Core Of Oxygen Minimum Zone Research Articles

Unique ocean circulation pathways reshape the Indian Ocean oxygen minimum zone with warming

Abstract. The global ocean is losing oxygen with warming. Observations and Earth system model projections, however, suggest that this global ocean deoxygenation does not equate to a simple and systematic expansion of tropical oxygen minimum zones (OMZs). Previous studies have focused on the Pacific Ocean; they showed that the outer OMZ deoxygenates and expands as oxygen supply by advective transport weakens, the OMZ core oxygenates and contracts due to a shift in the composition of the source waters supplied by slow mixing, and in between these two regimes oxygen is redistributed with little effect on OMZ volume. Here, we examine the OMZ response to warming in the Indian Ocean using an ensemble of Earth system model high-emissions scenario experiments from the Coupled Model Intercomparison Project Phase 6. We find a similar expansion–redistribution–contraction response but show that the unique ocean circulation pathways of the Indian Ocean lead to far more prominent OMZ contraction and redistribution regimes than in the Pacific Ocean. As a result, only the outermost volumes (oxygen>180 µmol kg−1) expand. The Indian Ocean experiences a broad oxygenation in the southwest driven by a reduction in waters supplied by the Indonesian Throughflow in favor of high-oxygen waters supplied from the southern Indian Ocean gyre. Models also project a strong localized deoxygenation in the northern Arabian Sea due to the rapid warming and shoaling of marginal sea outflows (Red Sea and Persian Gulf) and increases in local stratification with warming. We extend the existing conceptual framework used to explain the Pacific OMZ response to interpret the response in the Indian Ocean.

On the interpretation of changes in the subtropical oxygen minimum zone volume off Chile during two La Niña events (2001 and 2007)

Oxygen minimum zones (OMZs) are extended oceanic regions for which dissolved oxygen concentration is extremely low. They are suspected to be expanding in response to global warming. However, currently, the mechanisms by which OMZ varies in response to climate variability are still uncertain. Here, the variability of the subtropical OMZ off central Chile of a regional coupled physical–biogeochemical regional model simulation was analyzed for the period 2000–2008, noting that its fluctuations were significant despite the relatively weak amplitude of the El Niño/Southern Oscillation (ENSO). In particular, the interannual variability in the OMZ volume (OMZVOL, defined as the volume with dissolved oxygen concentration (DO) ≤ 45μM) was approximately 38% larger than that of the seasonal cycle, with maximum and minimum anomalies of OMZVOL taking place during two cold La Niña (LN) years (2001 and 2007). The model analyses further reveal that these anomalies resulted from a combined effect of changes in (1) the oxygen-poor waters poleward transport by the Peru–Chile undercurrent (PCUC), (2) the intensity of quasi-zonal jets influencing the transport of water to and from the OMZ, and (3) the zonal DO transport related to mesoscale eddy activity. Specifically, the interannual variability of the PCUC modulated primarily the DO contents of the OMZ core [(DO) ≤ 20μM] and secondarily the OMZVOL, while cross-shore DO transport by the zonal jets and the eddy fluxes played a major role in ventilating and shaping the offshore extent of the OMZ. When the OMZVOL was maximum (minimum), the PCUC transport was slightly increased (reduced), which was associated with a reduction (increase) in the ventilation of the OMZ through negative (positive) anomalies of zonal advection and DO eddy fluxes. Our results demonstrate that significant natural interannual variability in the subtropical OMZ off Chile originates from the interplay between oceanic equatorial teleconnection (PCUC transport) and local non-linear dynamics (the zonal jets and mesoscale eddies).

