Oxygen Minimum Layer Research Articles

Unraveling prokaryotic diversity distribution and functional pattern on nitrogen and methane cycling in the subtropical Western North Pacific Ocean

Prokaryotes play an important role in marine nitrogen and methane cycles. However, their community changes and metabolic modifications to the concurrent impact of ocean warming (OW), acidification (OA), deoxygenation (OD), and anthropogenic‑nitrogen-deposition (AND) from the surface to the deep ocean remains unknown. We examined here the amplicon sequencing approach across the surface (0–200 m; SL), intermediate (200–1000 m; IL), and deep layers (1000–2200 m; DL), and characterized the simultaneous impacts of OW, OA, OD, and AND on the Western North Pacific Ocean prokaryotic changes and their functional pattern in nitrogen and methane cycles. Results showed that SL possesses higher ammonium oxidation community/metabolic composition assumably the reason for excess nitrogen input from AND and modification of their kinetic properties to OW adaptation. Expanding OD at IL showed hypoxic conditions in the oxygen minimum layer, inducing higher microbial respiration that elevates the dimerization of nitrification genes for higher nitrous oxide production. The aerobic methane-oxidation composition was dominant in SL presumably the reason for adjustment in prokaryotic optimal temperature to OW, while anaerobic oxidation composition was dominant at IL due to the evolutionary changes coupling with higher nitrification. Our findings refocus on climate-change impacts on the open ocean ecosystem from the surface to the deep-environment integrating climate-drivers as key factors for higher nitrous-oxide and methane emissions.

Species distribution modeling of deep‐diving cetaceans

AbstractSpecies distribution models (SDMs) have been developed and extensively validated for diverse cetaceans within the California Current Ecosystem off the West Coast of the United States. These studies have recognized the challenges associated with developing robust models for deep‐diving cetaceans—sperm whales and beaked whales—thus limiting the accuracy of predictions for management and ecological understanding. In this study, we explore whether additional biologically relevant predictor variables can improve models for deep‐divers. These variables are related to the oxygen minimum layer and phytoplankton and micronekton biomass and could influence prey availability for cetacean top predators. We found that the addition of these variables improved the performance of SDMs for sperm whales, as well as for some more common baleen whale and dolphin species, but that the accuracy of deep‐diver models was nevertheless poor. The sightings data sets for deep‐diving cetaceans have small sample sizes compared to other cetaceans, and sightings are distributed nearly randomly across the study area and model domain. These factors hinder the development of useful environmentally driven models of spatial distribution.

Temperature independence of haemoglobin-oxygen affinity in smalleye Pacific opah (Lampris incognitus) and swordfish (Xiphias gladius).

Smalleye Pacific opah and swordfish can conserve metabolic heat and maintain specific body regions warmer than ambient water temperature (i.e. regional heterothermy). Consequently, blood O2 uptake at the gills occurs at the environmental temperature at which the individual is found, but O2 offloading will occur at different temperatures in different tissues. While several regionally heterothermic fishes (e.g. billfishes, tunas and sharks) show a reduced temperature effect on haemoglobin (Hb)-O2 affinity, the temperature dependence of Hb-O2 affinity in opah and swordfish is unknown. We hypothesized that the Hb of opah and swordfish would also show a reduced temperature dependence. Opah whole-blood-O2 affinity exhibited a reverse temperature dependence above 50% Hb-O2 saturation (10-20°C, pH7.2-8.0), while the temperature dependence of swordfish blood-O2 affinity (10-25°C) was saturation and pH dependent, becoming temperature independent below 50% Hb-O2 saturation and pH7.4. Experiments on stripped haemolysates showed that adding ATP ([ATP]/[Hb]=30) decreased the temperature sensitivity of Hb-O2 affinity, changing the overall oxygenation enthalpy (ΔH') values of opah (10-20°C) and swordfish (10-25°C) Hbs at pH7.4 from -15 and -42 kJ mol-1 O2, respectively, to +84 and -9 kJ mol-1 O2. Swordfish blood-O2 affinity was high compared with that of other large, pelagic, marine teleosts, which may be the result of unusually low ATP/Hb levels, but might also enable swordfish to forage in the potentially low-oxygenated water of the upper reaches of the oxygen minimum layer. The existence of Hbs with reduced temperature sensitivity in regionally heterothermic fishes may prevent marked changes in Hb-O2 affinity between the cold and warm tissues.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.

