Posts Tagged 'biogeochemistry'

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Exploring the land-ocean biogeochemical and microbial connectivity in the Ría de Vigo (NW Iberian Peninsula) through submarine groundwater discharge

Published 20 May 2025 Science Closed
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Highlights

  • SGD affects the carbonate system, methane and nitrous oxide content of the embayment
  • Solute composition of SGD largely impacted by subterranean estuary reactivity
  • Contrasting poor microbial connectivity across the different aquatic environments
  • Subterranean estuaries may act as microbial boundaries in the aquatic continuum

Abstract

Increasing evidence demonstrates the widespread occurrence of submarine groundwater discharge (SGD) in coastal zones, where it may influence biogeochemistry and microbial ecology. Here, we analyze the biogeochemical composition and microbial communities across diverse aquatic environments in a highly productive coastal system (Ría de Vigo, NW Iberian Peninsula), influenced by significant fresh SGD, to assess the extent of microbial and biogeochemical connectivity—i.e., mass transfer—among them. Samples were collected from surface and deep porewaters from two subterranean estuaries (STEs), surface seawater, riverine water, and continental groundwater. These samples were analyzed for a comprehensive set of microbial and biogeochemical variables, including radioisotopes used as SGD tracers. A significant correlation between SGD tracers and carbonate system parameters, N2O, and CH4 concentrations in surface seawater indicates SGD influences biogeochemistry of the embayment. However, some of these solutes do not originate from continental groundwater but are produced in the local STEs, which act as biogeochemical reactors modifying fresh SGD. The findings also reveal highly diverse microbial communities, with higher diversity in STEs due to the variety of niches present. Indicator taxa included the phyla Euryarchaeota, Chloroflexi, Omnitrophicaeota, and the family Nitrosopumilaceae in STEs; the phylum Cyanobacteria and the family Burkholderiaceae in freshwater endmembers; and the Flavobacteriaceae and Cryomorphaceae families in seawater. Most operational taxonomic units (∼87%) were unique to a single environment (river, continental groundwater, coastal water, or STE), showing STEs limit subterranean microbial transfer between groundwater and marine ecosystems. Our results highlight STEs as reservoirs of diversity and zones of intense biogeochemical reactivity.

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Riverine freshwater outflow enhanced ocean acidification in an urbanized subtropical estuary

Published 24 April 2025 Science Closed
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Highlights

  • Riverine carbonate input reduces buffering, amplifying acidification from CO₂.
  • Respiration of riverine organic matter intensifies acidification in low-salinities.
  • Organic matter-driven acidification affects the entire water column year-round.

Abstract

Ocean acidification poses a growing environmental threat to estuarine ecosystems. Most research has focused on bottom water acidification driven by eutrophication from riverine nutrient inputs. In contrast, the impacts of other riverine components in estuarine systems have received less attention. This study investigates the impacts of riverine carbonate and organic matter input on acidification in the Pearl River estuary (PRE), using field data from April 2015 to January 2016. The results show that DIC and TA in the PRE are primarily governed by river-ocean mixing, while their seasonal variations are largely influenced by changes in the freshwater end-member. Compared to the marine carbonate system, the riverine DIC:TA ratio is significantly higher. Riverine carbonate input weakens the estuarine buffering capacity during mixing, thereby amplifying acidification from atmospheric anthropogenic CO2. The pH reduction in the estuary due to anthropogenic CO2 intrusion reached a maximum of 0.15 units at a salinity of around 15, exceeding the 0.12 reduction at the seawater end. With the urbanization of the Pearl River Basin, the increased input of anthropogenic organic matter enhanced aerobic respiration in the estuary, releasing CO2 that may further intensify acidification. Unlike eutrophication-induced bottom acidification, which is closely associated with water column stratification and mainly occurs in the outer estuary during summer, organic matter-driven acidification is most pronounced in the upper estuary and affects the entire water column year-round. Moreover, it caused a maximum pH decline of up to 1.01 units in the PRE, surpassing that induced by eutrophication.

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Cold spells, fresh waves, and the biogeochemical response in the North Atlantic cold anomaly region

Published 22 April 2025 Science Closed
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Regional effects of marine cold spells (MCS, periods of anomalous cooling), their impact on ecosystem biogeochemistry, and link to salinity extremes remain underexplored. A case in point is North Atlantic’s Cold Anomaly (CA) region (known as the “cold blob”), which hits record low temperatures during 2014–16 while most of the global ocean warmed. Using up to 42 years of observations, we characterize the CA as a manifestation of both MCS and Fresh Waves (FW, low salinity extremes) and analyze the surface biogeochemical response. We observe a quasiperiodic pattern of MCS from the 1980s and FW (at least) from the 1990s to early 2020s in the CA region with alternations from cool and freshwater to warm and saline conditions. Since 1990s, the CA region appears to be potentially undergoing MCS and FW compound events that are more frequent and prolonged but less intense than other North Atlantic areas. The 2014‐16 CA was among the most widespread and prolonged MCS and FW events associated with a deeper mixed layer and distinct biogeochemical signature, including elevated nutrients and oxygen, an overall increased chlorophyll‐a and intensified ocean acidification. These resultssuggest that MCS could mitigate certain climate change effects through cooling and enhanced productivity, while exacerbating others such as ocean acidification. We compare 2014–16 CA region effects with those of Pacific’s warm blob, identifying contrasting behaviors from physical processes to biogeochemical impacts and discussing a common atmospheric driver. Our findings emphasize the need to further study ecological responses to MCS in the North Atlantic.

