineated by using marine ecoregions and Food and Agriculture Organization fishing areas, leveraging spatially explicit fate and fate sensitivity factors. Endpoint CFs were calculated using species sensitivity distributions that include species across a range of calcification levels, climate zones, and trophic levels. Results demonstrate significant geographic variability in ocean acidification impacts, with polar regions showing heightened vulnerability. Our findings emphasize the need for spatially explicit modeling to account for the diverse biogeochemical and ecological responses to ocean acidification. This work advances marine impact assessment by integrating spatial and biological complexity, providing critical tools for quantifying ocean acidification’s global ecological and economic consequences.

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Coupled acidification-nitrification dynamics in eutrophic estuarine waters

Published 20 October 2025 Science Closed
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Highlights

  • Mid-estuary emerges as a hotspot for coupled acidification-nitrification, intensified by hydrology.
  • Nitrifier community structure adapts to acidification stress, while responds differently.
  • AOB is more sensitive to acidification in estuarine water compared to AOA.
  • Future climate change scenarios project intensified acidification and nitrification coupling in mid-estuary.

Abstract

The interplay between acidification and nitrification in estuarine systems could have profound effects on coastal biogeochemistry and ecosystem health. However, the lack of integrated field research risks oversimplifying their relationships in complex ecosystem dynamics. This study investigates the spatiotemporal covariations of acidification sensitivity and nitrification rates derived from observed inorganic carbon and nutrients data along a land-sea continuum. In the middle estuary, estuarine pH exhibited the highest sensitivity to ammonium concentration, coinciding with maximum nitrification rates. The coupling effect intensified by 40% during the transition from dry to wet hydrological conditions. Despite that microbial network complexity generally decreased with increased acidification sensitivity, ammonia-oxidizing bacterial communities are more sensitive to acidification in estuarine water compared to ammonia-oxidizing archaea. Conversely, in the lower estuary, acidification was associated with a decline in nitrification activities. Machine learning-based models suggest that climate change scenarios could exacerbate acidification and nitrification in the Pearl River Estuary, potentially amplifying their coupling effect in the middle estuary. This holistic approach not only advances our fundamental understanding of estuarine processes, also provides critical insights for policymakers and coastal managers striving to maintain the ecological integrity of these vital ecosystems in an era of rapid global change.

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Macroalgae farming increases DO and pH, reduces pCO2 and nutrients, and enhances blue carbon potential

Published 17 October 2025 Science Closed
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Edible macroalgal cultivation is increasingly promoted as a nature-based solution to mitigate coastal eutrophication and improve seawater quality. However, the species-specific impacts and spatial extent of these ecological effects remain poorly understood, particularly in semi-enclosed bays with complex hydrodynamics. This study aims to quantify the biogeochemical influence of two widely cultivated species—Porphyra haitanensis and Hizikia fusiformis—on seawater carbonate chemistry and nutrient levels in Yueqing Bay, eastern China. High-resolution field surveys were conducted at 52 stations, enabling direct comparisons between cultivated and non-cultivated waters. Geostatistical modeling, including spherical semivariograms and Empirical Bayesian Kriging, was applied to delineate species-specific influence zones and quantify changes in key water quality parameters. P. haitanensis farming induced broad, kilometer-scale improvements in seawater chemistry, including elevated dissolved oxygen (DO) (+ 2.72%) and pH (+ 0.09 units), and significantly lower partial pressure of CO2 (pCO2) (− 118 µatm), relative to distant reference sites (all p < 0.05). A slight increase in total phosphorus (TP) (+ 0.007 mg L− 1) was also observed, likely reflecting nearby riverine inputs. In contrast, H. fusiformis cultivation produced more localized (< 100 m) but significant changes, including reductions in dissolved inorganic carbon (DIC) (− 1.84 mg L− 1) and pCO2 (− 82.6 µatm), alongside increases in DO (+ 1.72%), pH (+ 0.02 units), and chlorophyll-a (Chl-a) (+ 0.72 µg L− 1) (all p < 0.05). These results provide the first fine-scale, species-resolved spatial assessment of macroalgal farming effects on water quality in a semi-enclosed bay. By quantifying distance-dependent ecological responses, this study offers science-based guidance for spatial planning, nutrient management, and blue carbon integration—particularly as the routine harvest of biomass facilitates net carbon export from coastal waters. These findings highlight the potential of macroalgal farming as a scalable, multifunctional nature-based solution for sustainable aquaculture and climate mitigation.

