Open Ocean Oxygen Minimum Zones Research Articles

Shrinking of the Arabian Sea oxygen minimum zone with climate change projected with a downscaled model

In Arabian Sea (AS), land-locked northern boundary and strong seasonal productivity lead to the formation of one of the most intense open ocean Oxygen Minimum Zones (OMZs). Presence of this perennial OMZ has significant consequences on adjacent coastal fisheries and ecosystem. Simulations from CMIP5 suggest significant weakening of both monsoonal winds and productivity under high emission scenario. But the fate of AS OMZ in this scenario - whether it will expand or shrink - still remains elusive, mainly due to poor representation of extent and strength of AS OMZ in CMIP5 present-day simulations. To address this, we analyze the distribution of O2 in AS from a subset of three contrasted CMIP5 simulations, and complemented with a set of regional downscaled model experiments which we forced at surface and open boundaries using information from those three CMIP5 models. We tested two regional downscaling approaches - with and without correction of CMIP5 biases with respect to observations. Using a set of sensitivity experiments, we disentangle the contributions of local (atmospheric) forcing vs. remote (at the lateral boundaries) forcing in driving the future projected O2 changes. While CMIP5 projects either shrinking or expansion of the AS OMZ depending on the model, our downscaling experiments consistently project a shrinking of AS OMZ. We show that projected O2 changes in OMZ layer are affected by both local and remote processes. In the southern AS, the main response to climate change is oxygenation that originates from the boundaries, and hence downscalled and CMIP5 model responses are similar. In contrast, in northern AS, downscaling yields a substantial reduction in O2 projection discrepancies because of a minimal influence of remote forcing there leading to a stronger sensitivity to improved local physics and improved model representation of present-day conditions. We find that when corrected for present-day biases, projected deoxygenation in the northern AS is shallower. Our findings indicate the importance of downscaling of global models in regions where local forcing is dominant, and the need for correcting global model biases with respect to observations to reduce uncertainties in future O2 projections.

Causes of uncertainties in the representation of the Arabian Sea oxygen minimum zone in CMIP5 models

Abstract. Open-ocean oxygen minimum zones (OMZs) occur in regions with high biological productivity and weak ventilation. They restrict marine habitats and alter biogeochemical cycles. Global models generally show a large model–data misfit with regard to oxygen. Reliable statements about the future development of OMZs and the quantification of their interaction with climate change are currently not possible. One of the most intense OMZs worldwide is located in the Arabian Sea (AS). We give an overview of the main model deficiencies with a detailed comparison of the historical state of 10 climate models from the 5th Coupled Model Intercomparison Project (CMIP5) that present our present-day understanding of physical and biogeochemical processes. Most of the models show a general underestimation of the OMZ volume in the AS compared to observations that is caused by an overly shallow layer of oxygen-poor water in the models. The deviation of oxygen values in the deep AS is the result of oxygen levels that are too high simulated in the Southern Ocean formation regions of Indian Ocean Deep Water in the models compared to observations and uncertainties in the deepwater mass transport from the Southern Ocean northward into the AS. Differences in simulated water mass properties and ventilation rates of Red Sea Water and Persian Gulf Water cause different mixing in the AS and thus influence the intensity of the OMZ. These differences in ventilation rates also point towards variations in the parameterizations of the overflow from the marginal seas among the models. The results of this study are intended to foster future model improvements regarding the OMZ in the AS.

Environmental association of heterotrophic micro-eukaryotes in the varying biogeochemical regimes of the Arabian Sea, resolved via high-throughput sequencing