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Long-term successional dynamics and response strategies of harmful algal blooms to environmental changes in Tolo Harbour

Published 18 April 2025 Science Closed
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Highlights

  • Long-term monitoring reveals significant shifts in harmful algal bloom species and toxin dynamics in Tolo Harbour.
  • Government actions reduced nutrient levels, but climate change and organic nutrients influenced HABs’ species succession.
  • Number of HABs decreased, meanwhile frequency and types of new toxin species emerged, highlighting complex ecological changes.
  • Balanced dual nutrient reduction strategies are essential for controlling HABs and restoring coastal ecosystem health.

ABSTRACT

The production and succession of harmful algae blooms (HABs) are attributed more to excessive nutrient concentrations and unbalanced nutrient stoichiometry than to other environmental drivers as the absence of long-term monitoring data. This study analyzed HABs succession patterns and key drivers in Tolo Harbour from 1986 to 2023, leveraging nearly 40 years of data. Effective governmental measures significantly improved water quality, with dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), 5-day biochemical oxygen demand (BOD5), and Escherichia coli (E. coli) concentrations decreasing by 53%, 80%, 45%, and 59%, respectively. Annual HABs events dropped from 28 to 3, and species diversity declined from 6 to 2. However, toxic species frequency rose from 21% to 46%. Dinoflagellates emerged as dominant initial species, with a shift in secondary dominance from diatoms to ochrophytes and toxin types from diarrhetic shellfish poisoning (DSP) to hemolytic toxins (HT). These shifts likely result from combined human and natural influences. Model simulations confirmed that red tide outbreaks, species succession, and shifts in toxin types were driven by declining pH, rising temperatures, unbalanced nitrogen-phosphorus ratios, organic nutrient increases, and algal antagonism. The study emphasizes the importance of the dual reduction of both DIN and DIP, meanwhile inorganic and organic nutrients, suggesting that overly focusing on or distract from one nutrient (e.g., DIP or DON) could lead to unintended ecological consequences, like the proliferation of rare and toxic species. We highlight the combined impacts of climate change (warming and ocean acidification) and anthropogenic activities (nutrient pollution and eutrophication) on HABs, particularly the number and toxin production. This research links policy changes to HAB dynamics, offering strategic recommendations for managing red tides and contribute novel perspectives on the impact of nutrient reduction in comparable bay ecosystems.

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Modeling terrestrial dissolved organic carbon and its effect on the carbonate system in the Sunda Shelf seas, Southeast Asia

Published 17 April 2025 Science Closed
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Abstract

The flux of dissolved organic carbon (DOC) from land to sea is an important transfer within the global carbon cycle. The biogeochemical fate of this terrestrial DOC (tDOC) remains poorly understood and is usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from tropical peatland-draining rivers discharging onto the Sunda Shelf. We developed a new light-driven parameterization of tDOC remineralization that accounts for photochemical, microbial, and interactive photochemical–microbial degradation, and simulated the transport and remineralization of tDOC through the Sunda Shelf seas using the regional 3D hydrodynamical HAMSOM and biogeochemical ECOHAM models (only for the carbonate system). Our realistic hindcast simulations for 1958–2022 show that about 50% of riverine tDOC is remineralized before leaving the shelf. This lowers seawater pH across the entire inner Sunda Shelf by an average of 0.005 (by up to 0.05 in the Malacca Strait). Correspondingly, seawater pCO2 is raised, increasing yearly CO2 outgassing from the shelf by 19% (3.1 Tg C yr−1, 0.14 mol m−2 yr−1) during 2013–2022. Even regional ocean acidification trends increase, because river discharge and tDOC flux increase. Our model reveals large spatial variability with greatest inputs and remineralization of tDOC close to major peatlands, especially off Sumatra and Borneo. The interannual variability in tDOC input and the monsoonal current reversal lead to strong temporal variability in carbonate system parameters in these areas. Our results highlight the importance of representing tDOC in ocean models, and reveal the fate of tropical peatland tDOC.

Key Points

  • We modeled terrestrial dissolved organic carbon (tDOC) using a new scheme for photo-, bio-, and interactive photo-bio-degradation
  • TDOC input to the Sunda Shelf in 2013–2022 is 15.9 Tg C yr−1. 50% is remineralized on the shelf, 28% directly exported to Indian Ocean
  • This drives shelf-wide outgassing of 3.1 Tg C yr−1 in 2013–2022, lowers pH and aragonite saturation, increases ocean acidification trends

Plain Language Summary

The transport of terrestrial dissolved organic carbon (tDOC) from land to sea via rivers is an important part within the global carbon cycle. The majority of this tDOC is remineralized by sunlight and marine bacteria, which produces CO2 leading to ocean acidification and CO2 outgassing into the atmosphere. The degradation process is poorly understood, usually neglected in ocean models. Southeast Asia accounts for around 10% of global tDOC flux, mostly from rivers with peatland areas in their catchment. We developed an equation of tDOC remineralization that depends on sunlight and accounts for photochemical, microbial, and interactive photochemical–microbial degradation. With this, we simulated the fate of tDOC using a 3D computer model system for the Southeast Asian region. Our realistic results for 1958–2022 show: 50% of the tDOC is remineralized before leaving the Sunda Shelf. As a result, the shelf water acidifies and emits more CO2 to the atmosphere. Because of increasing river freshwater runoff (from climate change), more tDOC is transported in time into the sea and more is remineralized, increasing the ocean acidification. This happens mostly in coastal seas close to rivers with much peatland in their catchment. This is harmful for calcifying marine organisms like corals.