Vertical Profiles of 226Ra and 228Ra Activity Concentrations in the Western Subarctic Gyre of the Pacific Ocean

The vertical activity concentration distributions of the radium isotopes, 226Ra and 228Ra, which are useful as radiotracers, were obtained for the first time in the western Subarctic Pacific Gyre (WSAG). It was possible to detect short-lived 228Ra present from the surface to 400 m depth by analyzing large seawater samples. Low 228Ra and high 226Ra activity concentrations in the surface layer in the WSAG were strongly influenced by upwelled deeper water with nutrients. The activity concentration distribution of 226Ra especially was in good agreement with the silicate concentration distribution, which was consistent with previous reported findings. These distributions were uniform from the surface to 100 m of the dichothermal layer due to vertical mixing in winter. 228Ra activity concentration decreased with water depth below the pycnocline and reached the undetectable level at 600 m which was within the oxygen minimum layer. Estimations of vertical fluxes of 228Ra and nitrate according to 228Ra decay indicated that the vertical transport by eddy diffusion was a minor process for the 228Ra and nitrate fluxes, and lateral transport mainly affected 228Ra in the intermediate warm water, that is, the mesothermal layer below the main pycnocline. Vertical mixing or submarine underground water and lateral transport to the WSAG could yield this 228Ra in the intermediate depth.

Vertical changes in abundance, biomass and community structure of pelagic polychaetes down to 1000 m depths at Station K2 in the western subarctic Pacific Ocean covering the four seasons and day–night

Abstract Pelagic polychaetes are distributed from the sea surface to greater depths of the oceans worldwide, but little information is available regarding their ecology. This study investigated the vertical distribution of abundance, biomass and community structure of pelagic polychaetes at a single station in the western subarctic Pacific based on day–night vertical stratified samples collected from a 0 to 1000 m water column during four seasons covering 1 year. The polychaete abundance and biomass ranged from 0 to 757 ind. 1000 m−3 and 0–6.1 mg WW m−3, respectively. Ten pelagic polychaete species belonging to nine genera and six families were identified. From cluster analysis based on abundance, the polychaete community was divided into five communities. Each community occurred at different depth layers. Two surface groups seen at 0–200 m were dominated by two carnivorous species: Tomopteris septentrionalis and Typhloscolex muelleri. The deepest group, dominated by the particle feeder Pelagobia longicirrata, was seen at 500–1000 m. Two transition groups occurred in the intermediate depths. These vertical distributions of the pelagic polychaete communities were common throughout the season and day. Water mass, food availability and the oxygen minimum layer are put forth as environmental factors that affect the pelagic polychaete community.

Eddies Affect Subsurface Phytoplankton and Oxygen Distributions in the North Pacific Subtropical Gyre

AbstractThe North Pacific Subtropical Gyre (NPSG) is an oligotrophic environment where a number of mesoscale eddies occur. With continuous measurements from Biogeochemical‐Argo (BGC‐Argo) floats, we showed that mesoscale eddies can significantly affect the subsurface chlorophyll maximum (SCM) and subsurface biogenic particles. Different responses of the SCM to the cyclonic and anticyclonic eddies and those between the eddy core and edge region were revealed by combining the results from tracked eddies. The variations in the SCM and subsurface particles were further shown to be statistically linked with changes in the dissolved oxygen in the upper oxygen minimum layer. Such an eddy‐induced oxygen change in the central gyre is unresolved in global‐scale coupled models but can contribute to the oxygen variability in oligotrophic environments.

