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Mechanistic description of the transition from aerobic to anaerobic metabolism is necessary for diagnostic and predictive modeling of fixed nitrogen loss in anoxic marine zones (AMZs). In a metabolic model where diverse oxygen- and nitrogen-cycling microbial metabolisms are described by underlying redox chemical reactions, we predict a transition from strictly aerobic to predominantly anaerobic regimes as the outcome of ecological interactions along an oxygen gradient, obviating the need for prescribed critical oxygen concentrations. Competing aerobic and anaerobic metabolisms can coexist in anoxic conditions whether these metabolisms represent obligate or facultative populations. In the coexistence regime, relative rates of aerobic and anaerobic activity are determined by the ratio of oxygen to electron donor supply. The model simulates key characteristics of AMZs, such as the accumulation of nitrite and the sustainability of anammox at higher oxygen concentrations than denitrification, and articulates how microbial biomass concentrations relate to associated water column transformation rates as a function of redox stoichiometry and energetics. Incorporating the metabolic model into an idealized two-dimensional ocean circulation results in a simulated AMZ, in which a secondary chlorophyll maximum emerges from oxygen-limited grazing, and where vertical mixing and dispersal in the oxycline also contribute to metabolic co-occurrence. The modeling approach is mechanistic yet computationally economical and suitable for global change applications.

Mechanistic description of the transition from aerobic to anaerobic metabolism is necessary for diagnostic and predictive modeling of fixed nitrogen loss in anoxic marine zones (AMZs). In a metabolic model where diverse oxygen- and nitrogen-cycling microbial metabolisms are described by underlying redox chemical reactions, we predict a transition from strictly aerobic to predominantly anaerobic regimes as the outcome of ecological interactions along an oxygen gradient, obviating the need for prescribed critical oxygen concentrations. Competing aerobic and anaerobic metabolisms can coexist in anoxic conditions whether these metabolisms represent obligate or facultative populations. In the coexistence regime, relative rates of aerobic and anaerobic activity are determined by the ratio of oxygen to electron donor supply. The model simulates key characteristics of AMZs, such as the accumulation of nitrite and the sustainability of anammox at higher oxygen concentrations than denitrification, and articulates how microbial biomass concentrations relate to associated water column transformation rates as a function of redox stoichiometry and energetics. Incorporating the metabolic model into an idealized two-dimensional ocean circulation results in a simulated AMZ, in which a secondary chlorophyll maximum emerges from oxygen-limited grazing, and where vertical mixing and dispersal in the oxycline also contribute to metabolic co-occurrence. The modeling approach is mechanistic yet computationally economical and suitable for global change applications.