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Benthic biogeochemical processes and fluxes in the hypoxic and acidified northern Gulf of Mexico (nGoM), part I: carbonate dissolution from in situ microprofiles

Published 14 April 2025 Science Closed
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Highlights

  • High resolution in situ pH and O₂ microprofiles at the sediment-water interface.
  • Benthic measurements of carbonate system parameters, including pH, DIC, TA, Ca2+.
  • Large decrease in oxygen concentrations in overlying water, accompanied by DIC accumulation and a pH drop.
  • Undersaturation with respect to aragonite and occasionally calcite.
  • No significant dissolution of calcium carbonate minerals detected.

Abstract

The northern Gulf of Mexico (nGoM) experiences seasonal coastal hypoxia due to nutrient enrichment from the Mississippi-Atchafalaya River basin, leading to one of the world’s largest hypoxic zones. In these shallow zones, benthic processes play an essential role in driving/maintaining deoxygenation and acidification of bottom waters. In this regard, this paper investigates carbonate dissolution processes in surface sediment of the nGoM during hypoxic conditions in summer 2022, as the main acidification feedback mechanism, with a specific focus on the effects of bottom water acidification. A strong linear relationship is observed between oxygen and pH, with a pH difference of 0.37 between the most oxygenated and the nearly anoxic station, reaching a value of 7.63. Using high-resolution techniques, this study combines pH and O₂ microprofiling (200 μm) with benthic measurements of carbonate system parameters (pH, DIC, TA, Ca2+) to assess carbonate dissolution at millimeter-scale resolution. The pH microprofiles reveal a significant decrease in the first 3 cm, with pore water pH reaching values of 6.90 at the most hypoxic station. Despite undersaturation with respect to aragonite and occasionally calcite, Ca2+ profiles indicate no significant carbonate dissolution, suggesting stability of calcium carbonate in these sediments during the summer. This lack of dissolution, likely influenced by the absence of aragonite, and possible inhibitory effects of dissolved organic carbon and orthophosphate, points to a limited buffering capacity in these sediments. These insights are essential for refining models predicting coastal acidification and hypoxia responses to environmental stressors in the nGoM and similar eutrophic systems.

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Long-term monitoring of hydrological dynamics and phytoplankton biomass indicator in three shellfish ecosystems of the English Channel (2000-2024)

Published 11 April 2025 Science Closed
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This study investigates the long-term monitoring of physico-chemical parameters and biogeochemical cycles in coastal ecosystems, focusing on three stations in Normandy: Blainville-sur-Mer, Saint-Vaast-la Hougue, and Utah Beach. Over a 24-year period, we analyzed trends in temperature, pH, chlorophyll a concentrations, and nutrient levels, aiming to assess the impacts of climate change and human activities on marine ecosystems. Results show a consistent rise in winter temperatures, particularly since 2013, alongside increasing ocean acidification, especially at Blainville-sur-Mer. These trends suggest potential consequences for planktonic communities and mollusk health. Nutrient analysis revealed significant variations, including high ammonium concentrations on the East coast and a gradual decline in phosphates over the last 15 years, highlighting the influence of anthropogenic activities. The study also identified nutrient limitations, with phosphorus dominating the East coast and nitrogen on the West coast. Our findings emphasize the need for effective nutrient management strategies to mitigate the effects of climate change and human impact, ensuring the sustainability of coastal ecosystems and aquaculture practices. This long-term monitoring is crucial for understanding ecological dynamics and guiding future coastal zone management in the face of global environmental changes.

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Cold seeps and coral reefs in northern Norway: carbon cycling in marine ecosystems with coexisting features

Published 31 March 2025 Science Closed
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Abstract

Cold seeps and cold-water corals (CWCs) coexist on Northern Norway’s continental shelf at the Hola trough between Lofoten and Vesterålen. Here, cold seeps release methane from the seabed, yet none reaches the sea surface. Instead, the methane dissolves and disperses in the ocean where it is ultimately consumed by methane-oxidizing microorganisms. These microorganisms metabolize methane and release carbon dioxide and dissolved organic matter (DOM), which may impact the biogeochemical habitat of CWCs in close vicinity of cold seeps. We investigated the biogeochemistry of carbon, carbon isotopes, nutrients, DOM compositions, and microbial diversity in the water column. Our results indicated that dissolved inorganic carbon concentrations were 29% higher near cold seeps with modified carbon’s isotopic compositions. The hydrophysical parameters and surface-to-bottom control of sinking particles mainly govern water column productivity and nutrient cycle. DOM compositions implied that the seep-associated microbiomes modify DOM’s chemical diversity and isotopic composition at CWCs and the entire water column near cold seeps. Cold seeps and CWCs coexist in Northern Norway’s continental shelves; however, enhanced water temperatures and consequent increase in methane release at cold seeps may modify the carbon cycling in the area, which could mitigate the ecological role and functioning of CWC reefs in the future.