Investigating the effect of El Niño on nitrous oxide distribution in the eastern tropical South Pacific

Abstract. The open ocean is a major source of nitrous oxide (N2O), an atmospheric trace gas attributable to global warming and ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic upwelling regions such as the eastern tropical South Pacific (ETSP). The ETSP is influenced by the El Niño–Southern Oscillation that leads to inter-annual variations in physical, chemical, and biological properties in the water column. In October 2015, a strong El Niño event was developing in the ETSP; we conduct field observations to investigate (1) the N2O production pathways and associated biogeochemical properties and (2) the effects of El Niño on water column N2O distributions and fluxes using data from previous non-El Niño years. Analysis of N2O natural abundance isotopomers suggested that nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were occurring in the near-surface waters; indicating that both pathways contributed to N2O effluxes. Higher-than-normal sea surface temperatures were associated with a deepening of the oxycline and the oxygen minimum layer. Within the shelf region, surface N2O supersaturation was nearly an order of magnitude lower than that of non-El Niño years. Therefore, a significant reduction of N2O efflux (75 %–95 %) in the ETSP occurred during the 2015 El Niño. At both offshore and coastal stations, the N2O concentration profiles during El Niño showed moderate N2O concentration gradients, and the peak N2O concentrations occurred at deeper depths during El Niño years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. At multiple stations, water-column inventories of N2O within the top 1000 m were up to 160 % higher than those measured in non-El Niño years, indicating that subsurface N2O during El Niño could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Niño.

A global meta‐analysis of marine predator nitrogen stable isotopes: Relationships between trophic structure and environmental conditions

AbstractAimWe examined potential environmental drivers of broad‐scale spatial patterns in the trophic structure of marine ecosystems as represented by nitrogen stable isotopes in globally distributed marine predators. Additionally, we assessed the effects of spatial scale on the predictive capabilities of environmental variables.LocationGlobal oceans.Time period2000 to 2015.Major taxa studiedTunas: Thunnus albacares, Thunnus obesus, Thunnus alalunga.MethodsWe undertook a global compilation and meta‐analysis of the bulk nitrogen stable isotope ratios (δ15N values) of three tuna species (n = 4,281). After adjusting for regional variations in baseline δ15N values using a global ocean biogeochemistry model, generalized additive mixed models were employed to infer global‐scale oceanographic controls of trophic structure, using cosmopolitan tuna species as a model.ResultsFor the three tuna species, variation in trophic position estimated using bulk δ15N values was largely explained by geographical location and the corresponding oxygen minimum layer depth. Tuna trophic positions declined in areas with reduced oxygen at depth. Food‐chain length, as captured by maximum trophic position, was longer in areas of the western Pacific Ocean and shorter in the northern Atlantic and eastern Pacific Oceans. Trophic adaptability of the tuna predators, as indicated by intraspecific variability, was highest in the western and central Pacific Ocean and lowest in the northern Atlantic Ocean. Our analysis demonstrated that while tunas share similar functional trophic roles, deeper‐foraging tuna species had higher trophic positions globally. The predictive capacity of environmental variables decreased at finer (regional) spatial scales.Main conclusionsOur work suggests that habitat compression resulting from the predicted global expansion of oxygen minimum zones with ocean warming will impact the trophic structure of marine food webs and the corresponding foraging habits of marine predators. Spatial scale analyses highlighted the importance of representing differences in regional ecological dynamics in global‐scale trophic and ecosystem models.

Temperature control of cold-water coral (Lophelia) mound growth by climate-cycle forcing, Northeast Gulf of Mexico