The diverse physicochemical conditions prevailing in the Arabian Sea are expected to result in marked spatial variations in heterotrophic flagellate (HF) and ciliate communities. Here, we report the environmental association of heterotrophic micro-eukaryotes, particularly the heterotrophic flagellates and ciliates, based on 18S rRNA gene survey in the region. High-throughput next-generation sequencing, using the V4 eukaryotic-specific primer, was employed to study the composition of these communities associated with low-O2 waters in both coastal and offshore settings. Canonical correspondence analysis (CCA) revealed a preference of the heterotrophic flagellates for nitrate- and nitrite-rich zones. Notably, the heterotrophic nanoflagellate genus Monosiga showed a strong positive correlation with NO3−, which suggests its potential denitrifying capability. Shannon’s entropy analysis revealed a higher HF diversity in the hypoxic waters of the open ocean (depth 103 m), whereas ciliates were more diverse at oxygenated coastal stations. The estuarine waters exhibited a low diversity of both ciliates and flagellates. The UPGMA clusters of heterotrophic flagellates and ciliates in suboxic waters of the open ocean oxygen minimum zone were distinct from those found at other sites. Overall, CCA revealed the important relationship between nitrite, nitrate, salinity and chlorophyll a, which could be important factors for the partitioning of different ecological niches for specific HF and ciliate communities in the Arabian Sea. The community of heterotrophic protists that can adapt to varying biogeochemical regimes has been identified.

Potential role of priming effect in the open ocean oxygen minimum zones: an outlook

In contrast to earlier postulation that enhanced surface production is the sole source of organic carbon to heterotrophic respiration in open ocean as well as in oxygen minimum zones, recent studies provided evidences for the lateral transport of terrestrial organic carbon from continental margin to open ocean, supporting terrestrial organic carbon subsidized open ocean heterotrophy. However, actual sink mechanism of terrestrial organic carbon in the ocean remains to be resolved. “Priming effect” has been speculated to be possible sink mechanism of terrestrial organic carbon in ocean; however, realm in the ocean where the priming effect occurs is unknown. Since “elevated organic carbon re-mineralization” is one of the dominant causes of sustenance and expansion of oxygen minimum zones, here it is hypothesized that priming effect could be a significant contributor for the same. Based on the studies conducted so far, it appears that nutrient scavenging could be the dominant mechanism of priming effect in oxygen minimum zones.

BOUNDARY CONDITIONS OF ATLANTIC EOCENE OXYGEN MINIMUM ZONES

Biserial heterohelicid foraminifera are paleohydrographic indices for open ocean oxygen minimum zones. Comparison of the depletion of biserial heterohelicid carbon isotope ratios with the carbon isotope contrast through the water column is used to describe the strength of Eocene oxygen minima. Combining stable isotope values and percentages of planktonic foraminiferal index species, we describe the following boundary condition for early, middle and latest Eocene oxygen minimum zones in the Atlantic Ocean. Early Eocene - low meridional and vertical thermal contrasts, reduced upwelling and circulation vigor in a moderately war m, stable thermospheric ocean accompany a poorly developed, spatially restricted oxygen minimum. Middle Eocene - invigorated upwelling, large vertical and spatial thermal contrasts caused by cooling at the bottom but not at the surface together with increased ventilation of the water column accompany a strong, widely developed oxygen minimum; and Late Eocene - in a psychrospheric ocean low meridional and vertical thermal contrasts, reduced equatorial upwelling and atmospheric circulation with decreased current circulation vigor correlate with a homogeneous mixed layer in which an oxygen minimum is poorly developed above the thermocline, but not at mid depths.

Halogens (Cl, Br, and I) geochemistry in Middle Triassic carbonates: Implications for salinity and diagenetic alteration of I/(Ca + Mg) ratios

As an important family of elements, halogens (Cl, Br, and I) have been widely used to study geological processes, particularly in high-temperature environments, showing great prospects either in fundamental or in applied studies. However, little work has been conducted for sedimentary and low-temperature diagenetic environments re-construction although these elements have the potential in theory. Here we perform a pilot study on this issue using a case study of Middle Triassic marine dolomitization in South China. Our results show that the average Cl concentrations in limestone, dolomitic limestone, calcitic dolomite and dolomite are 88.3 ppm, 114.7 ppm, 121.3 ppm and 142.9 ppm respectively, and the corresponding Br concentrations are 1.15 ppm, 1.42 ppm, 1.59 ppm and 2.13 ppm, respectively. Both the concentrations of Cl and Br increased with the increasing dolomitization degree. The increasing tendency of Cl and Br is similar to the seawater evaporation trajectory before halite precipitation, indicating that dolomitization occurs in fluids with elevated salinity. Meanwhile, the difference of Cl/Br ratios between our carbonate samples and seawater may be ascribed to the input of terrestrial water, which has variable Cl/Br ratios. As for the concentration of I, the mean values sharply decreased from 0.36 ppm in limestone to 0.13 ppm in dolomitic limestone samples, and it further lowered to 0.09 ppm and 0.08 ppm in calcitic dolomite and dolomite samples, respectively. The calculated I/(Ca + Mg) ratios in limestones vary between 0.05 and 0.58 μmol/mol with an average value of 0.29 μmol/mol, indicating that limestone formed in open ocean oxygen-minimum zone (OMZs) or anoxic basin environment. The average I/(Ca + Mg) ratio decreases from 0.29 μmol/mol in limestone to 0.06 μmol/mol in dolomite, serving as an important case that dolomitization decreases I/(Ca + Mg) ratios of carbonates rocks. Halogens in carbonates therefore have promising potential for investigating the sedimentary and diagenetic environments of carbonate rocks.