Plain Language Summary

Cold seeps are geological features that release methane from the seabed to the water column. In oxygenated sea water column, seeping methane is consumed by specialized microbes that convert it into carbon dioxide. Although the increase in carbon dioxide can lead to ocean acidification, cold seeps are often found in the Hola trough of Northern Norway near cold-water corals (CWCs), which are vulnerable to changes in ocean acidity. This raises questions about how these features coexist in the same marine ecosystem and how they impact each other. We investigated the carbon exchange between cold seeps and CWCs by analyzing seawater samples. Our data on nutrients, organic matter, and microbial compositions implied cooccurring carbon processes such as methane oxidation and organic matter synthesis. Notably, cold seeps might support CWCs by producing dissolved organic matter that corals feed on. However, this relationship may be valid for a moderate amount of methane release. If methane release increases, consequent acidification may influence the CWC reefs in the future.

Key Points

  • Hola trough of Northern Norway is a natural system offering insights into carbon cycling within coexisting marine ecosystems
  • Despite their proximity, cold seeps and coral reefs reveal distinct biogeochemical characteristics in the Hola trough
  • Cold seeps and methane oxidation impact the inorganic carbon cycle, yet the exchange of organic carbon might be just as significant
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Physical and biogeochemical controls of sea surface pCO2 along the North American Atlantic coastal ocean margins on different time scales

Published 24 March 2025 Science Closed
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Coastal ocean margins play a crucial role in the global carbon cycle, acting as significant carbon sinks and highly productive ecosystems while being particularly vulnerable to climate change and ocean acidification. Despite their importance, there remains a critical knowledge gap in understanding the complex carbon dynamics and acidification processes in these regions, especially in the North American Atlantic Coastal Ocean Margin (NAACOM), which encompasses diverse and economically vital coastal ecosystems. This dissertation aims to investigate the complex physical and biogeochemical processes controlling inorganic carbon chemistry and ocean acidification processes along the NAACOM on different spatial and temporal scales. Through a series of studies, I address critical gaps in understanding the spatial and temporal variability of surface ocean partial pressure of CO2 (pCO2) and air-sea CO2 fluxes in these regions.

I first evaluate the reliability of multiple pCO2 products in the Gulf of Mexico (GoMx). Our analysis reveals significant spatial and seasonal variations in pCO2 due to complex local non-thermal dynamics, particularly in the Louisiana Shelf and Western Florida Shelf. I find that the GoMx is CO2-neutral overall, but with notable spatial differences in pCO2 trends. However, despite the fact that all the reconstructed data products are capable of providing a reasonable estimated annual mean pCO2 climatology for the GoMx, they are unable to simulate the long-term trend of pCO2 within the GoMx.

To address the limitations in existing pCO2 products, I develop a new regional reconstructed pCO2 product (ReCAD-NAACOM-pCO2) for the entire NAACOM. This product uses a novel two-step approach combining random forest and linear regression. Trained on Surface Ocean CO2 Atlas (SOCAT) observations and various environmental variables, ReCAD-NAACOM-pCO2 demonstrates high accuracy and robustness in capturing regional-scale variations, seasonal cycles, and decadal trends of pCO2 from 1993 to 2021.

Utilizing this new product, I investigate the spatial and seasonal variability of pCO2 and air-sea fluxes across the NAACOM. I reveal a south-to-north decreasing gradient in pCO2, primarily driven by the meridional temperature gradient and modulated by regional processes. In southern areas, seasonal thermal cycles dominate pCO2 seasonality, while in northern areas, biological processes play a more significant role. Our analysis identifies the NAACOM as a CO2 sink, with a mean flux of -0.63 ± 0.19 mol C m-2 yr-1 (-10.14 ± 3.00 Tg C yr-1) in the narrow margin (depth < 200m) and -0.60 ± 0.21 mol C m-2 yr-1 (-24.24 ± 8.31 Tg C yr-1) in the wide margin (distance from coast < 400km). This wide-margin CO2 uptake is around 60% lower than previously reported values, emphasizing the importance of accurate regional assessments.

Finally, I explore the impact of Gulf Stream variability on coastal acidification along the U.S. East Coast. Our findings show unexpectedly rapid acidification, with pCO2 increasing faster than atmospheric CO2 in both the Mid and South Atlantic Bights (MAB and SAB) during 2002-2021, accompanied by a rapid decline in the dissolution saturation of pH and calcium carbonate saturation state (Ω). Specifically, during this period, the sea surface pCO2 of the SAB shelf grew 60% faster than the atmosphere pCO2 and 28% faster than the MAB. I attribute this acceleration to an overall extraordinary regional warming background, and additionally to reduced biological carbon removal and decreased buffer capacity on the SAB shelf — all linked to Gulf Stream dynamics. By incorporating Gulf Stream dynamic parameters into a multiple linear regression model and Earth System Models, I provide revised estimates for past and future pCO2 levels in these regions. I show that the rapid pCO2 contrast between SAB and MAB can be traced back to the 1990s and will continue to be larger in the future due to a weakening Gulf Stream.

