Eastern Tropical North Pacific Research Articles

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.

Seasonality and Formation of Barrier Layers and Associated Temperature Inversions in the Eastern Tropical North Pacific

AbstractSeasonality and formation of barrier layers (BLs) and associated temperature inversions (TIs) in the eastern tropical North Pacific Ocean were investigated using raw and gridded Argo profiling float data, satellite data, and various sea surface flux data. BLs were observed frequently in boreal summer and autumn along the sea surface salinity (SSS) front south of the eastern Pacific fresh pool. TIs were found within the gap between the western and eastern Pacific warm pools in autumn when BLs were thickest. A mixed layer salinity budget was constructed to determine the formation mechanism responsible for BLs with TIs. This budget revealed that Ekman advection works to both freshen and cool the eastern tropical North Pacific in autumn and contributes to the formation of the thickest BLs with the warmest TIs through the tilting of the SSS front. Precipitation is a secondary contributor to BL formation in autumn. The BLs are also prevalent during summer but are thinner, are without associated TIs, and are primarily formed through precipitation. The largest rainfall associated with the intertropical convergence zone mostly occurred north of the band of thickest BLs in both summer and autumn. The geostrophic advection of salinity did not coherently contribute to the formation of BLs or TIs. The idea that Ekman advection contributes most to the formation of the thickest BLs with warm TIs was further corroborated because the horizontal salinity gradient was the dominant contributor to the density gradient and so is favorable for BL and TI formation.

The role of water masses in shaping the distribution of redox active compounds in the Eastern Tropical North Pacific oxygen deficient zone and influencing low oxygen concentrations in the eastern Pacific Ocean

AbstractOceanic oxygen deficient zones (ODZs) influence global biogeochemical cycles in a variety of ways, most notably by acting as a sink for fixed nitrogen (Codispoti et al. 2001). Optimum multiparameter analysis of data from two cruises in the Eastern Tropical North Pacific (ETNP) was implemented to develop a water mass analysis for the large ODZ in this region. This analysis reveals that the most pronounced oxygen deficient conditions are within the 13°C water (13CW) mass, which is distributed via subsurface mesoscale features such as eddies branching from the California Undercurrent. Nitrite accumulates within these eddies and slightly below the core of the 13CW. This water mass analysis also reveals that the 13CW and deeper Northern Equatorial Pacific Intermediate Water (NEPIW) act as the two Pacific Equatorial source waters to the California Current System. The Equatorial Subsurface Water and Subtropical Subsurface Water are synonymous with the 13CW and this study refers to this water mass as the 13CW based on its history. Since the 13CW has been found to dominate the most pronounced oxygen deficient conditions within the Eastern Tropical South Pacific ODZ and the Peru‐Chile Undercurrent, the 13CW and the NEPIW define boundaries for oxygen minimum conditions across the entire eastern Pacific Ocean.

The Distribution and Redox Speciation of Iodine in the Eastern Tropical North Pacific Ocean

AbstractThe distributions of iodate (IO3−), iodide (I−), nitrite (NO2−), and oxygen (O2) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centered within a potential density of 26.2–26.6 kg/m3, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP.

Investigating the cycling of chromium in the oxygen deficient waters of the Eastern Tropical North Pacific Ocean and the Santa Barbara Basin using stable isotopes