Nitrogen and carbon cycling over the western continental shelf of India during seasonal anoxia: A stable isotope approach

The western continental shelf of India (eastern Arabian Sea) houses the world's largest seasonally-occurring coastal low-oxygen zone. The natural abundance of nitrogen and oxygen isotopes in dissolved nitrate were measured along four transects of the shelf (off Mumbai, Goa, Mangalore and Calicut) to gain insights into nitrogen cycling in this system. Carbon and nitrogen isotope abundances in particulate organic matter were also measured to understand sources and cycling of organic matter. Three of the four transects (with the exception of Mumbai transect) experienced fully anoxic (sulphidic) conditions over the inner shelf. Denitrification signals, as indicated by a linear increase of δ15N and δ18O in nitrate in oxygen-deficient waters, were observed off Mumbai, Goa and Mangalore. The occurrence of nitrite oxidation was indicated by a decoupling of stable N and O isotopes of nitrate. The fractionation factor computed was found to be quite low (6.1‰), as compared to open ocean oxygen minimum zones, presumably due to a large contribution (>49%) of benthic denitrification to N-loss. Autochthonous production and phytoplankton composition are proposed to exert major controls on the C isotopic composition of particulate organic matter while nitrate and ammonium seem to control the N-isotope variability. Overall, this study attempts to fill the knowledge gap in terms of N and C stable isotope studies in a system that is otherwise well documented from the biogeochemical point of view.

Intact polar lipids in the water column of the eastern tropical North Pacific: abundance and structural variety of non-phosphorus lipids

Abstract. Intact polar lipids (IPLs) are the main building blocks of cellular membranes and contain chemotaxonomic, ecophysiological and metabolic information, making them valuable biomarkers in microbial ecology and biogeochemistry. This study investigates IPLs in suspended particulate matter (SPM) in the water column of the eastern tropical North Pacific Ocean (ETNP), one of the most extensive open-ocean oxygen minimum zones (OMZs) in the world, with strong gradients of nutrients, temperature and redox conditions. A wide structural variety in polar lipid head-group composition and core structures exists along physical and geochemical gradients within the water column, from the oxygenated photic zone to the aphotic OMZ. We use this structural diversity in IPLs to evaluate the ecology and ecophysiological adaptations that affect organisms inhabiting the water column, especially the mid-depth OMZ in the context of biogeochemical cycles. Diacylglycerol phospholipids are present at all depths, but exhibit the highest relative abundance and compositional variety (including mixed acyl/ether core structures) in the upper and core OMZ where prokaryotic biomass was enriched. Surface ocean SPM is dominated by diacylglycerol glycolipids that are found in photosynthetic membranes. These and other glycolipids with varying core structures composed of ceramides and hydroxylated fatty acids are also detected with varying relative abundances in the OMZ and deep oxycline, signifying additional non-phototrophic bacterial sources for these lipids. Betaine lipids (with zero or multiple hydroxylations in the core structures) that are typically assigned to microalgae are found throughout the water column down to the deep oxycline but do not show a depth-related trend in relative abundance. Archaeal IPLs comprised of glycosidic and mixed glycosidic-phosphatidic glycerol dibiphytanyl glycerol tetraethers (GDGTs) are most abundant in the upper OMZ, where nitrate maxima point to ammonium oxidation but increase in relative abundance in the core OMZ and deep oxycline. The presence of non-phosphorus “substitute” lipids within the OMZ suggest that the indigenous microbes might be phosphorus limited (P starved) at ambient phosphate concentrations of 1 to 3.5 µM, although specific microbial sources for many of these lipids still remain unknown.