This dissertation significantly advances our understanding of carbon dynamics in the NAACOM, providing crucial insights for improved coastal carbon cycle modeling. Our findings highlight the complex interplay of physical, biological, and chemical processes in shaping coastal carbon dynamics and acidification patterns. The development of the ReCAD-NAACOM-pCO2 product offers a valuable tool for future research in this area.

Furthermore, our research underscores the urgent need for enhanced ocean acidification monitoring in Gulf Stream-influenced areas and other Western Boundary Current regions. The rapid coastal acidification observed along the U.S. East Coast has important implications for marine ecosystem management and policy-making. As coastal regions face increasing pressures from climate change, our work provides critical information to support informed decision-making and adaptation strategies.

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Groundwater carbon biogeochemistry of coastal confined aquifers and its implications after discharge

Published 28 February 2025 Science Closed
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Highlights

  • Carbonate weathering and methanogenesis affect DIC production in deep groundwater.
  • Deep groundwater discharge serves as a buffer against ocean acidification.
  • Neglecting deep groundwater overestimates the effect of TSGD on ocean acidification.

Abstract

Shallow groundwater discharge from unconfined aquifers is known to affect ocean carbon budget and ocean acidification. However, the role of deeper groundwater discharge from confined aquifers in ocean carbonate chemistry was previously overlooked. Here, we characterized dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations and estimated DIC and TA fluxes from confined aquifers (50 ∼ 520 m depth) in the northern Yangtze River Delta. The results show that deep groundwater generally exhibits high DIC concentrations, primarily influenced by carbonate weathering and methanogenesis. The DIC concentrations in the deep groundwater (50 ∼ 520 m depth) are about 14 % lower than the TA concentrations, indicating that the deep groundwater discharge mitigates ocean acidification. However, the DIC concentrations in shallow groundwater (< 30 m depth) are much higher than the TA concentrations, indicating the strengthening of ocean acidification after discharging to the sea. The mean [TA-DIC] flux from the deep groundwater accounts for approximately 8 % of that from the shallow groundwater, but overlooking the deep groundwater will considerably overestimate the effect of terrestrial groundwater on ocean acidification.

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Drivers and variability of ocean carbonate chemistry near Hawai‘i

Published 27 February 2025 Science Closed
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Ocean carbon uptake, cycling and sequestration are variable on all time scales, and modulated by an interplay of complex physical and biogeochemical drivers, including anthropogenic CO2 increase and associated ocean acidification. This dissertation disentangles the contributions of and feedbacks between these drivers to assess past changes and potential future shifts in biological production, carbon sequestration and the interplay between ocean physics and carbon speciation. The variability of the CO2-carbonate system is investigated from seasonal to interannual time scales at two different locations in the North Pacific that represent crucial environments to study in the face of global change. The first two studies investigate dynamics in the oligotrophic North Pacific Subtropical Gyre near Hawai‘i, which is the largest ecosystem on earth, and a significant sink for anthropogenic carbon from the atmosphere. A seasonal mixed layer carbon budget stresses the importance of the (relatively steady) carbon supply from horizontal transport balanced by the (spring to summer) biological drawdown over a year. Long-term changes over 35 years are then explored in the mixed layer, as well as subsurface water masses, at the same location. A substantial enhancement of ocean acidification is detected in several subsurface layers, driven by multiple combinations of source water changes from atmospheric forcing (with a focus on freshwater forcing), and/or biological produc1vity, as well as increasing ingrowth of respired carbon and alkalinity during subduction. The influence of salinity changes in addition to biological and temperature changes on modulation of ocean acidification is then further examined in a coastal coral reef environment on the shores of O‘ahu, Hawai‘i, where an increase in seawater carbon dioxide has not led to a measurable decline in calcium carbonate saturation state. Here, global temperature and regional salinity changes exert opposing influences on coastal acidification. This is additionally exacerbated by local high respiration, but also buffered by dissolution of calcium carbonate in sediments and/or the water column. Insights from all three chapters shed a light on the complex interplay of physical, geochemical, and biological drivers of marine CO2-carbonate chemistry in these locations, and on the implications for global carbon cycling and the fate of anthropogenic CO2.

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Spatiotemporal variability of seawater carbonate chemistry in diverse coral reef environments in South East Asia and Australia

Published 26 February 2025 Science Closed
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Coral reefs are under threat from global environmental perturbations including ocean warming, acidification, and deoxygenation. The degree to which these perturbations will affect coral reefs is dependent on a range of factors including the local hydrodynamics and biogeochemical processes, both which vary widely across space and time. Consequently, not all coral reefs will be affected equally due to differences in local properties and processes. To develop predictive capacity for how coral reefs will be affected by ocean acidification (OA) it is necessary to first understand the current seawater CO2 chemistry variability and drivers. In this dissertation, autonomous sensors and discrete seawater samples were used to characterize the natural spatial and temporal variability of seawater CO2 chemistry across different reef scales and habitats in Australia and South East Asia (i.e., Heron Island, Great Barrier Reef; Dongsha Atoll and Taiping Island, South China Sea; and Onna-son Reef, Okinawa). In Heron Island, the largest spatial and temporal variability in seawater chemistry was associated with the most shallow, western region of the platform that also had the longest residence time. Interactions between reef geomorphology and timing of the tidal cycle greatly influenced chemical gradients and variability leading to some unexpected trends and patterns. On Dongsha, elevated pH and aragonite saturation state (ΩAr) were observed inside a semi-enclosed lagoon during both day and night. Future projections showed that this environment will not cross, detrimental aragonite thresholds (e.g., ΩAr < 2.92) as frequently as patch reefs and the large-scale lagoon of Dongsha. However, concurrent high water temperature and hypoxia indicated that this environment will not offer respite to taxa sensitive to OA. In Onna-son, the influence of seaweed cultivation on seawater chemistry during spring was compared to times of no cultivation during fall and winter. pH elevation was observed during both spring (+0.13 units) and fall (+0.10 units), but it was not possible to separate the role of cultivated seaweed from natural taxa. This dissertation demonstrates current coral reef seawater CO2 chemistry conditions and biochemical function, information that will be critical for making rigorous projections for the future.