Gradients and instability: Macrozoobenthic communities in the Benguela Upwelling System off Namibia

This is the most comprehensive study to date on the distribution of macrofaunal assemblages of the Oxygen Minimum Zone (OMZ) off Namibia, one of the least studied benthic marine macrofaunas in the world. OMZs are mid- and bottom-waters at continental margins characterized by dissolved oxygen concentrations lower than 0.5 ml O2 l−1. We present the distribution of macrofaunal assemblages between 17°S and 25°S latitudes and between 25 m and 1523 m water depth with a total of 534 taxa. We performed both abundance-based and biomass-based hierarchical clustering analysis, which differ slightly in the resulting assemblages. This difference is due to the greater importance of molluscs in the biomass-based clustering, while the abundance-based analysis seems to shift the importance to the short-lived crustaceans. The Namibian benthic macrofauna, in general, shows high total biomasses and high representativeness of molluscs compared to OMZs worldwide. Evidences suggest that temperature, depth, diffusive hydrogen sulphide flux, interseasonal dissolved oxygen average and dissolved oxygen seasonal variability as significant forces structuring benthic macrofauna in the Namibian shelf. Multivariate analysis defined 6 faunal assemblages. The deepest and shallowest assemblages were the most diverse, but biomass is low in the first and the highest in the second. The major assemblages within the OMZ core areas are found at an interseasonal average dissolved oxygen below 0.4 ml l−1, all of which co-occur quite frequently with diffusive hydrogen sulphide fluxes. Dead spots are found at an interseasonal average dissolved oxygen level below 0.05 ml l−1.

Diverging Fates of the Pacific Ocean Oxygen Minimum Zone and Its Core in a Warming World

AbstractGlobal ocean oxygen loss is projected to persist in the future, but Earth system models (ESMs) have not yet provided a consistent picture of how it will influence the largest oxygen minimum zone (OMZ) in the tropical Pacific. We examine the change in the Pacific OMZ volume in an ensemble of ESMs from the CMIP6 archive, considering a broad range of oxygen (O2) thresholds relevant to biogeochemical cycles and ecosystems (5–160 µmol/kg). Despite OMZ biases in the historical period of the simulations, the ESM ensemble projections consistently fall into three regimes across ESMs: an expansion of low oxygenated waters (+0.8 [0.6, 1.0] × 1016 m3/century for O2 ≤ 120 µmol/kg, ESM median and interquartile range); a slight contraction of the OMZ core although more uncertain across ESMs (−0.1 [−0.5, 0.0] × 1016 m3/century for O2 ≤ 20 µmol/kg); and at the transition from contraction to expansion regimes, a spatial redistribution but near‐zero change in the volume of hypoxic waters (0.0 [−0.3, +0.1] × 1016 m3/century for O2 ≤ 60 µmol/kg). Changes in circulation and biology dictate the shift from expansion to contraction. Specifically, reduced subtropical ventilation controls the expansion of low oxygenated waters, while a combination of circulation and biological changes explains the contraction of the core (likely changes in mixing, reduced intermediate ventilation and oxygen demand). Increased model complexity (e.g., ecosystem dynamics and equatorial circulation) likely stabilize the OMZ response, suggesting that future changes might lie in the lower bound of current projections. The expansion of low oxygenated waters which delimit the optimum habitat of numerous marine species would severely impact ecosystems and ecosystem services.

Anaerobic methane oxidation in a coastal oxygen minimum zone: spatial and temporal dynamics

SummaryCoastal waters are a major source of marine methane to the atmosphere. Particularly high concentrations of this potent greenhouse gas are found in anoxic waters, but it remains unclear if and to what extent anaerobic methanotrophs mitigate the methane flux. Here we investigate the long‐term dynamics in methanotrophic activity and the methanotroph community in the coastal oxygen minimum zone (OMZ) of Golfo Dulce, Costa Rica, combining biogeochemical analyses, experimental incubations and 16S rRNA gene sequencing over 3 consecutive years. Our results demonstrate a stable redox zonation across the years with high concentrations of methane (up to 1.7 μmol L−1) in anoxic bottom waters. However, we also measured high activities of anaerobic methane oxidation in the OMZ core (rate constant, k, averaging 30 yr−1 in 2018 and 8 yr−1 in 2019–2020). The OPU3 and Deep Sea‐1 clades of the Methylococcales were implicated as conveyors of the activity, peaking in relative abundance 5–25 m below the oxic–anoxic interface and in the deep anoxic water respectively. Although their genetic capacity for anaerobic methane oxidation remains unexplored, their sustained high relative abundance indicates an adaptation of these clades to the anoxic, methane‐rich OMZ environment, allowing them to play major roles in mitigating methane fluxes.

