A global coordination and continuous synthesis of interoperable data related to biogeochemical Essential Ocean Variables (EOVs) is critically needed to enhance the creation of information products and services to sustainably manage the climate system and ocean health. Among the existing biogeochemical EOVs, data synthesis products—which demonstrate the immense value of data coordination—already exist for carbon-relevant data (e.g. SOCAT, Global Ocean Data Analysis Project), and for methane and nitrous oxide (MEMENTO). The roadmap for building a Global Ocean Oxygen Database and ATlas (GO _2 DAT) (Grégoire et al (2021 Front. Mar. Sci. 1638 )) provides the theoretical basis to increase the interoperability of ocean oxygen data sets, without creating yet another separate repository. The goal is now to advance from the idea of GO _2 DAT to its implementation, building a sustainable, interoperable, and inclusive digital ecosystem for all stakeholders who may use ocean oxygen data. Successful implementation will require (I) the provision of guidance on data acquisition/ocean oxygen measurements, (II) recommended practices for ocean oxygen data management, including metadata requirements, uncertainty and data quality control attribution, (III) development of the ocean oxygen data platform including data flow and application of the recommended practices introduced in I and II, as well as its deep integration with cross-domain data federations such as the Ocean Data and Information System. This document provides an outline of GO _2 DAT’s objective and progress since 2021 and contributes to addressing these three requirements, synthesizing a series of global consultations on recommended practices for marine dissolved oxygen measurements, a working definition of ocean oxygen metadata, proposed data quality control levels and flags, a described novel mechanism for uncertainty attribution to allow the determination of data suitability for different scientific applications, and it concludes with an illustration of the data flow for implementation.

Coastal seas are especially vulnerable for eutrophication and deoxygenation. These processes can lead to major changes in oxygen concentrations within both coastal bottom waters and surface sediments. The dynamics of the greenhouse gas nitrous oxide (N2O) are strongly coupled to oxygen dynamics, and changes in oxygenation can thus lead to changes in N2O production and consumption patterns. Deoxygenation therefore has the potential to affect the exchange of N2O between surface sediments and bottom waters, which in turn might affect the water column budget of N2O and eventually its exchange across the sea-air interface. Here, we present depth profiles of oxygen and N2O as determined with microsensors within coastal marine surface sediments from Lake Grevelingen (The Netherlands) in March 2023. Lake Grevelingen is a eutrophic coastal system suffering from seasonal bottom water anoxia and euxinia. We present results for several field locations and results of laboratory experiments in which oxygen concentrations were manipulated. On-board measurements revealed high concentrations of N2O in the top sediments (~1 mm), where oxygen was also present. We find that increased depths of oxygen penetration in the sediment lead to higher penetration depths of N2O. We discuss the potential drivers of N2O fluxes between the surface sediments and bottom waters under different oxygenation conditions.

Coastal margins play a crucial role for greenhouse gases (GHG) budgets because biological cycling and sediment-water-air exchanges are more intense than in the open ocean. Given the tight connection between marine GHG cycling (production, consumption, fluxes) and dissolved oxygen dynamics, ongoing deoxygenation is of particular concern in coastal systems that experience seasonal or perennial hypoxia, and could therefore have a large impact on regional GHG budgets. N2O stands out among other long-lived GHG because of its role as ozone-depleting compound and its effectiveness in enhancing Earth’s warming. Based on the vast majority of studies, marine coastal margins are expected to be hotspots of N2O emissions to the atmosphere, with nitrification and partial denitrification as the main sources. However, despite significant advances in constraining the marine budget of N2O over the last decade, the magnitude and seasonal variability of coastal emissions are still highly uncertain. While N2O depletion in marine settings is usually associated to complete denitrification, recent evidence indicates the possibility of consumption at oxic-hypoxic interfaces or under fully oxic conditions. Moreover, a mechanistic understanding on the benthic-pelagic coupling of N2O fluxes and its potential changes with deoxygenation is still rudimentary. To amend this deficit, we conducted a comprehensive study in Lake Grevelingen (Netherlands), a marine coastal reservoir characterized by seasonally hypoxic/anoxic conditions resulting from limited water exchange with the North Sea and eutrophication. Our study combined biogeochemical and microbial analyses and comprised multiyear (2020–2023) shipboard observations. We observed that unlike most coastal systems, surface waters of Lake Grevelingen are a rather weak source of atmospheric N2O, with annual sea-air fluxes that represent