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Climate-driven shifts in Southern Ocean primary producers and biogeochemistry in CMIP6 models

Published 26 February 2025 Science Closed
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As a net source of nutrients fuelling global primary production, changes in Southern Ocean productivity are expected to influence biological carbon storage across the global ocean. Following a high-emission, low-mitigation pathway (SSP5-8.5), we show that primary productivity in the Antarctic zone of the Southern Ocean is predicted to increase by up to 30 % over the 21st century. The ecophysiological response of marine phytoplankton experiencing climate change will be a key determinant in understanding the impact of Southern Ocean productivity shifts on the carbon cycle. Yet, phytoplankton ecophysiology is poorly represented in Coupled Model Intercomparison Project phase 6 (CMIP6) climate models, leading to substantial uncertainty in the representation of its role in carbon sequestration. Here we synthesise the existing spatial and temporal projections of Southern Ocean productivity from CMIP6 models, separated by phytoplankton functional type, and identify key processes where greater observational data coverage can help to improve future model performance. We find substantial variability between models in projections of light concentration (>15 000 (µE m−2 s−1)2) across much of the iron- and light-limited Antarctic zone. Projections of iron and light limitation of phytoplankton vary by up to 10 % across latitudinal zones, while the greatest increases in productivity occurs close to the coast. Temperature, pH and nutrients are less spatially variable – projections for 2090–2100 under SSP5-8.5 show zonally averaged changes of +1.6 °C and −0.45 pH units and Si* ([Si(OH)4]–[NO3–]) decreases by 8.5 µmol L−1. Diatoms and picophytoplankton and/or miscellaneous phytoplankton are equally responsible for driving productivity increases across the subantarctic and transitional zones, but picophytoplankton and miscellaneous phytoplankton increase at a greater rate than diatoms in the Antarctic zone. Despite the variability in productivity with different phytoplankton types, we show that the most complex models disagree on the ecological mechanisms behind these productivity changes. We propose that a sampling approach targeting the regions with the greatest rates of climate-driven change in ocean biogeochemistry and community assemblages would help to resolve the empirical principles underlying the phytoplankton community structure in the Southern Ocean.

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Carbonate and nutrient dynamics in a Mississippi River influenced eutrophic estuary

Published 25 February 2025 Science Closed
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There is limited information on how the nutrient and freshwater input affects water column carbonate chemistry in the estuaries along the northern Gulf of Mexico. In this study, we assess the seasonal and spatial variability in carbonate chemistry in the Barataria Basin, a eutrophic estuary adjacent to the mouth of the Mississippi River. Eleven stations were sampled along a salinity gradient during the winter (January), spring (April), summer (July), and fall (October) of 2021. Surface and bottom water samples were collected for the analyses of dissolved inorganic carbon (DIC); total alkalinity (TA); and nitrite plus nitrate (NO2 + NO3), phosphate (PO4), and dissolved silica (SiO4). Dissolved CO2 (pCO2) was measured in the surface water. Seasonal surface DIC and TA values ranged from 1553 to 2582 μmol kg−1 and 1217 to 2217 μmol kg−1, respectively. DIC and TA varied seasonally and showed an increasing trend from fresh stations to saline stations. The highest DIC and TA values were observed during the fall season, likely due to the increased contribution of DIC and TA from adjacent marshes as a result of enhanced porewater exchange. In contrast to DIC and TA, pCO2 decreased with the increase of salinity. The seasonal and spatial patterns in carbonate chemistry could not be explained solely by physical mixing and reflected complex interactions between biogeochemical processes driven by nutrient supply and temperature as well as tidal flushing and material exchanges with adjacent marshes.

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Effect of thermal and non-thermal processes on the variability of ocean surface pCO2 and buffering capacity in the North Indian Ocean

Published 24 February 2025 Science Closed
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Highlights

  • A coupled atmosphere–ocean-biogeochemistry model is customised for NIO.
  • High surface pCO2 in upwelling regions of NIO are controlled by non-thermal processes.
  • Surface pCO2 changes in upwelling regions of NIO are more sensitive to changes in DIC.
  • Diffusion, CO2 flux and Phytoplankton uptake primarily control DIC variability in NIO.