Application of Cd as a paleo-environment indicator

Cadmium is a micronutrient element and a tracer of biological productivity in the ocean. The contrasting Cd distributions in sediments from modern restricted basins, continental oxygen minimum zones (OMZs), and oxic settings make it possible to establish accurate thresholds to distinguish between various environments. By receiver operating characteristic analysis, we propose that Cd > 14.5 μg/g and Cd/Al > 4.33 × 10 −4 provide strong evidence for deposition within perennial OMZs, while Cd < 0.52 μg/g and Cd/Al < 0.08 × 10 −4 separate oxic sediments from anoxic sediments. With the modern Cd cycle as a guide, we present a case study of sedimentary Cd enrichment through a slope–basin transect in the early Cambrian Nanhua Basin, South China. With increasing water depth, the patterns of Cd concentrations and Cd/Al ratios are similar to those of the modern Peruvian margin, which are characterized by highest Cd enrichments in the OMZ core and muted Cd enrichments at the upper and lower edges of the OMZ. These data, combined with previous studies, indicate that the OMZ-like structure might be common in early Cambrian oceans. Furthermore, we analyzed Cd contents of shales from the Mesoproterozoic and assemble them with published data to explore the secular evolution of seawater Cd inventory. We find that a significant increase in Cd abundance and Cd/Al ratio occurs through the Ediacaran–Cambrian transition, reflecting enlarged Cd reservoir and widespread ocean oxygenation. Overall, Cd geochemistry can provide unique information concerning the redox state and hydrological condition of paleo-environment. • Contrasting Cd enrichments distinguish restricted basins, OMZs, and oxic settings. • Sedimentary Cd pattern reveals active upwelling in the early Cambrian Nanhua Basin. • Cd record reflects an increase in marine Cd reservoir through the E–C transition.

Last 10000 years Variation in the Intensity of OMZ-Core Reconstructed from Sediment of the Eastern Arabian Sea

The Arabian Sea Oxygen Minimum Zone (OMZ) is one of the most intense of the world ocean OMZs witnessing denitrification, hence, has implications to global climate. In order to understand the Holocene variations in the OMZ of this region, a sediment core (BP3-GCR3) from a water depth of 500 m was retrieved, which is presently swept by nearly anoxic waters of the core of the OMZ. The sedimentary oxide-bound Mn, a reliable redox proxy, exhibits a gradual decrease from ∼14 ppm during the early-Holocene to ∼8 ppm during the late-Holocene, suggesting a relatively less intense OMZ during the former period. The concentration of a suite of other redox-sensitive elements (Fe, Co and Ce) associated with the dispersed sedimentary oxide particulates also show a gradual and coherent decrease through the Holocene suggesting reduction of dissolved oxygen (DO) in the core of the OMZ towards the modern times. This intensification of OMZ seems to be a result of increased export production coupled with decreasing ventilation of thermocline waters through the Holocene. A one-time positive shift is observed at ∼5.5 ka BP in the organic carbon content (OC) from ∼2 % to ∼10 % in core-top. This shift in OC content is also marked by rapid decrease of CaCO3 by ∼20 %, suggesting enhanced carbonate dissolution due to increased anoxia in the OMZ core caused by increased export production in the late Holocene.

In situ observations show vertical community structure of pelagic fauna in the eastern tropical North Atlantic off Cape Verde

Distribution patterns of fragile gelatinous fauna in the open ocean remain scarcely documented. Using epi-and mesopelagic video transects in the eastern tropical North Atlantic, which features a mild but intensifying midwater oxygen minimum zone (OMZ), we established one of the first regional observations of diversity and abundance of large gelatinous zooplankton. We quantified the day and night vertical distribution of 46 taxa in relation to environmental conditions. While distribution may be driven by multiple factors, abundance peaks of individual taxa were observed in the OMZ core, both above and below the OMZ, only above, or only below the OMZ whereas some taxa did not have an obvious distribution pattern. In the eastern eropical North Atlantic, OMZ expansion in the course of global climate change may detrimentally impact taxa that avoid low oxygen concentrations (Beroe, doliolids), but favour taxa that occur in the OMZ (Lilyopsis, phaeodarians, Cydippida, Colobonema, Haliscera conica and Halitrephes) as their habitat volume might increase. While future efforts need to focus on physiology and taxonomy of pelagic fauna in the study region, our study presents biodiversity and distribution data for the regional epi- and mesopelagic zones of Cape Verde providing a regional baseline to monitor how climate change may impact the largest habitat on the planet, the deep pelagic realm.