Manganese (Mn) is an essential micronutrient and key redox intermediate in marine systems. The role of organically complexed dissolved Mn(III) (dMn(III)-L) as an electron acceptor and donor in marine environments is still incompletely understood. Here, we use geochemical depth profiles of solutes and solids for the sediment and overlying waters and a reactive transport model to reconstruct the seasonality in sedimentary dMn(III)-L dynamics and benthic Mn release in a eutrophic, seasonally euxinic coastal basin (Lake Grevelingen, the Netherlands). Our model results suggest that dMn(III)-L is a major component of the dissolved Mn pool throughout the year. According to the model, there are three major sources of pore water dMn(III)-L when oxygen (O2) is present in the bottom water, namely reduction of Mn oxides coupled to the oxidation of Fe(II), reduction of Mn oxides coupled to organic matter degradation, and oxidation of Mn(II) with O2. Removal of pore water dMn(III)-L is inferred to primarily take place through reduction by dissolved Fe(II). When bottom waters are euxinic in summer, model-calculated rates of sedimentary Mn cycling decrease strongly because of a lower supply of Mn oxides. The dMn(III)-L transformations in summer mostly involve reactions with Fe(II) and organic matter. Modeled benthic release of Mn mainly occurs as dMn(III)-L when bottom waters are oxic, as Mn(II) upon initial bottom-water euxinia and as both Mn(II) and dMn(III)-L when the euxinia becomes persistent. Our model findings highlight strong interactions between the sedimentary Fe and Mn cycles. Dissolved Mn(III)-L is a relatively stable and mobile Mn species, compared to Mn(II), and is therefore more easily transported laterally throughout the coastal zone and possibly also to open marine waters.

Iron (Fe), manganese (Mn), and ammonium (NH4+) removal from groundwater using rapid sand filtration is a widely employed method in drinking water production. Over time, Fe and Mn oxides accumulate in the filter, which necessitates frequent backwashing to avoid clogging. In this study, we investigated the impact of backwashing on the microbial community and filter chemistry in a dual-media filter comprising anthracite and sand layers. Specifically, we focused on the removal of Fe, Mn, and NH4+ over the runtime of the filter. With increasing runtime, depth profiles of dissolved and particulate Fe revealed the buildup of Fe oxide flocs, causing Fe2+ and Mn2+ oxidation and nitrification to occur at greater depths within the filter. Towards the end of the filter runtime, breakthrough of suspended Fe oxides was observed, likely due to preferential flow. Backwashing effectively removed metal oxide flocs and restored the Fe removal efficiency in the top layer of the filter. While the two layers remained separate, the anthracite and sand layers themselves fully mixed during backwashing, leading to a homogenous distribution of the microbial community within each layer. Methyloglobulus and Gallionella were the predominant organisms in the anthracite layer, likely catalyzing methane and Fe2+ oxidation, respectively. The nitrifying community of the anthracite consisted of Nitrosomonas, Candidatus Nitrotoga, and Nitrospira. In contrast, the nitrifying community in the sand layer was dominated by Nitrospira. Backwashing minimally affected the microbial community composition of the filter medium except for Gallionella, which were preferentially washed out. In conclusion, our research offers a molecular and geochemical basis for understanding how backwashing influences the performance of rapid sand filters.