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Insights from a changing ocean: evolving biogeochemistry and its impacts on marine ecosystems and climate

Published 13 October 2025 Science Closed
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Marine biogeochemistry integrates chemical, biological, geological, and physical processes that are fundamental to Earth’s climate and ecosystems. As elements cycle through the ocean, atmosphere, and biosphere, they leave behind biogeochemical fingerprints that serve as proxies to track environmental change. Over the industrial era, anthropogenic CO2 emissions and other human activities have caused the oceans to change rapidly, perturbing this biogeochemical landscape. Characterizing biogeochemical shifts is critical to advance our understanding of climate-driven impacts, assess marine ecosystem health, and evaluate climate solutions. Recent advancements in biogeochemical tools and technologies have deepened our insights into oceanic change. The development of high-precision paleoproxies has extended records of ocean conditions into the pre-industrial era, while the Argo float array has enabled four-dimensional monitoring of biogeochemistry globally. High-resolution numerical modeling has also improved our ability to capture complex interactions at fine spatial and temporal scales, offering a holistic framework to understand anthropogenic impacts from past to future. Together, these technologies provide a comprehensive toolkit to characterize shifts in ocean biogeochemistry in unprecedented detail and advance our understanding of global environmental change. This thesis weaves together applications of novel biogeochemical tools to examine the drivers, impacts, and mitigation strategies of a rapidly changing ocean. Each chapter leverages diverse datasets and multiple tools to provide new insights on ocean change based on marine biogeochemistry. In Chapter 2, I combine boron-isotope measurements from cold-water corals with a biogeochemical model to reconstruct and investigate subsurface acidification trends over the industrial era in the California Current System. In Chapter 3, I combine Argo-based biogeochemical data products, archival tagging records, and machine learning methods to develop a four-dimensional species distribution model for an economically important fishery species, revealing biogeochemical constraints on its migration. In Chapter 4, I employ a high-resolution biogeochemical model of the Salish Sea to evaluate the detectability of ocean alkalinity enhancement, a marine carbon dioxide removal strategy for climate mitigation. These studies provide new frameworks and tools to investigate, monitor, and respond to a changing ocean.

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Research progress on responses of upper-ocean nitrogen uptake and nitrification to ocean acidification and warming (in Chinese)

Published 10 October 2025 Science Closed
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Nitrogen uptake by phytoplankton and nitrification mediated by nitrifying microorganisms in the upper ocean are key processes affecting marine productivity and carbon sequestration. How these two critical nitrogen cycle processes respond to the dual stressors of ocean acidification and warming represents a pressing research frontier in marine biogeochemical cycles and global change. Elucidating this issue will provide a theoretical foundation for accurately assessing future changes in ocean productivity and the efficiency of the biological pump. However, most existing studies rely on laboratory pure culture experiments, which may fail to adequately reflect the complex interactions between phytoplankton and nitrifying microorganisms in natural marine ecosystems and their responses to changes in environmental factors. The impacts and mechanisms of ocean acidification and warming on nitrogen uptake and nitrification are systematically summarized. In addition, more attention needs to be paid to other factors, such as strengthened ocean stratification and decreased dissolved oxygen contents, induced by ocean acidification and warming, which could indirectly affect nitrogen uptake and nitrification. Existing problems, such as insufficient in-situ monitoring of ecosystems, limited synergistic studies on multiple processes and stresses, and inadequate understanding of long-term adaptation processes, are highlighted. Finally, three key areas of research that need to be focused on in the future were prospected: ① to conduct the synchronous coupling analysis of nitrogen uptake and nitrification processes and clarify the interactive effects of acidification and warming, ② to explore the vertical differentiation response mechanisms of the above processes in the upper ocean, particularly in oligotrophic oceans, where critical knowledge gaps exist, and ③ to elucidate the long-term adaptation processes and nonlinear response laws of phytoplankton and nitrifying microorganisms. A three-in-one research framework is constructed in the spatial dimension, temporal scale and the experimental system to provide a scientific basis for evaluating the evolution of key nitrogen processes and marine productivity under global change.