Potential mechanisms responsible for occurrence of core oxygen minimum zone in the north-eastern Arabian Sea

The thickness of oxygen minimum zone (OMZ) is estimated for the first time using dissolved oxygen (DO) profiles obtained from the Biogeochemical Argo floats collected between 2013 and 2019 in the Arabian Sea. The depth of upper boundary of the OMZ varied narrowly between 70 and 220 m in the entire Arabian Sea whereas the lower boundary of OMZ significantly deepened from south (500 m) to northern Arabian Sea (1200 m). The thickness of OMZ decreased from north (>1050 m) to south (400 m) with the thickest OMZ in the northeastern Arabian Sea (950–1050 m). The thick OMZ in the northeastern Arabian Sea is associated with low concentration of depth integrated Chlorophyll-a, primary production in the upper 100 m and sinking carbon fluxes at 100 m depth than other regions. The particle back-scatter, proxy for particulate organic matter, is higher in the northeastern than other regions in the Arabian Sea. The high particle back-scatter data is found in the core of OMZ and it is increased from shelf to offshore indicating that cross-shelf transport of organic matter may be supporting bacterial carbon demand in the OMZ in the northeastern Arabian Sea. The eastward shift in the OMZ is attributed to weak mixing, high penetration time of intermediate water masses, and organic matter transport from the shelf region. Numerical models estimated negligible changes in DO in the OMZ since past several decades, whereas long-term observational data indicates decline in DO levels in the OMZ. Such contrasting results may be caused by lack of cross-shelf transport of organic matter in the models. Though this study identifies the occurrence of cross-shelf transport, the nature, quality and composition of organic matter transported from shelf is unknown. Nevertheless, inclusion of such processes in the models may improve predictability of possible changes in OMZ in future in the Arabian Sea.

Imprint of Trace Dissolved Oxygen on Prokaryoplankton Community Structure in an Oxygen Minimum Zone

The Eastern Tropical North Pacific (ETNP) is a large, persistent, and intensifying oxygen minimum zone (OMZ) that accounts for almost half of the total area of global OMZs. Within the OMZ core (~350-700 m depth), dissolved oxygen is typically near or below the analytical detection limit. Steep oxygen gradients above and below the OMZ core lead to vertical structuring of microbial communities that also vary between particle-associated (PA) and free-living (FL) size fractions. Here, we use 16S amplicon sequencing (iTags) coupled with multivariate statistics to analyze the diversity and distribution of prokaryotic populations between FL and PA size fractions and among the range of existing redox conditions. Nonmetric Dimensional Scaling (nMDS) revealed that bacterial and archaeal phylogenetic representations were significantly different between size fractions. Based on analysis of similarities (ANOSIM) and iTag profiles, composition of PA assemblages was less influenced by the prevalent depth-dependent biogeochemical regime than the FL fraction. Trace oxygen concentrations (~0.33 M) were present throughout the OMZ core at our sampling location. Consequently, nitrite accumulations typically reported for OMZ cores were absent. Furthermore, sequences for anammox bacteria (Brocadiales genus Candidatus Scalindua), commonly found at oxic-anoxic boundaries in other systems, were not recovered here. However, ammonia-oxidizing bacteria (AOB) and archaea (AOA) distributions and maximal autotrophic carbon assimilation rates (1.4 µM C d-1) coincided with a pronounced ammonium concentration maximum near the top of the OMZ core. In addition, members of the genus Nitrospina, a dominant nitrite-oxidizing bacteria (NOB) were present suggesting that both ammonia and nitrite oxidation occur at almost undetectable oxygen concentrations. We conclude that the presence of trace oxygen in the OMZ core of this region of the ETNP OMZ is sufficient to support active nitrification.

Spatial Distribution Patterns of Bacterioplankton in the Oxygen Minimum Zone of the Tropical Mexican Pacific.