Chronostratigraphy, of a northern Gulf of Mexico Lophelia pertusa (cold-water coral) mound (Roberts Reef), indicated that mound development included periods of growth and non-growth that are closely tied to Late Pleistocene climate cycles. U/Th and 14C dating of coral fragments in two ~ 16 m cores (mound crest and mound flank) and foraminiferal biostratigraphy provided a stratigraphic framework that identified five mound-growth sequences separated by four hiatuses, a total of ~ 300 ka. Mound growth occurred during interglacials while non-growth and unconformity formation correlates to glacial periods. In each cycle, growth on the mound flank persisted after growth at the crest ceased.Results of our study suggest warming bottom-water as the most probable environmental condition for limiting L. pertusa growth and mound accretion as sea level fell toward a glacial maximum. At highstands of sea level, the mound resided in the Oxygen Minimum Layer with a present temperature range of ~ 9.0–12.8 °C. During falling-to-lowstands of sea level, isotherms of warm, Subtropical Underwater (~ 12.8–21 °C) progressively exposed the mound to water above the thermal range for L. pertusa.A comparison of non-growth periods in the mound cores indicates that coral mortality and suppression of mound growth started at the crest and moved down the flank, a trend that supports progressive immersion of the mound in warm Subtropical Underwater. Multiday episodes of vertical down-mixing may have also transported warm ocean water, above the tolerance level of Lophelia, to the mound.Present average bottom-water temperatures at the mound crest range between 9 and 10.5 °C, ideal for L. pertusa growth in the Gulf of Mexico. Experimental work indicates that relatively short periods (weeks) of L. pertusa exposure to water of ~ 12–14 °C results in mortality. At periods of lowered sea level, the mound is projected to have been immersed in ocean water of ~ 12–14 °C for millennial scale time periods.

Diversity of culturable nitrate-reducing bacteria from the Arabian Sea oxygen minimum zone

The subsurface waters of the northern Arabian Sea display a pronounced oxygen minimum layer associated with high nitrite maxima which provide an ideal niche for organisms that can respire nitrate. Culture-based studies elaborate the physiological characteristics of the organisms and their metabolic activities in biogeochemical cycles. In this study, the bacterial diversity and nitrate utilizing activity of the culturable heterotrophic bacteria inhabiting the water column oxygen minimum zone of Arabian Sea were investigated. Nitrate-reducing bacteria were isolated from the water column in the central Arabian Sea. Genotypic characterization of the isolates using 16 S rDNA gene sequencing grouped them into three phylogenetic groups i.e. Proteobacteria, Firmicutes, and Actinobacteria. Out of the 56 isolated bacteria, 45 strains belonged to Proteobacteria, 6 to Firmicutes and 5 to Actinobacteria. The nitrate reducing ability of the isolates was tested using Griess test. Thirty-six species belonging to genera Alcanivorax, Alteromonas, Halomonas, Pseudoalteromonas, Marinobacter, Bacillus, and Vibrio were positive for NO2-/NO3- reduction. Our results imply that cultivable bacteria capable of utilizing NO3- available in the system are present in the Arabian Sea oxygen minimum zone and the conditions existing therein must be favorable for their growth and functionality.

Influence of temperature and oxygen on the distribution of blue marlin (Makaira nigricans) in the Central Pacific

AbstractThe blue marlin (Makaira nigricans) is a highly migratory pelagic predator of tropical and subtropical seas. Information on the habitat use of marine species is fundamental to understanding their ecology and population dynamics and is needed to inform responsible management strategies. Using a long‐term satellite tagging data set from The International Game Fish Association Great Marlin Race, we examined habitat use and how oxygen and temperature influence the horizontal and vertical distributions of blue marlin in the Central Pacific. Blue marlin primarily occurred in warm waters (26–30°C) and exhibited a diel bimodal depth distribution across the 5‐year data record (2009–2013), with fish spending the majority of their time near the surface at night and at deeper depths during the day (25–100 m). The depth distribution of blue marlin was limited in areas where low oxygen and/or temperature conditions occur closer to the surface, with the extent of habitat compression being greatest when both oxygen and temperature were limiting. The migrations of blue marlin appeared restricted during the 2010 La Niña, when increased equatorial upwelling resulted in an extension of the cold, low oxygen waters of the cold tongue into the Central Pacific, creating a barrier to the trans‐equatorial migrations that occurred during all other tagging years. If the frequency and intensity of La Niña events increases and the oxygen minimum layer continues to expand as has been predicted under certain climate change scenarios, the migratory behavior and habitat availability of blue marlin may be impacted.