Distribution and phylogeny of anaerobic ammonium-oxidizing (anammox) bacteria in the water column of the central Pacific Ocean

Anaerobic ammonium-oxidizing (anammox) bacteria produce dinitrogen gas from nitrite and ammonium under anoxic conditions and significantly contribute nitrogen removal processes in marine environments. Distribution and activity of anammox bacteria in a marine water column have been well studied in some oxygen deficient waters such as Black Sea, Arabian Sea, Namibian upwelling and Peruvian upwelling systems. However, the habitable area of anammox bacteria in the Pacific Ocean is still an open question. Here, we report distribution, abundance and phylogeny of anammox bacteria in the water column along the north-south transect (40⁰S to 55⁰N, 170⁰W) of the Pacific Ocean in order to determine their habitable area in the open ocean. Anammox bacteria-specific 16S rRNA gene was detected at 10 locations mostly from oxygen minimum depths (OMD) where dissolved oxygen (DO) concentration ranged from 10.9 to 46.2 µmol Kg-1. Their abundance measured by qPCR ranged from 3.69 ± 1.35 × 102 copies L-1 to 474 ± 15 × 102 copies L-1. The phylogenetic analysis showed that anammox bacteria detected in this study were all related to Candidatus Scalindua species and clustered into three subgroups: the Scalindua brodae/sorokinii cluster, the sediment cluster and the Arabian Sea cluster. Relationship between the abundance of anammox bacteria and DO concentration varied among these 3 clusters, suggesting their difference in oxygen sensitivity. Although rate measurement of anammox reaction using 15N-labeled substrates failed for all OMD samples tested during the cruise, this study unveiled the distribution of anammox bacteria in the open ocean oxygen minimum zone (OMZ) where oxygen deficiency is not extreme but moderate, which provides an insight into the potential distribution of anammox bacteria in marine environments.

A multi‐year time‐series of N2O dynamics in a seasonally anoxic fjord: Saanich Inlet, British Columbia

AbstractWe present an 8‐yr time‐series of monthly water column N2O measurements from Saanich Inlet, a seasonally anoxic fjord in southern British Columbia. We document seasonal and inter‐annual variability in N2O concentrations driven by physical and biological forcing, and examine the relationship between N2O and O2 concentrations across the redoxcline in this system. Near‐surface N2O concentrations were typically supersaturated, and increased with depth to a maximum near the oxic‐anoxic transition. Genes associated with both the nitrification and denitrification pathways were widely distributed throughout the water column, suggesting the potential for simultaneous N2O production from nitrification and incomplete denitrification. Maximum N2O concentrations in Saanich Inlet were similar to other anoxic basins, but lower than those previously documented in coastal upwelling and open ocean oxygen minimum zones. N2O and O2 were inversely correlated throughout the water column, except below 10 μmol O2 L−1 where net N2O consumption appears to occur. In surface waters, maximum N2O concentrations and sea–air fluxes occurred during late summer/early fall, when O2 levels in the oxycline were low. Annual deep basin renewals often persisted over multiple months, supplying O2, , and N2O to the deep basin, with N2O subsequently declining to undetectable levels (< 0.4 nmol N2O L−1) by late spring. An unusually weak renewal was observed during 2009, possibly linked to reduced upwelling off the coast of British Columbia during El Niño. Our results can be used as a baseline to identify the effect of longer‐term changes in Saanich Inlet, such as deoxygenation and its effect on N2O concentrations.

Patterns of deoxygenation: sensitivity to natural and anthropogenic drivers.