The inventory and variability of oceanic dissolved inorganic carbon (DIC) is driven by the interplay of physical, chemical, and biological processes. Quantifying the spatiotemporal variability of these drivers is crucial for a mechanistic understanding of the ocean carbon sink and its future trajectory. Here, we use the Estimating the Circulation and Climate of the Ocean‐Darwin ocean biogeochemistry state estimate to generate a global‐ocean, data‐constrained DIC budget and investigate how spatial and seasonal‐to‐interannual variability in three‐dimensional circulation, air‐sea CO2flux, and biological processes have modulated the ocean sink for 1995–2018. Our results demonstrate substantial compensation between budget terms, resulting in distinct upper‐ocean carbon regimes. For example, boundary current regions have strong contributions from vertical diffusion while equatorial regions exhibit compensation between upwelling and biological processes. When integrated across the full ocean depth, the 24‐year DIC mass increase of 64 Pg C (2.7 Pg C year−1) primarily tracks the anthropogenic CO2growth rate, with biological processes providing a small contribution of 2% (1.4 Pg C). In the upper 100 m, which stores roughly 13% (8.1 Pg C) of the global increase, we find that circulation provides the largest DIC gain (6.3 Pg C year−1) and biological processes are the largest loss (8.6 Pg C year−1). Interannual variability is dominated by vertical advection in equatorial regions, with the 1997–1998 El Niño‐Southern Oscillation causing the largest year‐to‐year change in upper‐ocean DIC (2.1 Pg C). Our results provide a novel, data‐constrained framework for an improved mechanistic understanding of natural and anthropogenic perturbations to the ocean sink. The ocean has absorbed roughly 40% of fossil fuel carbon dioxide (CO2) emissions since the beginning of the industrial era. This so‐called “ocean carbon sink,” which primarily sequesters emissions in the form of dissolved inorganic carbon (DIC), plays a key role in regulating climate and mitigating global warming. However, we still lack a mechanistic understanding of how physical, chemical, and biological processes impact the ocean DIC reservoir in both space and time, and hence how the storage rates of emissions may change in the future. Here we use a global‐ocean biogeochemistry model Estimating the Circulation and Climate of the Ocean‐Darwin, which ingests both physical and biogeochemical observations to improve its accuracy, to map how ocean circulation, air‐sea CO2exchange, and marine ecosystems have modulated the combined natural and anthropogenic ocean DIC budget for 1995–2018. We find that in the upper ocean, circulation provides the largest supply of DIC while biological processes drive the largest loss. Year‐to‐year changes in the ocean carbon sink are dominated by El Niño‐Southern Oscillation events in the equatorial Pacific Ocean, which then affect DIC globally. In summary, our data‐constrained, global‐ocean DIC budget constitutes a significant step forward toward understanding climate‐related changes to the ocean DIC reservoir. We evaluate the global dissolved inorganic carbon (DIC) budget for 1995–2018 using an ocean biogeochemistry state estimate Estimating the Circulation and Climate of the Ocean‐DarwinIn the upper ocean, circulation provides the largest gain of DIC and biological processes are the dominant lossInterannual variability is greatest in equatorial regions and is associated with El Niño‐Southern Oscillation We evaluate the global dissolved inorganic carbon (DIC) budget for 1995–2018 using an ocean biogeochemistry state estimate Estimating the Circulation and Climate of the Ocean‐Darwin In the upper ocean, circulation provides the largest gain of DIC and biological processes are the dominant loss Interannual variability is greatest in equatorial regions and is associated with El Niño‐Southern Oscillation

Anaerobic microbial activity in the ocean causes losses of bioavailable nitrogen and emission of nitrous oxide to the atmosphere, but its predictability at global scales remains limited. Resource ratio theory suggests that anaerobic activity becomes sustainable when the ratio of oxygen to organic matter supply is below the ratio required by aerobic metabolisms. Here, we demonstrate the relevance of this framework at the global scale using three‐dimensional ocean datasets, providing a new interpretation of existing observations. Evaluations of the location and extent of anoxic zones and a diagnostic rate of pelagic nitrogen loss are consistent with previous estimates. However, we demonstrate that a threshold based on substrate‐supply fluxes is qualitatively different from a threshold based solely on the ambient oxygen concentration. This implies that use of the flux‐based threshold in global biogeochemical models can result in different predictions of anaerobic activity and nitrogen loss. Where oxygen concentrations are sufficiently low in the ocean, nutrients are lost to the atmosphere because of specialized activity by microorganisms. Typically, biogeochemical models prescribe a critical oxygen concentration as a threshold to predict this activity. Here, we explain and demonstrate that an alternative threshold is more appropriate. The new threshold takes into account the supply flux of oxygen, rather than the concentration, as well as the supply flux of organic matter that is consumed by the microorganisms. We use global datasets to demonstrate that this threshold is relevant at the global scale. Since it is distinct from and more mechanistic than the typical oxygen concentration threshold, it may be used in global biogeochemical climate models to improve their ability to predict the loss of nutrients to the atmosphere and the impact on the climate system. We show that a threshold for anaerobic activity based on oxygen and organic matter fluxes is consistent with global ocean observationsOur diagnostic calculation uses global datasets to define the three major pelagic anoxic zones in a new wayThe flux‐based threshold is qualitatively different than a threshold based on the oxygen concentration We show that a threshold for anaerobic activity based on oxygen and organic matter fluxes is consistent with global ocean observations Our diagnostic calculation uses global datasets to define the three major pelagic anoxic zones in a new way The flux‐based threshold is qualitatively different than a threshold based on the oxygen concentration