Multidecadal trends of oxygen and their controlling factors in the western North Pacific

AbstractThe rate of change of dissolved oxygen (O2) concentrations was analyzed over 1987–2011 for the high‐frequency repeat section along 165°E in the western North Pacific. Significant trends toward decreasing O2 were detected in the northern subtropical to subtropical‐subarctic transition zones over a broad range of isopycnal horizons. On 25.3σθ between 25°N and 30°N in North Pacific Subtropical Mode Water, the rate of O2 decrease reached −0.45 ± 0.16 µmol kg−1 yr−1. It is largely attributed to a deepening of isopycnal horizons and to a reduction in oxygen solubility associated with ocean warming. In North Pacific Intermediate Water, the rate of O2 decrease was elevated (−0.44 ± 0.14 µmol kg−1 yr−1 on 26.8σθ) and was associated with net increases in apparent oxygen utilization in the source waters. On 27.3σθ in the subtropical Oxygen Minimum Layer (OML) between 32.5°N and 35°N, the rate of O2 decrease was significant (−0.22 ± 0.05 µmol kg−1 yr−1). It was likely due to the increases in westward transport of low‐oxygen water. These various drivers controlling changes in O2 along the 165°E section are the same as those acting along 137°E (analyzed previously) and also account for the differences in the rate of O2 decrease between these sections. Additionally, in the tropical OML near 26.8σθ between 5°N and 10°N, significant trends toward increasing O2 were detected in both sections (+0.36 ± 0.04 µmol kg−1 yr−1 in the 165°E section). These results demonstrate that warming and circulation changes are causing multidecadal changes in dissolved O2 over wide expanses of the western North Pacific.

Processes controlling mid‐water column oxygen minima over the Texas‐Louisiana shelf

AbstractWe investigate distributions of dissolved oxygen over the Texas‐Louisiana shelf using spatially highly resolved observations in combination with a regional circulation model with simple oxygen dynamics. The observations were collected using a towed, undulating CTD during the Mechanisms Controlling Hypoxia (MCH) program. Mid‐water oxygen minimum layers (dissolved oxygen lower than 3.2 mL L−1) were detected in many transects. These oxygen minimum layers are connected with the bottom boundary layer and follow the pycnocline seaward as a tongue of low oxygen into the mid‐water column. T‐S diagrams highlighting the low oxygen minima in both observations and simulations imply direct connections between low‐oxygen bottom water and the oxygen minimum layer. The dynamics of these oxygen minimum layers in the mid‐water column are examined using a three‐dimensional hydrodynamic model, based on the Regional Ocean Modeling System (ROMS). Convergence within the bottom boundary layer relative to density surfaces is calculated, results show that there is a convergence in the bottom boundary layer at the location where the pycnocline intersects the bottom. Buoyancy advection forced by bottom Ekman transport creates this convergent flow, and the corresponding low‐oxygen intrusion. Similar intrusions of near‐bottom water into the pycnocline are observed in other regions. The presence of hypoxia within the bottom boundary layer in the northern Gulf of Mexico creates a unique situation in which these intrusions are also associated with low dissolved oxygen.

The Silurian of south-western margin of the East European Platform (Ukraine, Moldova and Romania): lithofacies and palaeoenvironments

Silurian strata, stretching along the western margin of the East European Platform from the Baltic to the Black Sea, represent a potential target for both conventional and unconventional hydrocarbon exploration. Distribution of the black shale facies, prospective for shale gas, and the reef facies, prospective for oil, has been studied in respect of palaeoenvironments. The Silurian sequence has been investigated in the territory of Ukraine (Volyn-Podillyan Plate, Dobrogean Foredeep) and correlated with the data on Moldova and Romania (Moldovian Platform). The occurrence of Silurian strata, their thickness, and petrographic and lithological characteristics allowed reconstructing the distribution of open-shelf, reef and lagoonal facies. The reef facies migrated during the Wenlock–Middle Pridoli, shifting towards the open sea and back towards the shore, and therefore has been termed a migrating reef facies. Correspondingly, the boundary between the open-shelf and reef facies was shifting. The facies distribution was controlled by the transgressive-regressive cycles, which caused the fluctuations of the shelf water depth in different time intervals of the Silurian. The shelf water depth of about 100 m, where the top of the oxygen-minimum layer impinged on the sea bottom, was the boundary between the open-shelf facies, represented by organic-rich sediments, and the reef buildups.