Cr isotope geochemistry is being explored in the context of a variety of geological problems as well as the environmental remediation of pollutant Cr(VI). There is a strong Cr isotope fractionation during reduction of oxidized Cr(VI) to reduced Cr(III). We present chromium concentration and Cr isotope data for samples from highly reducing environments ([O2] < 2 μmol/kg) in the Eastern Tropical North Pacific (ETNP) Oxygen Deficient Zone (ODZ) off of Mexico and the deep Santa Barbara Basin off of California. Total dissolvable Cr in the upper ETNP ODZ is slightly depleted (by up to 0.8 nmol/kg) and δ53Cr is up to 0.1–0.2‰ heavier compared to oxic waters of the same density seen at the SAFe station (30°N, 140°W), presumably both a result of reduction of Cr(VI) and removal of light Cr(III) by sinking particles. The Cr depletion and Cr isotope fractionation peak at the same depth as the highest δ15N of NO3− and decrease within the equally oxygen-deficient waters below, implying that microbial reduction dependent on the sinking organic matter flux may be the mechanism of Cr reduction. These data are consistent with a fractionation mechanism with a net isotope fractionation factor of ε ≈ −0.44‰. In the deepest anoxic waters of the Santa Barbara Basin in July 2014, dissolved (<0.2 μm) Cr is depleted by up to 1.8 nmol/kg and δ53Cr is up to 0.5‰ heavier compared to SAFe station waters of the same density. This is consistent with a net isotope fractionation factor of ε ≈ −0.65‰. At the Santa Barbara Basin site, it is possible that abiotic Fe(II) reduction (from Fe(II) diffusing out of reducing continental shelf sediments) also contributes to Cr reduction in addition to the microbial reduction mechanism.

Amperometric sensor for nanomolar nitrous oxide analysis

Nitrous oxide is an important greenhouse gas and there is a need for sensitive techniques to study its distribution in the environment at concentrations near equilibrium with the atmosphere (9.6 nM in water at 20 °C). Here we present an electrochemical sensor that can quantify N2O in the nanomolar range. The sensor principle relies on a front guard cathode placed in front of the measuring cathode. This cathode is used to periodically block the flux of N2O towards the measuring cathode, thereby creating an amplitude in the signal. This signal amplitude is unaffected by drift in the baseline current and can be read at very high resolution, resulting in a sensitivity of 2 nM N2O for newly constructed sensors. Interference from oxygen is prevented by placing the front guard cathode in oxygen-consuming electrolyte. The sensor was field tested by measuring an N2O profile to a depth of 120 m in the oxygen minimum zone of the Eastern Tropical North Pacific Ocean (ETNP) off the coast of Mexico.

Subfootprint Variability of Sea Surface Salinity Observed during the SPURS-1 and SPURS-2 Field Campaigns

Subfootprint variability (SFV), variability within the footprint of a satellite measurement, is a source of error associated with the validation process, especially for a satellite measurement with a large footprint such as those measuring sea surface salinity (SSS). This type of error has not been adequately quantified in the past. In this study, I have examined SFV using in situ ocean data from the SPURS-1 (Salinity Processes in the Upper ocean Regional Studies-1) and SPURS-2 field campaigns in the subtropical North Atlantic and eastern tropical North Pacific respectively. I computed SFV from these data over two one-year periods of intense sampling. The results show that SFV is highly seasonal. I have computed SFV errors in several different forms, a median value of the weekly snapshot error, a total snapshot error, an absolute error of the Aquarius and SMAP (Soil Moisture Active Passive) measurement, a part of that error associated with SFV and a bias due to the skewness of the distribution of SSS. These results are characteristic only of the particular regions studied. However, comparison of the results with high resolution models, and in situ data from moorings gives the possibility of getting global estimates of SFV from these other more common sources of SSS data.

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.

Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

Diverse studies predict global expansion of Oxygen Minimum Zones (OMZs) as a consequence of anthropogenic global warming. While the observed dissolved oxygen concentrations in many coastal regions are slowly decreasing, sediment core paleorecords often show contradictory trends. This is the case for numerous high-resolution reconstructions of oxygenation in the Eastern Tropical North Pacific (ETNP). While major shifts in redox conditions of the ETNP are dominated by glacial-interglacial cycling, important fluctuations also occur in response to minor climatic and oceanographic perturbations. It is important to understand these scenarios of past redox variation, as they are the closest analogues for near future climate and oceanographic change. We present recently collected sediment core proxy records from the Gulf of California in which we reproduce the variability of productivity and oxygenation of the ETNP OMZ during the past millennium. We emphasize paleoproductivity (Corg, Ni, Baexcess) and paleoredox indicators (Mo, V, Uauth) in sediment cores collected in Alfonso and La Paz basins and compare these OMZ records with other archives of the Eastern Pacific. Our findings indicate that the OMZ expanded in response to increased upwelling and productivity during cold intervals around 1830, 1680, and 1500 years AD (evidenced by higher P, Ni, Mo, V, and Uauth). The most hypoxic times corresponded to the beginning of the Little Ice Age (expressed in elevated Mo). Significant OMZ contractions occurred around 1300, 1650, and 1900s AD after reoxygenation events that were instigated by low productivity (lower Ni, Mo, V, and Uauth). The mechanisms that control decadal-to-centennial oxygen variability in the ETNP remain unidentified but are likely influenced by solar forcing not only driving migrations of the Intertropical Convergence Zone (ITCZ) but more importantly changes in the intensity of the Pacific Walker Circulation (PWC). During the Little Ice Age solar irradiance was at its lowest for the past millennium, which strengthened the PWC. This pattern contributed to more frequent La Niña-like conditions, which enhanced upwelling of nutrient-rich waters in the west coast of North America, driving productivity and reducing bottom oxygen levels, as seen in our ETNP records.

Nitrogen assimilation in picocyanobacteria inhabiting the oxygen‐deficient waters of the eastern tropical North and South Pacific

AbstractProchlorococcus and Synechococcus are the most abundant free‐living photosynthetic microorganisms in the ocean. Uncultivated lineages of these picocyanobacteria also thrive in the dimly illuminated upper part of oxygen‐deficient zones (ODZs), where an important portion of ocean nitrogen (N) loss takes place via denitrification and anaerobic ammonium oxidation. Recent metagenomic studies revealed that ODZ Prochlorococcus have the genetic potential for using different N forms, including nitrate and nitrite, uncommon N sources for Prochlorococcus, but common for Synechococcus. To determine which N sources ODZ picocyanobacteria are actually using in nature, the cellular 15N natural abundance (δ15N) and assimilation rates of different N compounds were determined using cell sorting by flow cytometry and mass spectrometry. The natural δ15N of the ODZ Prochlorococcus varied from −4.0‰ to 13.0‰ (n = 9), with 50% of the values in the range of −2.1–2.6‰. While the highest values suggest nitrate use, most observations indicate the use of nitrite, ammonium, or a mixture of N sources. Meanwhile, incubation experiments revealed potential assimilation rates of ammonium and urea in the same order of magnitude as that expected for total N in several environments including ODZs, whereas rates of nitrite and nitrate assimilation were very low. Our results thus indicate that reduced forms of N and nitrite are the dominant sources for ODZ picocyanobacteria, although nitrate might be important on some occasions. ODZ picocyanobacteria might thus represent potential competitors with anammox bacteria for ammonium and nitrite, with ammonia‐oxidizing archaea for ammonium, and with nitrite‐oxidizing bacteria for nitrite.

Molecular and isotopic evidence of the distribution of nitrogen-cycling microbial communities in the oxygen minimum zone of the Tropical Mexican Pacific.

Microbial communities play a central role in the N cycle of oceanic oxygen minimum zones (OMZs), such as in the Eastern Tropical North Pacific (ETNP). We explored the spatial distribution of the genetic potential for ammonia oxidation (amoA gene for bacteria and archaea), denitrification (nirS and nirK), anaerobic ammonium oxidation (anammox) (hzo) and dissimilarity nitrate reduction to ammonia (DNRA) (nrfA) and their relationships with the hydrological variables and isotopic composition of nitrate in the ETNP off Mexico. Oxygen concentration, the availability of N chemical forms and the upwelling shaped the distribution of N cycling genes along the water column. The high abundance of N genes and the isotopic composition of nitrate suggest the N cycling is very dynamic in the OMZ core. The accumulation of nitrite, the high abundance of archaeal amoA genes, and the deviation of the N and O isotopes of nitrate from the expected 1:1 ratio for nitrate reduction in the upper portion of this OMZ indicate that nitrification is a relevant process that fuels the denitrifier community. Conversely, the high abundances of nitrate, ammonium and nrfA genes in the deeper layer indicate that DNRA is a crucial process enhancing anammox there. These results show the need for more detailed studies of the N processes in OMZs.

Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ15N Record

AbstractDecades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ15N. We combine two δ15N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ15N variability on seasonal to centennial timescales. Seasonal variability in SBB δ15N is consistent in timing with the poleward advection of a high δ15N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ15N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ15N. Seasonal and interannual variability in sediment trap δ13Corg is consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ15N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ15N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ15N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific.

Dinitrogen Fixation Across Physico‐Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone

AbstractThe Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (&lt;20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (&gt;9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

AbstractSedimentary δ15N (δ15Nsed) has been widely applied as a proxy for water column denitrification. When combined with additional productivity proxies, it provides insights into the driving forces behind long‐term changes in water column oxygenation. High‐resolution (~2 years) δ15Nsed and productivity proxy records (total organic carbon [TOC], Si/Ti, and Ca/Ti) from Santa Barbara Basin, California, were generated from a well‐dated Kasten core (SPR0901‐03KC). These records reveal the relationship between Southern California upwelling and oxygenation over the past 2,000 years. Inconsistencies between Si/Ti (coastal upwelling proxy) and TOC (total export productivity proxy) suggest wind curl upwelling influenced Southern California primary productivity, especially during intervals of weak coastal upwelling. Coherence between δ15Nsed, TOC, and drought indicators supports a local control of δ15Nsed by atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, δ15Nsed is used as a water mass tracer of denitrification signals transported north from the eastern tropical North Pacific (ETNP) via the California Undercurrent. A 1,200‐year δ15Nsed record from the Pescadero slope, Gulf of California, lies between denitrifying subsurface waters in the ETNP and Southern California. During the Medieval Climate Anomaly, coherence between Pescadero and Santa Barbara Basin δ15Nsed indicates connections between ETNP and Southern California on centennial timescales. Yet an out‐of‐phase relationship occurred when the Aleutian Low was anomalously strong during the Little Ice Age. We suggest intensified nutrient‐rich subarctic water advection might have transported high‐15N nitrate into Southern California when the California Undercurrent and ETNP denitrification weakened.

Cyanobacteria and cyanophage contributions to carbon and nitrogen cycling in an oligotrophic oxygen-deficient zone

Up to half of marine N losses occur in oxygen-deficient zones (ODZs). Organic matter flux from productive surface waters is considered a primary control on N2 production. Here we investigate the offshore Eastern Tropical North Pacific (ETNP) where a secondary chlorophyll a maximum resides within the ODZ. Rates of primary production and carbon export from the mixed layer and productivity in the primary chlorophyll a maximum were consistent with oligotrophic waters. However, sediment trap carbon and nitrogen fluxes increased between 105 and 150 m, indicating organic matter production within the ODZ. Metagenomic and metaproteomic characterization indicated that the secondary chlorophyll a maximum was attributable to the cyanobacterium Prochlorococcus, and numerous photosynthesis and carbon fixation proteins were detected. The presence of chemoautotrophic ammonia-oxidizing archaea and the nitrite oxidizer Nitrospina and detection of nitrate oxidoreductase was consistent with cyanobacterial oxygen production within the ODZ. Cyanobacteria and cyanophage were also present on large (>30 μm) particles and in sediment trap material. Particle cyanophage-to-host ratio exceeded 50, suggesting that viruses help convert cyanobacteria into sinking organic matter. Nitrate reduction and anammox proteins were detected, congruent with previously reported N2 production. We suggest that autochthonous organic matter production within the ODZ contributes to N2 production in the offshore ETNP.