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Environmental conditions and carbonate chemistry variability influencing coral reef composition along the Pacific coast of Costa Rica

Published 10 October 2025 Science Closed
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Coral reef development is influenced by a wide variety of factors, including temperature, salinity, nutrient concentrations, and carbonate chemistry. Studies focusing on physicochemical drivers of coral reef distribution and composition in the Eastern Tropical Pacific (ETP) are scarce, and carbonate chemistry and nutrient data for this region are limited. This study measured coral reef composition and physicochemical parameters along the Pacific coast of Costa Rica, over a one-year period at three locations: Santa Elena and Matapalo in the north, and Parque Nacional Marino Ballena in the south. Our results show high seasonal and spatial variability of physicochemical conditions with significant differences mainly explained by inorganic nutrient concentrations, with driving processes also having a strong influence on the variability of carbonate chemistry parameters. Coastal upwelling is the main driver of the seasonal variability in Santa Elena. Comparison of seasonal dissimilarity within locations confirms the presence of a geographical gradient, with stronger influence of the upwelling in Santa Elena relative to Matapalo, where several parameters displayed a lower seasonality and a carbonate system that supports reef development throughout the year. Conversely, in Marino Ballena the river discharges during rainy season exerted a strong control on the seasonal variability. The integrated analysis of coral reef composition and physicochemical parameters suggests that in addition to inorganic nutrients carbonate chemistry also plays a key role in coral distribution. Analyzing the spatial distribution of the main reef builders provides insights into the species-specific tolerance to varying conditions. Pavona clavus is widely distributed in both the northern and southern locations, suggesting that this massive coral is very tolerant to the high variability of physicochemical conditions. The dominant corals in the north (Pavona gigantea and Pocillopora spp.) are highly tolerant to nutrient-enriched cold waters with low aragonite saturation, while one of the main reef-builders in southern locations (Porites cf. lobata) cope better with low salinity, low aragonite saturation and low light intensity caused by river discharges. Understanding the preferences of individual coral species at our study locations can shed light on the environmental factors driving coral reef distribution in other locations of the ETP.

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Relative enrichment of ammonium and its impacts on open-ocean phytoplankton community composition under a high-emissions scenario

Published 7 October 2025 Science Closed
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Ammonium (NH4+) is an important component of the ocean’s dissolved inorganic nitrogen (DIN) pool, especially in stratified marine environments where intense recycling of organic matter elevates its supply over other forms. Using a global-ocean biogeochemical model with good fidelity to the sparse NH4+ data that are available, we project increases in the NH4+: DIN ratio in over 98 % of the ocean by the end of the 21st century under a high-emission scenario. This relative enrichment of NH4+ is driven largely by circulation changes and secondarily by warming-induced increases in microbial metabolism, as well as reduced nitrification rates due to pH decreases. Supplementing our model projections with geochemical measurements and phytoplankton abundance data from Tara Oceans, we demonstrate that shifts in the form of DIN to NH4+ may impact phytoplankton communities by disadvantaging nitrate-dependent taxa like diatoms while promoting taxa better adapted to NH4+. This could have cascading effects on marine food webs, carbon cycling and fishery productivity. Overall, the form of bioavailable nitrogen emerges as a potentially underappreciated driver of ecosystem structure and function in the changing ocean.

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Remote sensing of coastal acidification: UAS and satellite-based estimation in the Mississippi Sound and landscape change impact assessment

Published 7 October 2025 Science Closed
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Ocean acidification results from atmospheric CO₂ absorption, while coastal acidification is more localized, influenced by nutrient runoff, freshwater input, and organic matter decomposition. Due to its complexity, specialized monitoring is essential. The present research estimated two key carbonate system parameters total alkalinity (TA) and partial pressure of carbon dioxide (pCO₂) using uncrewed aircraft systems (UAS) imagery and autonomous surface vessel (ASV) observations over an oyster reef in the Western Mississippi Sound (WMS). Field campaigns were conducted from 2018 to 2022 to collect high resolution aerial imagery over the largest oyster reef in WMS, utilizing a multispectral sensor mounted on a drone. An ASV was deployed during June, July, and September 2021 UAS missions over the same sites to collect in situ data, including pH, partial pressure of carbon dioxide (pCO2), sea surface temperature (SST), sea surface salinity (SSS), colored dissolved organic matter (CDOM), and chlorophyll-a (Chl-a). Random forest models developed and accurately estimated TA and pCO₂ (R² > 0.91). Time-series maps were generated using Chl-a images derived from UAS imagery and SSS images derived from CDOM maps, employing salinity-CDOM linear regression model developed in this study. Results demonstrate UAS effectiveness in small-scale coastal monitoring due to its high spatial resolution. However, UAS lacks spatial coverage needed for broader regions like Mississippi Sound. To address this, MODIS imagery and HYCOM model outputs were integrated with ASV data collected in June and August 2023 in this research. Random forest models using SST, SSS, and Chl-a performed well (R² = 0.81 for TA, 0.87 for pCO₂). By incorporating MODIS Level 3 SST and Chl-a (1 km) and HYCOM SSS (downscaled 4 km to 1 km), this research generated annual and monthly time-series maps of mean TA and pCO₂ over the entire Mississippi Sound for the period 2002–2020. These maps reveal spatial seasonal dynamics and long-term trends. This research also investigated how land use and land cover (LULC) changes influenced TA and pCO₂ across the entire Mississippi Sound from 2002 to 2020. Spatial correlation and trend maps revealed associations between eight LULC class type changes and TA and pCO₂ patterns. The findings suggest connections between environmental changes and carbonate system responses but do not confirm causation, instead providing a basis for hypothesis generation and further study of biogeochemical processes. Overall, this dissertation highlights how combining remote sensing, in situ measurements, machine learning technique, and LULC analysis improves coastal acidification assessment in the Mississippi Sound.