Some aspects of excellent marine source rock formation: implications on enrichment regularity of organic matter in continental margin basins

Through the analysis of ocean organisms, the distribution characteristics and enrichment of organic matters in modern marine sediments and ancient marine strata, this paper shows that the main factors influencing the formation of excellent marine source rocks are the paleoclimate, biologic productivity, terrestrial organic matter, oxidation–reduction environment, sedimentation rate, and the type of the basin. In addition to those factors, high biologic productivity or high content of terrestrial organic matter input is a requirement for the enrichment of the organic matter in a marine environment. Reducing environment was favorable for organic matter accumulation and preservation in depositing and early diagenesis stage, which is an important element for the formation of high-quality marine source rocks. Paleoclimate also influences the marine source rocks formation, as humid subtropical and tropical climates are the most favorable regimes for the formation of marine source rocks. Wind transports some vascular plant materials into the marine environment. Furthermore, upwellings driven by steady wind can cause high biologic productivity, thus forming organic-C-rich mud. Suitable sedimentation rate is beneficial for marine organic matter accumulation. Moreover, the type of the basin also plays an important role in the development of marine source rocks. Silled basins with a positive water balance often act as nutrient traps, thus enhancing both productivity and organic matter preservations, while in open oceans, organic matter enrichment in sediments has just been found in the oxygen minimum layers.

Ultrahigh resolution mass spectrometric differentiation of dissolved organic matter isolated by coupled reverse osmosis-electrodialysis from various major oceanic water masses

A high-recovery technique of dissolved organic matter (DOM) isolation – reverse osmosis coupled with electrodialysis (RO/ED) – was used to isolate DOM from the North Atlantic Senegal-Mauritanian upwelling area surface water (5m), North Atlantic oxygen minimum water (415m) and deep water (3000m), North Pacific subtropical gyre surface water (5m), and North Pacific intermediate water (674m) and deep water (3500m). Samples were characterized by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization (ESI FTICR-MS). RO/ED isolated DOM samples share a significant number of common formulas accounting for 54–79% of formulas in each sample. Total dissolved carbohydrate (TCHO) concentrations in RO/ED isolated DOM were specifically measured using a colorimetric method, and were found to have higher contribution to DOC than estimated by FTICR-MS data. Percentages of TCHO-C in DOC are in the range of 3.7–19.6% in all samples, with the North Pacific deep (3500m) water having the lowest % and the North Atlantic upwelling core surface water having the highest %. Principal component analysis (PCA) using the relative magnitudes of MS peaks facilitated identification of specific peaks that are enriched in different samples. Peaks enriched in surface samples have higher H/C values than peaks enriched in deep samples, in both the North Atlantic DOM and the North Pacific DOM. This enrichment pattern is likely due to the selective photo-degradation of aromatic compounds and the bio-production of aliphatic and carbohydrate-like compounds in surface waters, and the selective bio-degradation of aliphatic and carbohydrate-like compounds with increasing depth. In further support of a photo-degraded signature for DOM in surface waters, photo-resistant and photo-produced molecular formulas were present in the highest numbers in the surface North Pacific subtropical gyre DOM. Peaks enriched in the North Pacific intermediate and deep DOM have significantly higher O/C values than the North Atlantic oxygen minimum layer and deep DOM, for both CHO formula compounds and CHON formula compounds. This difference in O/C values observed for the deep Pacific vs. Atlantic suggests oxidation of DOM, possibly via microbial activity during the ageing of DOM or the preferential remineralization of DOM from sinking particles at depth in the Pacific.