A water mass history of the Southern California current system

AbstractThe California Current System represents a confluence of different water masses originating in the subarctic, subtropical, and tropical eastern Pacific. Variations in their relative influence can alter regional biogeochemistry and ecosystem structure. We perform an optimum multiparameter analysis on historical hydrographic data to quantify the spatiotemporal variability of water mass contributions to the California Current. Within the pycnocline, a strong cross‐shore gradient in the primary water mass source reflects the dominant advective pathways within the California Current and California Undercurrent. The El Niño Southern Oscillation imparts variability in the relative contributions and depth structure of source waters, allowing stronger upwelling during La Niña to more effectively tap nutrient‐rich, oxygen‐poor waters originating in the eastern tropical North Pacific. This regional water mass history provides context for understanding the drivers and pathways of biogeochemical variability in the California Current and demonstrates that oceanic changes occurring far afield can have regionally heterogeneous impacts.

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.

Identifying the origin of nitrous oxide dissolved in deep ocean by concentration and isotopocule analyses

Nitrous oxide (N2O) contributes to global warming and stratospheric ozone depletion. Although its major sources are regarded as bacterial or archaeal nitrification and denitrification in soil and water, the origins of ubiquitous marine N2O maximum at depths of 100–800 m and N2O dissolved in deeper seawater have not been identified. We examined N2O production processes in the middle and deep sea by analyzing vertical profiles of N2O concentration and isotopocule ratios, abundance ratios of molecules substituted with rare stable isotopes 15N or 18O to common molecules 14N14N16O, in the Atlantic, Pacific, Indian, and Southern oceans. Isotopocule ratios suggest that the N2O concentration maxima is generated by in situ microbial processes rather than lateral advection or diffusion from biologically active sea areas such as the eastern tropical North Pacific. Major production process is nitrification by ammonia-oxidizing archaea (AOA) in the North Pacific although other processes such as bacterial nitrification/denitrification and nitrifier-denitrification also significantly contribute in the equatorial Pacific, eastern South Pacific, Southern Ocean/southeastern Indian Ocean, and tropical South Atlantic. Concentrations of N2O below 2000 m show significant correlation with the water mass age, which supports an earlier report suggesting production of N2O during deep water circulation. Furthermore, the isotopocule ratios suggest that AOA produce N2O in deep waters. These facts indicate that AOA have a more important role in marine N2O production than bacteria and that change in global deep water circulation could affect concentration and isotopocule ratios of atmospheric N2O in a millennium time scale.

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.

The regulation of oxygen to low concentrations in marine oxygen-minimum zones

The Bay of Bengal hosts persistent, measurable, but sub-micromolar, concentrations of oxygen in its oxygen-minimum zone (OMZ). Such low-oxygen conditions are not necessarily rare in the global ocean and seem also to characterize the OMZ of the Pescadero Basin in the Gulf of California, as well as the outer edges of otherwise anoxic OMZs, such as can be found, for example, in the Eastern Tropical North Pacific. We show here that biological controls on oxygen consumption are required to allow the semistable persistence of low-oxygen conditions in OMZ settings; otherwise, only small changes in physical mixing or rates of primary production would drive the OMZ between anoxic and oxic states with potentially large swings in oxygen concentration. We propose that two controls are active: an oxygen-dependent control on oxygen respiration and an oxygen inhibition of denitrification. These controls, working alone and together, can generate low-oxygen concentrations over a wide variability in ocean mixing parameters. More broadly, we discuss the oxygen regulation of organic matter cycling and N2 production in OMZ settings. Modern biogeochemical models of nitrogen and oxygen cycling in OMZ settings do contain some of the parameterizations that we explore here. However, these models have not been applied to understanding the persistence of low, but measurable, concentrations of oxygen in settings like the Bay of Bengal, nor have they been applied to understanding what biological/physical processes control the transition from a weakly oxygenated state to a “functionally” anoxic state with implications for nitrogen cycling. Therefore, we believe that the approach here illuminates the relationship between oxygen and the biogeochemical cycling of carbon and nitrogen in settings like the Bay of Bengal. Furthermore, we believe that our results could further inform large-scale ocean models seeking to explore how global warming might influence the spread of low-oxygen waters, influencing the cycles of oxygen, carbon, and nitrogen in OMZ settings.