Abstract

The oceans have absorbed nearly 30% of the anthropogenic CO2 that alters the ocean carbon chemistry. The oceanic processes are highly complex, which mandate approaches that couple its physical, chemical and biological states. Here, we use a coupled atmosphere–ocean-biogeochemistry model, incorporating spatially and temporally varying atmospheric CO2 to simulate the north Indian Ocean (NIO) carbon dynamics for the period 2013–2020. We assess the seasonal variability of Dissolved Inorganic Carbon (DIC), total Alkalinity (ALK), ocean surface pCO2 and buffering capacity. To assess the mechanisms that control carbon dynamics in the region, we segregate the ocean surface pCO2 into temperature-driven (thermal) and bio-physical processes induced (non-thermal) pCO2. We find that the thermally driven pCO2 is dominant in summer (June, July, August and September; JJAS), but the non-thermal component in winter (December, January and February; DJF) in the northern Arabian Sea (AS). The northern AS is characterised by a deep mixed layer and convection-induced vertical mixing during winter. DIC from the subsurface layer is uplifted to the surface, which results in high ocean surface pCO2 in winter. Off the Oman coast, the non-thermal processes control the surface pCO2 in summer. In the northern bay, the thermal component of pCO2 is dominant in summer and non-thermal component is prominent in winter as in northern AS, but their magnitudes are lower due to large riverine flux. The budget analysis reveals strong influence of diffusion, CO2 flux and biological processes in controlling DIC variability in NIO. Low buffering capacity in upwelling regions indicates that pCO2 changes are more sensitive to changes in DIC, primarily due to the upwelled DIC-rich surface waters. Therefore, it results in a reduced ability to absorb CO2. This warrants the need to address recent changes in carbon dynamics in response to the increased levels of atmospheric CO2.

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Carbon dioxide–induced acidification enhances short-lived brominated hydrocarbons production in oligotrophic oceans

Published 18 February 2025 Science Closed
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Oceanic emission is a primary source of brominated very short-lived substances (BrVSLs) to the atmosphere, which have important effects on stratospheric ozone chemistry. Marine biogeochemical processes regulating BrVSLs are often sensitive to ocean acidification. Yet, the response of BrVSLs production to acidification remains poorly understood. Herein, the effects of acidification on the production of two main BrVSLs, dibromomethane (CH2Br2) and tribromomethane (CHBr3), were studied by ship-based incubation experiments at three stations in the South Atlantic and Indian Oceans. The average CH2Br2 and CHBr3 concentrations increased by 17.2–58.7% and 14.3–80.3% due to acidification under the in situ nutrient conditions with nutrient and/or iron limitation at the three stations, but the mechanisms driving these increases varied among different regions. The increased bromoperoxidase (BrPO) activity caused by acidification facilitated BrVSLs release in the Eastern Tropical Indian Ocean, where diatoms were dominant. CHBr3 increased due to acidification as a result of enhanced reactivity of dissolved organic matter (DOM) in the Eastern Tropical Atlantic, where dinoflagellates were dominant. Brominated very short-lived substances increased due to acidification as a result of a combined effect of the above two mechanisms in the Benguela Current Coastal with high phytoplankton abundance. Under the nutrient and/or iron addition conditions with nutrient and iron sufficiency, however, acidification did not promote BrVSLs production due to its only minor effect on the BrPO activity and reactivity of DOM, partly because the effect of increased oxidative stress was offset by that of changed phytoplankton composition. Our study provided a basis for future modeling on the impact of acidification on global BrVSLs emissions.

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Long-term biogeochemical variations in the southern South China Sea and adjacent seas: A model data analysis

Published 17 February 2025 Science Closed
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Highlights

  • Sea surface temperature in the southern South China Sea is rising by 0.2°C per decade, highlighting climate change’s impact on the region.
  • Significant declines in Dissolved Oxygen and pH levels highlight broader impacts on ocean health in the southern SCS.
  • SST anomalies strongly influence biogeochemical changes, highlighting biogeochemistry’s sensitivity to climate variability.

Abstract

The southern portion of the South China Sea (SCS), which constitutes the largest semi-enclosed marginal sea in Southeast Asia, is currently undergoing unusual surface warming, which may significantly impact its marine ecosystems. This study examines the long-term variations of biogeochemistry in the southern SCS and its adjacent seas from 1993 to 2020, using data from the Copernicus Marine Environment Monitoring Service (CMEMS) model to better understand the implications of these warming trends on nutrient dynamics and overall marine health. Analysis of interannual variations highlights a strong relationship between sea surface temperature (SST) anomalies and the occurrence of El Niño-Southern Oscillation and Indian Ocean Dipole events, leading to significant alterations in biogeochemistry in the southern SCS. These findings emphasize the sensitivity of biogeochemistry to climatic variability. Over the study period, a region-wide increasing SST trend (0.2 °C per decade) indicates the influence of climate change, while surface Chlorophyll concentration and Phytoplankton biomass show decreasing trends, suggesting potential shifts in primary productivity. Additionally, surface Dissolved Oxygen and pH levels have significantly declined, pointing to broader impacts on ocean health. These observed declining trends in biogeochemical variables are particularly pronounced in the southeastern Vietnam, East Coast of Peninsular Malaysia, and Java Sea regions, which are primarily driven by significant changes in SST and related wind patterns. The weakening of summer southwesterly winds and winter northeasterly winds, evidenced by linear trends in wind stress, has reduced nutrient supply through upwelling and decreased ocean heat loss, respectively. This disruption exacerbates surface ocean warming and limits the replenishment of essential nutrients, leading to diminished primary productivity and altering marine ecosystem dynamics. This study provides valuable insights into present biogeochemical trends under climate change, forming a crucial basis for future projection studies in this region.