Strong Depth-Related Zonation of Megabenthos on a Rocky Continental Margin (∼700–4000 m) off Southern Tasmania, Australia

Assemblages of megabenthos are structured in seven depth-related zones between ∼700 and 4000 m on the rocky and topographically complex continental margin south of Tasmania, southeastern Australia. These patterns emerge from analysis of imagery and specimen collections taken from a suite of surveys using photographic and in situ sampling by epibenthic sleds, towed video cameras, an autonomous underwater vehicle and a remotely operated vehicle (ROV). Seamount peaks in shallow zones had relatively low biomass and low diversity assemblages, which may be in part natural and in part due to effects of bottom trawl fishing. Species richness was highest at intermediate depths (1000–1300 m) as a result of an extensive coral reef community based on the bioherm-forming scleractinian Solenosmilia variabilis. However, megabenthos abundance peaked in a deeper, low diversity assemblage at 2000–2500 m. The S. variabilis reef and the deep biomass zone were separated by an extensive dead, sub-fossil S. variabilis reef and a relatively low biomass stratum on volcanic rock roughly coincident with the oxygen minimum layer. Below 2400 m, megabenthos was increasingly sparse, though punctuated by occasional small pockets of relatively high diversity and biomass. Nonetheless, megabenthic organisms were observed in the vast majority of photographs on all seabed habitats and to the maximum depths observed - a sandy plain below 3950 m. Taxonomic studies in progress suggest that the observed depth zonation is based in part on changing species mixes with depth, but also an underlying commonality to much of the seamount and rocky substrate biota across all depths. Although the mechanisms supporting the extraordinarily high biomass in 2000–2500 m depths remains obscure, plausible explanations include equatorwards lateral transport of polar production and/or a response to depth-stratified oxygen availability.

The Distribution of Vertically Migrating Fishes Across the Central Equatorial Pacific

AbstractMidwater trawl samples of vertically migrating fishes indicated four latitudinal faunal zones in the central equatorial Pacific between Hawaii and Tahiti. Boundaries of the northernmost and southernmost zones coincided with the boundaries between the equatorial and central water masses. The equatorial water mass was divided into two zones with distinctly different faunas. Of the 112 species considered, 20 occurred in all zones, and most of the remainder fell into one of six groups based upon pattern of occurrence or absence in each of the faunal zones. Correlation of faunal boundaries with oceanographic features in the study area and consideration of distributional data on the species in other areas indicate that differences in thermal structure, and consequent differences in productivity, between the major water masses could account for some barriers to distribution, but that differences in the intensity and vertical extent of the oxygen minimum layer constitute barriers both within and between w...

Water column denitrification rates in the oxygen minimum layer of the Pacific Ocean along 32°S

AbstractTo date, estimates of water column denitrification rates in global‐ and basin‐scale nitrogen budgets do not include any values obtained in the open ocean outside the oxygen minimum zones (OMZs). The high‐quality, 32°S Pacific repeat hydrographic surveys conducted in 1992, 2003, and 2010 offer an opportunity to estimate denitrification rates on basin scales in the open ocean away from the Eastern Tropical Pacific (ETP) oxygen minimum zones. Here, extended optimum multiparameter analysis and N* are used to estimate water column denitrification (ΔNdeni) in the oxygen minimum layer (OML; 1100–2000 dbar). Water column denitrification rates for 2003 and 2010 are estimated from the regression analysis using ΔNdeni and transit time distribution ages based on CFC12. Estimated mean denitrification rates in the OML at 32°S range from 3.1 to 7.5 µmol N m−3 yr−1 (=2.6–6.8 mmol N m−2 yr−1 with a thickness of 900 m). The OML denitrification rates are 2–3 orders of magnitudes lower than those estimated in the OMZs of ETP. However, their significance to Pacific nitrogen budget needs to be investigated in the future. Although uncertainties remain, our results may ultimately contribute to an improved understanding of marine nitrogen cycles.