Oxygenic photosynthesis in the ocean of the early Proterozoic may have been limited by the nutrient phosphorus. If so, precession-driven variations in riverine phosphorus input may have enhanced oxygenic photosynthesis and thereby contributed to the rise of atmospheric oxygen. Here, we combine geochemical analyses of 2.46-billion-year-old deposits of the Joffre Member of the Brockman Iron Formation (Australia) and results of a reactive transport model to reconstruct pathways of organic matter degradation and phosphorus cycling in oceanic sediments over a precession cycle. Our results support a conceptual model in which increased phosphorus availability during precession maxima at southern paleolatitudes drove net oxygen production by inducing increased reductant burial in the sediment (mainly as pyrite, vivianite and magnetite). During precession minima, legacy benthic release of methane may have enhanced photolysis of atmospheric methane, thereby allowing for additional net oxygen production. Hence, precession-driven variations in coupled carbon-phosphorus-oxygen cycling may have acted as an accelerator towards the Great Oxidation Event.

Global warming is the main cause for current ocean deoxygenation. Stopping further warming requires net-zero emissions. Achieving this will, in the foreseeable future, not be possible without the implementation of a portfolio of carbon dioxide removal (CDR) approaches. Given substantial uncertainties about the potential and durability of land-based approaches to deliver sufficient CDR, marine CDR options are receiving more and more interest. A deployment of marine CDR for mitigating global warming could therefore also be viewed as a measure for mitigating ocean deoxygenation if, and only if, the respective CDR measure itself does not lead to a larger oxygen loss than the reduction in atmospheric CO2, and hence warming, would have caused.We present the current state of knowledge regarding the potential impacts of various marine CDR options onto ocean oxygen, a key ocean state variable and an essential element for all higher forms of marine life. We show that particularly biotic approaches, such as ocean fertilization, macroalgae cultivation and organic matter dumping, can have significant impacts on ocean oxygen, whereas geochemical approaches may be applied in ways that could have only small additional impacts on dissolved oxygen beyond global warming. We suggest that marine oxygen should be considered in environmental impact assessments of marine CDR field experiments, that the change in oxygen levels should be accounted for in assessing the suitability of mCDR, and that oxygen should be measured prior to, during and after any potential deployment.

Marine hypoxia (low or depleted oxygen in a water body) is a transboundary issue - it is not confined to a single country or a single region, but is a global concern with global impacts. Consequently, a programmatic approach that supports the identification and implementation of collaborative solutions at global, regional, and national levels is considered the most effective way to address the problem over the long-term. In recognition of this, the Clean and Healthy Ocean Integrated Program (CHO-IP) has been established by the Global Environment Facility (GEF) to address marine hypoxia through science-based action.The CHO-IP offers an opportunity to implement the science-policy-society value chain approach, with a Global Coordination Project led by FAO, together with three multilateral development banks (ADB, CAF, EBRD) in partnership with IOC-UNESCO and GWP, as well as 14 Country Projects spanning Latin America, Africa, Asia and the Middle East spanning 9 Large Marine Ecosystems. The overall goal of the program is to curb coastal hypoxia via the reduction of coastal pollution from agriculture, industrial, and municipal sources through policy, regulatory measures, infrastructure investments and nature-based solutions. This is the first time a GEF program focuses on the relationship between hypoxia and nutrient pollution. The Global Ocean Oxygen Network, an IOC-UNESCO working group, will be key to providing the scientific data, advice and connections required to achieve this goal and address challenges hampering action at this point, such as: - a lack of comprehensive data on the causes and impacts of, and solutions to, marine hypoxia;- policies and investments that do not sufficiently address nutrient pollution; - limited access to innovative solutions, best management practices, and financing, restricting the adoption of cost-effective measures; - fragmented and unconnected global efforts to tackle marine hypoxia fail to achieve significant transformative impacts.We will present updated best practices for ocean oxygen measurement, data quality assessment and management, tailored and available for both scientific and practical applications. These practices are crucial for developing new methods to enhance the establishment, improvement, and sustainability of systems for monitoring nutrient pollution, oxygen levels, related data collection and reporting systems. We will also demonstrate how this links to the Global Ocean Oxygen Database and Atlas, which is expected to improve the interoperability of data produced via the CHO-IP and established databases worldwide.