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Long-term variations in pH in coastal waters along the Korean Peninsula

Published 12 February 2025 Science Closed
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Declining seawater pH, associated with rising atmospheric CO2 levels, adversely affects marine organisms and ecosystems, thereby posing a considerable risk to coastal fisheries and economies. However, the effects of long-term pH variations in coastal waters remain poorly understood. In this study, we investigated the variability in pH in the coastal waters of Korea over an 11-year period (2010–2020) and sought to identify the principal drivers of pH fluctuations. Unlike the persistent pH declines observed in the open oceans and other coastal systems, Korean coastal waters showed no persistent pH variation, thus indicating that local biogeochemical processes may have a greater influence than atmospheric CO2 in determining aquatic pH. Analysis of environmental data including temperature, salinity, chlorophyll a, and dissolved oxygen (DO) revealed a strong correlation between pH and DO. However, instances of pH changes exceeding those predicted by DO depletion alone indicate the influence of additional biogeochemical factors. As global seawaters warm, reduction in DO is anticipated to cause a further decline in the pH of coastal waters. This trend could have a pronounced influence on Korean coastal waters, which support extensive aquaculture operations integral to the local and national economies. Consequently, high-frequency monitoring is essential for extending the current time series and predicting future water quality.

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The effect of carbonate mineral additions on biogeochemical conditions in surface sediments and benthic–pelagic exchange fluxes (update)

Published 11 February 2025 Science Closed
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Coastal sediments are hotspots of biogeochemical processes that are impacting subsurface and overlying water conditions. Fluid composition in sediments is altered through the mineralization of organic matter which, under oxic conditions, further lowers both pH and the carbonate saturation state. As a potential mitigation strategy for this sediment acidification, we explored the effects of mineral additions to coastal sediments. We experimentally quantified carbonate mineral dissolution kinetics of carbonate shells suitable for field application and then integrated these data into a reactive transport model that represents early diagenetic cycling of C, O, N, S, and Fe and traces total alkalinity, pH, and saturation state of CaCO3. Model simulations were carried out to delineate the impact of mineral type and amount added, porewater mixing, and organic matter mineralization rates on sediment alkalinity and its flux to the overlying water. Model results showed that the added minerals undergo initial rapid dissolution and generate saturated conditions demonstrating the potential of alkalinity enhancement in mitigating surface sediment acidification. Aragonite dissolution led to higher total alkalinity concentrations than calcite. Simulations of carbonate mineral additions to sediment environments with low rates of organic matter mineralization exhibited a substantial increase in mineral saturation state compared to sediments with high CO2 production rates, highlighting the environment-specific extent of the effect of mineral addition. Our work indicates that carbonate additions have the potential to effectively buffer surficial sediments over multiple years, yielding biogeochemical conditions that counteract the detrimental effect of low-pH sediment conditions on larval recruitment and potentially increase benthic alkalinity fluxes to support marine carbon dioxide removal (mCDR) in the overlying water.

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Innovations in ocean biogeochemical instrumentation and monitoring

Published 11 February 2025 Science Closed
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Anthropogenic activities are driving changes in ocean biogeochemistry, which can be monitored through instruments and sensors deployed across diverse platforms in even the harshest marine environments. Continued monitoring of these changes demands innovations in instrumentation, calibration and quality control to effectively capture dynamic signals and ensure comprehensive ocean coverage. This dissertation focuses on advancements in oceanographic pH sensors, starting with the longest near-continuous ocean pH dataset collected using ionsensitive field effect transistor (ISFET) technology at Scripps Pier. A new in situ calibration approach, based on direct tris buffer injection, was compared to the traditional bottle collection method, yielding a fourfold improvement in repeatability with an uncertainty of 0.006 pH. Additionally, an automated calibration system integrated into the sensor package was evaluated, offering near real-time, self-calibrating capability for ocean acidification and biogeochemical monitoring programs. To continue the discourse of pH sensor technology in the second section of this dissertation, a novel optical pH sensor was evaluated in laboratory settings to establish its accuracy and precision, response time, temperature and pressure sensitivity, and calibration techniques which improved accuracy over factory methods. Field tests of the optical pH sensor across diverse marine environments—deep ocean, dynamic nearshore, and open ocean profiling—provided guidelines for field calibration, correction and optimal field use. In a scaled-up sense, the final section of this dissertation leveraged pH and other biogeochemical sensors on BGC-Argo profiling floats to explore biogeochemical variability in the equatorial Pacific from 2019 to 2024. While the region has extensive physical data, subsurface biogeochemical observations and their links to El Niño and La Niña cycles are sparse. These floats revealed distinct biogeochemical patterns driven by vertical movement of the mixed layer depth, meridional subtropical water transport and primary production shifts associated with ENSO phases. Overall, this work combines new sensor technologies and analytical methods to provide essential data, instrument guidelines and reveal insights into ocean biogeochemical phenomena. Ongoing instrumentation development and monitoring will be critical to expand and deepen our understanding of how human-driven impacts are transforming our oceans.

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