Microbial communities within oxygen minimum zones (OMZs) are crucial drivers of marine biogeochemical cycles; however, we still lack an understanding of how these communities are distributed across an OMZ. We explored vertical (from 5 to 500m depth) and horizontal (coast to open ocean) distribution of bacterioplankton and its relationships with the main oceanographic conditions in three transects of the tropical Mexican Pacific OMZ. The distribution of the microbial diversity and the main clades changed along the transition from oxygen-rich surface water to the OMZ core, demonstrating the sensitivity of key bacterial groups to deoxygenation. The euphotic zone was dominated by Synechococcales, followed by Flavobacteriales, Verrucomicrobiales, Rhodobacterales, SAR86, and Cellvibrionales, whereas the OMZ core was dominated by SAR11, followed by SAR406, SAR324, SAR202, UBA10353 marine group, Thiomicrospirales and Nitrospinales. The marked environmental gradients along the water column also supported a high potential for niche partitioning among OMZ microorganisms. Additionally, in the OMZ core, bacterial assemblages from the same water mass were more similar to each other than those from another water mass. There were also important differences between coastal and open-ocean communities: Flavobacteriales, Verrucomicrobiales, Rhodobacterales, SAR86, and Cellvibrionales were more abundant in coastal areas, while Synechococcales, SAR406, SAR324, SAR202, UBA10353 marine group, and Thiomicrospirales were more abundant in the open ocean. Our results suggest a biogeographic structure of the bacterioplankton in this OMZ region, with limited community mixing across water masses, except in upwelling events, and little dispersion of the community by currents in the euphotic zone.

Ocean deoxygenation and copepods: coping with oxygen minimum zone variability

Abstract. Increasing deoxygenation (loss of oxygen) of the ocean, including expansion of oxygen minimum zones (OMZs), is a potentially important consequence of global warming. We examined present-day variability of vertical distributions of 23 calanoid copepod species in the Eastern Tropical North Pacific (ETNP) living in locations with different water column oxygen profiles and OMZ intensity (lowest oxygen concentration and its vertical extent in a profile). Copepods and hydrographic data were collected in vertically stratified day and night MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) tows (0–1000 m) during four cruises over a decade (2007–2017) that sampled four ETNP locations: Costa Rica Dome, Tehuantepec Bowl, and two oceanic sites further north (21–22∘ N) off Mexico. The sites had different vertical oxygen profiles: some with a shallow mixed layer, abrupt thermocline, and extensive very low oxygen OMZ core; and others with a more gradual vertical development of the OMZ (broad mixed layer and upper oxycline zone) and a less extensive OMZ core where oxygen was not as low. Calanoid copepod species (including examples from the genera Eucalanus, Pleuromamma, and Lucicutia) demonstrated different distributional strategies (implying different physiological characteristics) associated with this variability. We identified sets of species that (1) changed their vertical distributions and depth of maximum abundance associated with the depth and intensity of the OMZ and its oxycline inflection points; (2) shifted their depth of diapause; (3) adjusted their diel vertical migration, especially the nighttime upper depth; or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept). These distribution depths changed by tens to hundreds of meters depending on the species, oxygen profile, and phenomenon. For example, at the lower oxycline, the depth of maximum abundance for Lucicutia hulsemannae shifted from ∼600 to ∼800 m, and the depth of diapause for Eucalanus inermis shifted from ∼500 to ∼775 m, in an expanded OMZ compared to a thinner OMZ, but remained at similar low oxygen levels in both situations. These species or life stages are examples of “hypoxiphilic” taxa. For the migrating copepod Pleuromamma abdominalis, its nighttime depth was shallow (∼20 m) when the aerobic mixed layer was thin and the low-oxygen OMZ broad, but it was much deeper (∼100 m) when the mixed layer and higher oxygen extended deeper; daytime depth in both situations was ∼300 m. Because temperature decreased with depth, these distributional depth shifts had metabolic implications. The upper ocean to mesopelagic depth range encompasses a complex interwoven ecosystem characterized by intricate relationships among its inhabitants and their environment. It is a critically important zone for oceanic biogeochemical and export processes and hosts key food web components for commercial fisheries. Among the zooplankton, there will likely be winners and losers with increasing ocean deoxygenation as species cope with environmental change. Changes in individual copepod species abundances, vertical distributions, and life history strategies may create potential perturbations to these intricate food webs and processes. Present-day variability provides a window into future scenarios and potential effects of deoxygenation.