Observational estimates and numerical models both indicate a significant overall decline in marine oxygen levels over the past few decades. Spatial patterns of oxygen change, however, differ considerably between observed and modelled estimates. Particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, observations indicate a general oxygen decline, whereas most of the state-of-the-art models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, we here study mechanisms of changes in oxygen supply and consumption with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, of changes in the wind-driven circulation and atmospheric nutrient supply, and of some poorly constrained biogeochemical processes are investigated. Predominantly wind-driven changes in the low-latitude oceanic ventilation are identified as a possible factor contributing to observed oxygen changes in the low-latitude thermocline during the past decades, while the potential role of biogeochemical processes remains difficult to constrain. We discuss implications for the attribution of observed oxygen changes to anthropogenic impacts and research priorities that may help to improve our mechanistic understanding of oxygen changes and the quality of projections into a changing future.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

A model study of warming-induced phosphorus–oxygen feedbacks in open-ocean oxygen minimum zones on millennial timescales

Abstract. Observations indicate an expansion of oxygen minimum zones (OMZs) over the past 50 years, likely related to ongoing deoxygenation caused by reduced oxygen solubility, changes in stratification and circulation, and a potential acceleration of organic matter turnover in a warming climate. The overall area of ocean sediments that are in direct contact with low-oxygen bottom waters also increases with expanding OMZs. This leads to a release of phosphorus from ocean sediments. If anthropogenic carbon dioxide emissions continue unabated, higher temperatures will cause enhanced weathering on land, which, in turn, will increase the phosphorus and alkalinity fluxes into the ocean and therefore raise the ocean's phosphorus inventory even further. A higher availability of phosphorus enhances biological production, remineralisation and oxygen consumption, and might therefore lead to further expansions of OMZs, representing a positive feedback. A negative feedback arises from the enhanced productivity-induced drawdown of carbon and also increased uptake of CO2 due to weathering-induced alkalinity input. This feedback leads to a decrease in atmospheric CO2 and weathering rates. Here, we quantify these two competing feedbacks on millennial timescales for a high CO2 emission scenario. Using the University of Victoria (UVic) Earth System Climate Model of intermediate complexity, our model results suggest that the positive benthic phosphorus release feedback has only a minor impact on the size of OMZs in the next 1000 years. The increase in the marine phosphorus inventory under assumed business-as-usual global warming conditions originates, on millennial timescales, almost exclusively (> 80 %) from the input via terrestrial weathering and causes a 4- to 5-fold expansion of the suboxic water volume in the model.

Methanotrophic Community Dynamics in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia

Marine oxygen minimum zones (OMZs) play disproportionate roles in nutrient and climate active trace gas cycling including nitrous oxide and methane, in the ocean. OMZs are currently expanding due to climate change making it increasingly important to identify microbial controls on trace gas cycling at the individual, population and community levels. Here we present a two-year survey of the microbial community along seasonal redox gradients in Saanich Inlet focused on identifying microbial agents driving methane oxidation. Although methanotrophs were rare, we identified three uncultivated groups affiliated with particulate methane monooxygenase (pMMO) encoding phylogenetic groups (OPU), and methanotrophic symbionts as primary drivers of methane oxidation in Saanich Inlet. Distribution and activity patterns for these three groups were consistent with niche partitioning that became increasingly resolved during water column stratification. Moreover co-occurrence analysis combined with multi-level indicator species analysis revealed significant correlations between operational taxonomic units affiliated with Methylophaga, Methylophilales, SAR324, Verrucomicrobia, and Planctomycetes with OPUs and methanotrophic symbiont groups. Taken together these observations shed new light on the composition, dynamics, and potential interspecific interactions of microbes associated with CH4 cycling in the Saanich Inlet water column, provide a baseline for comparison between coastal and open ocean OMZs and support the potential role of OPUs, and methanotrophic symbiont groups as a widely distributed pelagic sink for CH4 along continental margins.

What prevents nitrogen depletion in the oxygen minimum zone of the eastern tropical South Pacific?