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The evolution of ocean carbon cycle feedbacks in observations and models

Published 7 October 2025 Science Closed
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Since the Industrial Revolution, the ocean has absorbed a cumulative ~40% of the anthropogenic carbon (Cant) released into the atmosphere by fossil fuel emissions. Cant accumulation in the upper ocean has driven an increase in the partial pressure of carbon dioxide gas (pCO2) and associated declines in pH and carbonate ion concentration. These chemical changes, collectively referred to as ocean acidification (OA), progressively weaken the ocean’s buffer capacity and reflect the evolution of a positive marine carbon cycle feedback that reduces the efficiency of future Cant uptake and amplifies the influence of natural variability on the carbonate system. This dissertation investigates the spatial and temporal changes in the ocean carbon cycle caused by Cant using a combination of in situ observations, data synthesis products, and output from regional and global ocean models to improve our understanding of the processes governing the ocean carbon sink and its evolving feedbacks. Chapter 1 evaluates the impact of Cant accumulation on multiple OA metrics throughout the water column in the North Pacific Ocean and California Current Large Marine Ecosystem using ship-based observations. Results indicate that the greatest increases in pCO2 occur subsurface, where Cant content is moderate and pCO2 change can exceed overlying surface change by ≥100%. Amplified pCO2 responses in the interior ocean are related to background ocean carbonate chemistry, with the greatest subsurface changes associated with poorly buffered waters that have experienced substantial organic matter remineralization. Chapter 2 evaluates the impact of Cant on the seasonal variability of pCO2 in the surface ocean using output from global ocean biogeochemical models (GOBMs) used by global carbon budgeting efforts to estimate the historical ocean carbon sink strength. Results indicate that dissimilar model representations of surface ocean pCO2 seasonality, particularly during winter, lead to increasing disagreement in annual ocean carbon sink strength estimates over time. Chapter 3 examines how differences in representations of interior ocean Cant and natural carbon influence patterns of amplified subsurface pCO2 change using the same set of GOBMs, in addition to observation-based data products. Results indicate that GOBMs dissimilarly simulate subsurface Cant-induced pCO2 changes, particularly at the depth of maximum winter mixing, when these signals can re-emerge at the surface and bias estimates of the annual ocean carbon sink strength. This research contributes to ongoing international efforts to better constrain the global ocean carbon sink. Discrepancies between observation- and model-based estimates of the modern ocean carbon sink have grown over time, with across-model disagreements compounding in future climate projections. This points to an outstanding need to constrain sources of model discrepancies. This work helps to address this by clarifying: (1) a model’s projected end-of-century ocean carbon sink magnitude is highly dependent on its post-spin-up seasonal and annual mean-state; (2) a more realistic representation of interior ocean carbon distributions and ecosystem processes is needed to achieve a more realistic representation of ocean carbon cycle change and the evolution of its feedbacks.

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Ocean acidification, iodine bioavailability, and cardiovascular health: a review of possible emerging risks

Published 16 September 2025 Science Closed
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Anthropogenic climate change drives ocean acidification, which alters marine iodine cycling and increases bioaccumulation in marine ecosystems. This environmental shift elevates dietary and atmospheric iodine exposure, particularly in coastal populations, posing risks for thyroid dysfunction and downstream cardiovascular complications. Acidification enhances iodine uptake in marine species, such as kelp and seafood, thereby amplifying human intake. Chronic iodine excess can induce hypothyroidism or hyperthyroidism, both linked to cardiovascular diseases, including heart failure, atrial fibrillation, and atherosclerosis. This narrative review synthesizes the mechanistic pathways connecting ocean acidification, iodine bioavailability, thyroid dysfunction, and cardiovascular health. We emphasize the need for proactive clinical screening, dietary interventions, environmental monitoring, international collaboration, and inter-disciplinary research to address this climate-sensitive public health challenge. Coastal communities, reliant on marine diets, require targeted strategies to mitigate these emerging risks.

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