On the Estimation of Zooplankton-Mediated Active Fluxes in Oxygen Minimum Zone Regions

In the Peruvian upwelling system, the mesopelagic oxygen minimum zone (OMZ) is the main vertically structuring feature of the pelagic habitat. Several zooplankton and nekton species undertake diel vertical migrations (DVMs) into anoxic depths. It has been argued that these migrations contribute substantially to the oxygen consumption as well as excretion of dissolved compounds (in particular ammonium) in subsurface waters. However, metabolic suppression as a response to low ambient oxygen partial pressure ($pO_2$) has not been accounted for in these estimates. Here, we present estimates of zooplankton- and nekton-mediated oxygen consumption and ammonium release based on vertically stratified net hauls (day/night, upper 1000m). Samples were scanned, followed by image analysis and size-/taxon-specific estimation of metabolic rates of all identified organisms as a function of their biomass as well as ambient temperature and $pO_2$. The main crustacean migrants were euphausiids (mainly \textit{E. mucronata}) on offshore stations and the commercially exploited squat lobster \textit{Pleuroncodes monodon} on the upper shelf, where it often undertakes migration to the seafloor during the day. Correction for metabolic suppression results in a substantial reduction of both respiration and ammonium excretion within the OMZ core. Ignoring this mechanism leads to tenfold higher estimate of DVM-mediated active export of carbon by respiration to below 100 m depth at deep-water stations. The DVM-mediated release of ammonium by euphausiids into the 200 to 400 m depth layer ranges between $0$ and $36.81 \mu mol NH_4 m^{-2} d^{-1}$, which is insufficient to balance published estimates of ammonium uptake rates due to anammox. It seems critical to account for the modulation of zooplankton metabolic activity at low oxygen in order to correctly represent the contribution of migrating species to the biological pump.

Pteropods and heteropods (Mollusca, Gastropoda, Thecosomata) and the Miocene Oxygen Minimum Zone, Trinidad

Fossil holoplanktonic mollusks (pteropods and heteropods) are so far little known from Trinidad and have not yet been employed in paleoenvironmental studies in this region. Specimens >125 µm were examined in samples from two sections of the Middle Miocene Brasso Formation of Central Trinidad. Samples from the type section at Brasso Village were younger (planktonic foraminiferal Zones N11–N12) than those from Nollis Tunnel, near Tabaquite (N8). Both sections yielded abundant holoplanktonic molluscs, but all were juveniles. At Brasso Village, yields were highest in samples collected near a bathyal oxygen minimum zone (OMZ) that was revealed using benthic foraminifera. The percentage of the fauna as Vaginella sp. did not vary significantly throughout this section, while Heliconoides sp. was concentrated in rocks deposited near the OMZ base and Creseis spina above the OMZ core. This implies depth stratification among the pteropods that limited the flux of Heliconoides sp. to shallower water samples. The fauna from Nollis Tunnel, where recovery was restricted to eight out of nine samples, was used to infer the position of the OMZ at this site. The assemblage was more abundant at Nollis Tunnel than at Brasso Village. Simultaneous confidence intervals show that the proportional abundance of Vaginella sp. did not vary significantly throughout the Nollis Tunnel section, while Creseis roesti and Heliconoides inflatus inflatus? showed depth stratification. This demonstrates that fossil holoplanktonic gastropods have potential as a tool for paleoenvironmental interpretations, especially in detecting fossil OMZs.

Differential Distribution and Determinants of Ammonia Oxidizing Archaea Sublineages in the Oxygen Minimum Zone off Costa Rica.