Abstract. Local coupling between nitrogen fixation and denitrification in current biogeochemical models could result in runaway feedback in open-ocean oxygen minimum zones (OMZs), eventually stripping OMZ waters of all fixed nitrogen. This feedback does not seem to operate at full strength in the ocean, as nitrate does not generally become depleted in open-ocean OMZs. To explore in detail the possible mechanisms that prevent nitrogen depletion in the OMZ of the eastern tropical South Pacific (ETSP), we develop a box model with fully prognostic cycles of carbon, nutrients and oxygen in the upwelling region and its adjacent open ocean. Ocean circulation is calibrated with Δ14C data of the ETSP. The sensitivity of the simulated nitrogen cycle to nutrient and oxygen exchange and ventilation from outside the model domain and to remineralization scales inside an OMZ is analysed. For the entire range of model configurations explored, we find that the fixed-N inventory can be stabilized at non-zero levels in the ETSP OMZ only if the remineralization rate via denitrification is slower than that via aerobic respiration. In our optimum model configuration, lateral oxygen supply into the model domain is required at rates sufficient to oxidize at least about one fifth of the export production in the model domain to prevent anoxia in the deep ocean. Under these conditions, our model is in line with the view of phosphate as the ultimate limiting nutrient for phytoplankton, and implies that for the current notion of nitrogen fixation being favoured in N-deficit waters, the water column of the ETSP could even be a small net source of nitrate.

Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

Abstract. The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500 m is estimated to be 1 × 10−5 m2 s−1, with lower and upper 95% confidence limits of 0.8 × 10−5 m2 s−1 and 1.4 × 10−5 m2 s−1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 μmol kg−1 yr−1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper Central Water layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100 m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

Ammonia oxidation rates and nitrification in the Arabian Sea

[1] Nitrification rates, as well as the relationships between rates and ammonia oxidizer abundance (both archaeal and bacterial), were investigated in the Arabian Sea. Ammonia oxidation rates were measured directly using 15N-NH4+stable isotope additions in gas-impermeable, trace metal clean trilaminate bags (500 mL) at in situ temperature. Tracer incubations were performed at three stations at depths above, below, and within the oxycline of the open-ocean oxygen minimum zone (OMZ). Ammonia oxidation rates were similar to previous open-ocean measurements, ranging from undetectable to 21.6 ± 0.1 nmol L−1 d−1. The highest rates at each station occurred at the primary nitrite maximum (above the OMZ), and rates were very low at depths greater than 900 m. The abundances of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were estimated using theamoA gene by quantitative polymerase chain reaction (qPCR). Both AOA and AOB amoA were detected above, within, and below the OMZ, although the AOA were always more abundant than the AOB, by a factor of 35–216. Nitrification rates were not directly correlated to AOA or AOB amoA abundance. These rates offer new insight into the role of nitrification in the mesopelagic zone. The abundance of AOA amoA genes at 1000 m suggests that ∼50% of the microbial biomass could be autotrophic. Additionally, the integrated nitrification rate at depth implies that nitrification could consume most of the ammonium produced by the flux of organic carbon in the mesopelagic zone.

Oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands

The open‐ocean oxygen minimum zone (OMZ) south and east of the Cape Verde Islands is studied from CTD hydrography, ADCP velocities, Argo float trajectories, and historical data, with a focus on the zonal supply and drainage paths. The strongest oxygen minimum is located north of the North Equatorial Countercurrent (NECC) at about 400 to 500‐m depth just above the boundary between Central Water and Antarctic Intermediate Water (AAIW). It is shown that the NECC, the North Equatorial Undercurrent at 4 to 6°N, and a northern branch of the NECC at 8 to 10°N are the sources for oxygen‐rich water supplied to the OMZ in summer and fall. A weak eastward NECC at 200‐m depth also exists in winter and spring as derived from Argo floats drifting at shallow levels. Historical oxygen data from 200‐m depth confirm this seasonality showing high (low) oxygen content in summer and fall (spring) within the supply paths. Compared to the strong oxygen supply at 150 to 300‐m depth, the ventilation of the OMZ at 300 to 600‐m depth is weaker. Westward drainage of oxygen‐poor water takes place north of the Guinea Dome, i.e., north of 10°N, most pronounced at 400 to 600‐m depth. In July 2006 the total eastward transport of both NECC bands above σθ = 27.1 kg m−3 at 23°W was about 13 Sv (1 Sv = 106 m3 s−1). About half of this water volume circulates within the Guinea Dome or recirculates westward north of the Guinea Dome.