Increased anthropogenic activities are affecting water quality, e.g. leading to eutrophication and deoxygenation, culminating in biodiversity loss in coastal ecosystems globally. In the Southwest Delta in the Netherlands, large scale engineering to protect coastal areas against storm surges has turned several tidal inlets and estuaries into coastal lagoons and (marine) lakes. The water quality in these ecosystems has strongly deteriorated as a result of stagnation of bottom waters in combination with eutrophication. One such ecosystem, Lake Veere, showed signs of recovery after restoration of water exchange with the adjacent tidal marine Eastern Scheldt in 2004. In recent years, regular water monitoring has revealed the return of low-oxygen conditions, however, along with other signs of worsening water quality such as fish kills and jellyfish blooms. Here, we assess the role of the sediments in the (re)occurrence of low-oxygen conditions in Lake Veere. During two sampling campaigns in 2022, water column and sediment samples were collected. Geochemical analysis, including direct in-situ flux measurements with a benthic lander, revealed an increasing sedimentary oxygen demand (SOD) from the western (sea-side) part of the lake to the east, from ~10 to >100 mmol O2 m-2 d-1. This gradient in SOD opposes the observed trend in water column deoxygenation, with low-oxygen conditions predominantly prevailing in the central and western part of the lake and not in the east. This indicates that, despite restoration efforts, large parts of the lake are still highly sensitive to deoxygenation. Sediment analyses show the near-absence of iron-oxides, hence little capacity to buffer toxic hydrogen sulfide, which indeed accumulated in pore waters, reaching concentrations of up to 10 mmol L-1. In the central part of the lake, hydrogen sulfide even accumulated in the bottom waters, pointing towards its potential involvement in the observed fish kills in the region. Our results illustrate the difficulty of improving water quality through changes in water exchange alone because of strong legacy effects of eutrophication and deoxygenation in the sediment.

Reoxygenation approaches have shown some success in lakes, but their potential risks must be examined carefully before they’re implemented as solutions to improve the health of coastal waters.

Chemical weathering of silicate rocks redistributes major, minor and trace elements through coupled dissolution–precipitation reactions. These weathering processes drive shifts in ocean acid–base chemistry, modulating atmospheric carbon dioxide levels and providing a stabilizing feedback in the carbon cycle. Silicate weathering occurs in both terrestrial and marine environments, releasing (‘forward’) or consuming alkalinity (‘reverse’), but these have largely been perceived as independent and studied in isolation. However, weathering products are transported downstream across terrestrial and to marine environments, suggesting a dynamic coupling of these weathering processes across scales. Here we propose that the Earth’s silicate weathering occurs along a continuum linking mountains to the deepest sedimentary environments and forward to reverse weathering. In this framework, the magnitude and direction of a local weathering flux depends on the materials’ origin, weathering–erosion history and environmental conditions. Consequently, global silicate weathering fluxes and the long-term carbon cycle feedback may be governed by the dynamic interplay of various environments along the silicate weathering continuum. Chemical weathering of silicate rocks occurs along a continuum from terrestrial to marine environments.