Ammonia oxidizing archaea (AOA) are microbes that are widely distributed in the ocean that convert ammonia to nitrite for energy acquisition in the presence of oxygen. Recent study has unraveled highly diverse sublineages within the previously defined AOA ecotypes (i.e., water column A (WCA) and water column B (WCB)), although the eco-physiology and environmental determinants of WCB subclades remain largely unclear. In this study, we examined the AOA communities along the water columns (40–3000 m depth) in the Costa Rica Dome (CRD) upwelling region in the eastern tropical North Pacific Ocean. Highly diverse AOA communities that were significantly different from those in oxygenated water layers were observed in the core layer of the oxygen minimum zone (OMZ), where the dissolved oxygen (DO) concentration was < 2μM. Moreover, a number of AOA phylotypes were found to be enriched in the OMZ core. Most of them were negatively correlated with DO and were also detected in other OMZs in the Arabian Sea and Gulf of California, which suggests low oxygen adaptation. This study provided the first insight into the differential niche partitioning and environmental determinants of various subclades within the ecotype WCB. Our results indicated that the ecotype WCB did indeed consist of various sublineages with different eco-physiologies, which should be further explored.

Anaerobic methane oxidation is an important sink for methane in the ocean's largest oxygen minimum zone

AbstractWe investigated methane oxidation in the oxygen minimum zone (OMZ) of the eastern tropical North Pacific (ETNP) off central Mexico. Methane concentrations in the anoxic core of the OMZ reached ~ 20 nmol L−1 at off shelf sites and 34 nmol L−1 at a shelf site. Rates of methane oxidation were determined in ship‐board incubations with 3H‐labeled methane at O2 concentrations 0–75 nmol L−1. In vertical profiles at off‐shelf stations, highest rates were found between the secondary nitrite maximum at ~ 130 m and the methane maximum at 300–400 m in the anoxic core. Methane oxidation was inhibited by addition of 1 μmol L−1 oxygen, which, together with the depth distribution, indicated an anaerobic pathway. A coupling to nitrite reduction was further indicated by the inhibitory effect of the nitric oxide scavenger 2‐phenyl‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (PTIO). Metatranscriptomes from the anoxic OMZ core supported the likely involvement of nitrite‐reducing bacteria of the NC10 clade in anaerobic methane oxidation, but also indicated a potential role for nitrate‐reducing euryarchaeotal methane oxidizers (ANME‐2d). Gammaproteobacteria of the Methanococcales were further detected in both 16S rRNA gene amplicons and metatranscriptomes, but the role of these presumed obligately aerobic methane oxidizers in the anoxic OMZ core is unclear. Given available estimates of water residence time, the measured rates and rate constants (up to ~ 1 yr−1) imply that anaerobic methane oxidation is a substantial methane sink in the ETNP OMZ and hence attenuates the emission of methane from this and possibly other OMZs.

Metabolic adaptations of the pelagic octopod Japetella diaphana to oxygen minimum zones

The pelagic octopod Japetella diaphana inhabits meso- and bathypelagic depths worldwide. Across its geographic and depth range, individuals encounter oxygen levels ranging from nearly air-saturated to nearly anoxic. In this study, we assessed the physiological adaptations of individuals from one of the largest and most extreme regions of ocean hypoxia, the Eastern Tropical North Pacific (ETNP) oxygen minimum zone (OMZ). Ship-board measurements of metabolic rate and hypoxia tolerance were conducted and a metabolic index was constructed to model suitable habitat for aerobic metabolism. We found that individuals from the ETNP had a low metabolic rate, yet higher than conspecifics from more oxygen-rich habitats. Despite their higher rates, hypoxia tolerance may be similar to or greater than in conspecifics from more oxygen-rich waters. Furthermore, hypoxia tolerance in J. diaphana has a reverse temperature-dependence from most marine ectotherms, a characteristic that uniquely suits the physical characteristics of the lower oxycline. Even with its high tolerance to hypoxia, J. diaphana is incapable of maintaining basal oxygen demand in the OMZ core, at a depth where populations are typically most abundant in more oxygenated regions. Despite the limited metabolically-suitable habitat, J. diaphana is abundant in the ETNP outside the OMZ core, suggesting that physiological and behavioral plasticity is sufficient to maintain species fitness in this region.

Distribution of Meiofauna in Bathyal Sediments Influenced by the Oxygen Minimum Zone Off Costa Rica

Ocean deoxygenation has become a topic of increasing concern because of its potential impacts on marine ecosystems, including oxygen minimum zone (OMZ) expansion and subsequent benthic effects. We investigated the influence of oxygen concentration and organic matter (OM) availability on metazoan meiofauna within and below an OMZ in bathyal sediments off Costa Rica, testing the hypothesis that oxygen and OM levels are reflected in meiofaunal community structures and distribution. Mean total densities in our sampling cores (400-1800 m water depth) were highest with 3688 ind. 10 cm-2 at the OMZ core at 400 m water depth, decreasing rapidly downslope. Nematodes were overall dominant, with a maximum of 99.9% in the OMZ core, followed by copepods (13%), nauplii (4.8%), and polychaetes (3%). Relative copepod and nauplii abundance increased consistently with depth and increasing bottom-water O2. Meiofaunal composition was significantly different among sites, with lower taxonomic diversity at OMZ sites relative to deeper, oxygenated sites. Vertical distribution patterns within sediments showed that in strongly oxygen-depleted sites less meiofauna was concentrated in the surface sediment than at deeper slope sites. Highest meiofaunal abundance and lowest diversity occurred under lowest oxygen and highest pigment levels, whereas highest diversity occurred under highest oxygen-concentrations and low CPE, as well as high quality of sedimentary pigment (chl a/phaeo) and organic carbon (C/N). The lower meiofaunal diversity, and hence lower structural and trophic complexity, at oxygen-depleted sites raises concerns about changes in the structure and function of benthic marine ecosystems in the face of OMZ expansions.

Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru

Abstract. The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O2 conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O2 series characterising two seasonal steady states at the upper trap: suboxic ([O2]&lt;25 µmol kg−1) and hypoxic–oxic (15&lt;[O2]&lt;160 µmol kg−1) in austral summer and winter–spring, respectively. The OMZ vertical transfer efficiency of particulate organic carbon (POC) between traps (Teff) can be classified into three main ranges (high, intermediate, low). These different Teff ranges suggest that both predominant preservation (high Teff&gt;50 %) and remineralisation (intermediate Teff 20 &lt; 50 % or low Teff&lt;6 %) configurations can occur. An efficient OMZ vertical transfer (Teff&gt;50 %) has been reported in summer and winter associated with extreme limitation in O2 concentrations or OM quantity for OM degradation. However, higher levels of O2 or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50 %. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (Teff&lt;6 %). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ15N suggests a complex pattern of 15N impoverishment or enrichment according to Teff modulation. This sensitivity of OM to O2 fluctuations and particle concentration calls for further investigation into OM and O2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections.

Response of Sea Urchin Fitness Traits to Environmental Gradients Across the Southern California Oxygen Minimum Zone

Marine calcifiers are considered to be among the most vulnerable taxa to climate-forced environmental changes occurring on continental margins with effects hypothesized to occur on microstructural, biomechanical, and geochemical properties of carbonate structures. Natural gradients in temperature, salinity, oxygen, and pH on an upwelling margin combined with the broad depth distribution (100-1100 m) of the pink fragile sea urchin, Strongylocentrotus (formerly Allocentrotus) fragilis, along the southern California shelf and slope provide an ideal system to evaluate potential effects of multiple climate variables on carbonate structures in situ. We measured for the first time trait variability across four distinct depth zones in a natural experiment to simulate species-specific implications of oxygen minimum zone (OMZ) expansion, deoxygenation and ocean acidification. Although S. fragilis may likely be tolerant of future oxygen and pH decreases predicted during the twenty-first century, we determine from adults collected across multiple depth zones that urchin size and potential reproductive fitness (gonad index) are drastically reduced in the OMZ core (450-900 m) compared to adjacent zones. Increases in porosity and mean pore size coupled with decreases in mechanical nanohardness and stiffness of the calcitic endoskeleton in individuals collected from lower pHTotal (7.57-7.59) and lower dissolved oxygen (13-42 µmol kg-1) environments suggest that S. fragilis may be potentially vulnerable to crushing predators if these conditions become more widespread in the future. In addition, elemental composition determined using inductively coupled plasma-mass spectrometry indicates that S. fragilis has a skeleton composed of the low Mg-calcite mineral phase of calcium carbonate (mean Mg/Ca= 0.02 mol mol-1), the lowest Mg/Ca values measured in sea urchins known to date. Together these findings suggest that ongoing declines in oxygen and pH will likely affect the ecology and fitness of a dominant echinoid on the California margin.