Anthropogenic activities are key drivers of eutrophication and deoxygenation in coastal marine ecosystems. This stimulates the anaerobic degradation of organic matter and the release of reduced products, such as ammonium, methane, and hydrogen sulfide, which may, in turn, exacerbate eutrophication and deoxygenation. In this study, using a combination of chemical and microbial analyses, we assess the nitrogen dynamics in the water column of a eutrophic coastal system (Stockholm Archipelago) at three sites with contrasting redox conditions (oxic to long-term euxinic). At the oxic site, counter gradients of ammonium and oxygen in the water column, low nitrate δ15N values in bottom waters, and the 16S rRNA gene-based presence of nitrifiers indicate nitrification near the sediment-water interface. At the seasonally and long-term euxinic sites, nitrification, as inferred from the water column oxygen and nutrient profiles and the relative abundance of nitrifiers, primarily occurred near the oxycline. At these two sites, nitrate was removed below the oxycline through denitrification linked to sulfide oxidation by Sulfurimonas . Nitrous oxide emissions from surface waters in the archipelago reached up to 40 µmol m-2 d-1 and were not directly related to water column redox conditions, indicating that multiple factors control coastal emissions of this greenhouse gas to the atmosphere. The relative abundance of 16S rRNA genes and of N-cycle genes in metagenomes was highest at the seasonally euxinic site. Importantly, nitrifiers were significantly less abundant at the long-term euxinic site. Our results highlight that prolonged euxinia promotes recycling of ammonium over its removal, likely due to sulfide inhibition of nitrification, which sustains eutrophication and deoxygenation of coastal systems. ### Competing Interest Statement The authors have declared no competing interest.

Coastal ecosystems serve as vital connectors between land and ocean, and their nitrogen cycle and ammonium removal can be affected by various factors. During our seasonal sampling campaign in 2021, the eutrophic marine Lake Grevelingen exhibited high ammonium concentrations and low nitrification rates in the water column, except for a brief period in late summer. Our study revealed ammonium accumulation in the anoxic middle and bottom water layers due to restricted transport caused by water column stratification. Only when ammonium reached the oxic part of the water column, was a short-lived peak in nitrification activity and nitrifier abundance observed at the end of August. Amplicon sequencing indicated very low abundances of Nitrosococcus and Nitrospira (<0.2%) in March, while Nitrosomonas and Nitrospina peaked at the end of August with relative abundances of 2.5% and 1.3%, respectively. Archaea and archaeal ammonium oxidizers were found in very low abundances. Anammox 16S rRNA genes were not detected. Together, these observations suggest a limited role for nitrification in ammonium removal in marine Lake Grevelingen. ### Competing Interest Statement The authors have declared no competing interest.

In eutrophic coastal waters, aerobic methane-oxidizing bacteria (MOB) mitigate methane emissions by oxidizing benthic methane even in the stratified, anoxic water column. However, ongoing warming and eutrophication lead to extended stratification periods, enhancing anoxic and sulfidic conditions (euxinia), potentially affecting methane removal capacity. Here we compared overall water column methane removal between sites with irregular, seasonal and longer-term euxinia in the Stockholm Archipelago during summer 2022. The highest water-air methane emissions, bottom water-methane and sulfide accumulation, and the lowest methane oxidation potential were observed under longer-term euxinic bottom water conditions. While MOB relative abundance and potential activity indicated high functioning of the methane biofilter in the seasonally euxinic bottom water layer, the methane-filtering potential was much lower in the longer-term euxinic bottom water. Under persistent euxinic conditions, overall bacterial diversity and microbial network connectivity were lower, likely following a simultaneous shift in redox conditions and a shift toward anaerobic sulfur-cycling. This shift may force MOB to retreat from the euxinic bottom water into the narrow oxycline, reducing the capacity of the methane biofilter and resulting in higher methane emissions. These findings highlight the positive feedback loop that can further amplify oceanic methane emissions, particularly from eutrophic and shallow coastal waters prone to prolonged stratification under global warming.

Coastal waters worldwide are increasingly affected by oxygen loss due to human-induced eutrophication and global warming. This coastal deoxygenation has dramatically altered biogeochemical processes with major consequences for marine life. Prominent examples of large anthropogenic coastal “dead zones” include the Gulf of Mexico, Baltic Sea and Chesapeake Bay but numerous small coastal systems are also strongly affected. Many efforts are currently underway to restore the water quality of these coastal waters, but these are not always effective. In this presentation, I will discuss how the interplay of biogeochemical processes and hydrodynamics may affect present-day restoration efforts in coastal systems. Using examples from a range of field and modelling studies performed by my group, I will specifically discuss legacy effects resulting from accumulation of organic-rich sediments, the potential for reoxygenation of coastal waters through increased water column mixing and/or lateral water exchange and the expected short-term and long-term effects of nutrient load reductions. Taken together, our results highlight that there is no one-size-fits-all approach to rapidly improve water quality in coastal waters suffering from eutrophication and deoxygenation.

Coastal ecosystems are susceptible to eutrophication and deoxygenation, which may alter their nitrogen cycle dynamics. Here, we investigated the microbial nitrogen cycling potential in the sediment of a seasonally euxinic coastal ecosystem (Lake Grevelingen, NL) in winter and summer. Activity tests revealed ammonium (NH4 +) oxidation potential with maximum potential rates up to 53 μmol g-1 day-1, even in anoxic sediment layers. A nitrifying microbial community was present in both oxic and anoxic sediment sections (up to 1.4% relative abundance). Nitrate (NO3 -), nitrite (NO2 -), and nitrous oxide (N2O) reduction potential were prominent across all sediment sections, with the highest potential rates (167 μmol NO3 -∙g-1 day-1) in the surface sediment in summer. Denitrification (79.3%-98.4%) and dissimilatory nitrate reduction to ammonium (DNRA; 1.6%-20.7%) were the major NO3 - removal pathways, as supported by the detection of the narG/napA, nirK/nirS, norB, nosZ and nrfA/otr genes in all sediment sections. The DNRA contribution increased with depth and with the addition of electron donors, such as monomethylamine. Anaerobic ammonium oxidation (anammox) was not detected in these eutrophic sediments. Combined, our results show that there is high potential for nitrogen removal in eutrophic coastal ecosystems, which may help further restoration measures.

Sedimentary concentrations of redox-sensitive trace metals are widely used to reconstruct past ocean redox conditions. Vanadium (V) has great potential as a (paleo)redox proxy, due to its strong redox-dependent speciation (+III, +IV, +V) and the increased sedimentary sequestration of its more reduced species. The geochemistry of V in sulfide-rich marine environments is not yet well understood, however, hampering the use of V as a (paleo)redox proxy. Here, we present V data for two coastal systems, with bottom water redox conditions ranging from oxic to euxinic, to further constrain V geochemistry. Our sedimentary record from a eutrophic coastal marine basin (Scharendijke basin, Lake Grevelingen, the Netherlands), covering the last decade, shows distinct enrichments in molybdenum (Mo) and organic carbon (C-org) but depletions in V during seasonal bottom water euxinia, which can be discerned due to the exceptionally high sedimentation rate at our study site (up to 20 cm yr(-1)). A seasonal study for the same coastal basin confirms this trend and reveals the accumulation of V, iron (Fe) and manganese (Mn) in the water column during summer euxinia. We conclude that the slow kinetics of V reduction to V(III) and subsequent precipitation as (oxy)hydroxide V(OH)(3(s)) likely provide the opportunity for V to escape sedimentary sequestration during summer euxinia, resulting in the observed sedimentary V depletion. Sediments from three sites with contrasting bottom water redox conditions (oxic, seasonally hypoxic, euxinic) in the eutrophic Stockholm Archipelago, show a similar trend as that of Lake Grevelingen, with decreasing V concentrations and increasing Mo and C-org concentrations as bottom water conditions become more reducing. This confirms that our findings for Lake Grevelingen are not site-specific and are likely a generic feature of euxinic coastal systems with high sulfide concentrations (> 0.5 mmol L-1) near the sediment surface and high rates of anaerobic degradation of organic matter. Our results show that co-occurring sedimentary Mo and C-org enrichments and V depletion (or absence or suppression of an enrichment) are indicators of strongly sulfidic conditions in such settings. Finally, we show that maxima in sedimentary molar V/Mn ratios correlate with strongly reducing conditions. This finding contrasts with prior work on V/Mn ratios as a (paleo)redox proxy, implying that further research is necessary.