Expansion of the eastern North Pacific OMZ and the associated denitrification regime

2020 ◽  
Author(s):  
Allan Devol ◽  
Wendi Ruef
Keyword(s):  
Pacific Decadal Oscillation ◽  
North Pacific ◽  
Arabian Sea ◽  
Thermocline Depth ◽  
Fixed Nitrogen ◽  
Potential Density ◽  
Hurricane Intensity ◽  
Oxygen Minimum Zones ◽  
Marine Areas ◽  
Oxygen Minimum

<p> </p><p>At this point ocean deoxygenation is well documented, including in oxygen minimum zones (OMZs).  Within the large OMZs of the Arabian Sea and eastern Pacific are imbedded areas where oxygen concentrations are so low that they are undetectable by routine CTD sensors (oxygen deficient zones, ODZs).  How do we determine if these ODZ are losing O<sub>2</sub>?  Furthermore, denitrification occurs in oxygen minimum zones (OMZs) so one might hypothesize that denitrification is likewise expanding if oxygen is decreasing.  This is important because the ocean's fixed nitrogen inventory limits the productivity over large marine areas.</p><p>We have investigated these questions in the largest OMZ, the eastern tropical North Pacific (ETNP) through an analysis of  6 repeats of a 1000 km transect along 110<sup>o</sup> West in the heart of the ETNP ODZ between 1971-2019.  We use N*, a stoichiometric parameter calculated from nitrate and phosphate, as our indicator of denitrification. The more Negative N* the more denitrification has occurred. After secondary QC the values of O<sub>2</sub> concentration between potential density 24.75 and 1000m along with N* were integrated across the transect and over the depth of the ODZ.  </p><p>The results show a clear decrease in oxygen inventory along with an increase in N*, suggesting deoxygenation and intensification of denitrification over during the 50 year period. We discuss potential mechanisms for denitrification signal increase including ENSO, Pacific Decadal Oscillation, tropical hurricane intensity, and variations in thermocline depth.</p>

scholarly journals Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

2021 ◽  
Author(s):  
Martin Tetard ◽  
Laetitia Licari ◽  
Kazuyo Tachikawa ◽  
Ekaterina Ovsepyan ◽  
Luc Beaufort
Keyword(s):  
North Pacific ◽  
Transfer Functions ◽  
Late Quaternary ◽  
Global Climate ◽  
Single Species ◽  
Pore Waters ◽  
Oxygen Minimum Zones ◽  
Global Calibration ◽  
Circularity Index ◽  
Oxygen Minimum

Abstract. Oxygen Minimum Zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. Their ecological and economic consequences are being debated. The investigation of past OMZ conditions allows us to better understand biological and physical mechanisms responsible for their variability with regards to climate change, carbon pump and carbonate system. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed, but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxonfree morphometric analysis of benthic foraminiferal tests, developed and calibrated using Eastern North Pacific (ENP) and the Eastern South Pacific (ESP) OMZs samples. This new approach is based on an average size and circularity index for each sample. This method, as well as two already published micropalaeontological approaches based on benthic foraminiferal assemblages variability and porosity investigation of a single species, were here calibrated based on availability of new data from 23 core tops recovered along an oxygen gradient (from 0.03 to 1.79 mL.L−1) from the ENP, ESP, AS (Arabian Sea) and WNP (Western North Pacific, including its marginal seas) OMZs. Global calibrated transfer functions are thus herein proposed for each of these methods. These micropalaeontological reconstruction approaches were then applied on a paleorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblages and morphometric approaches (that is also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) shown ambiguous estimations.

scholarly journals High-throughput screening of sediment bacterial communities from Oxygen Minimum Zones of the northern Indian Ocean

2019 ◽  
Author(s):  
Jovitha Lincy ◽  
Cathrine Manohar
Keyword(s):  
Indian Ocean ◽  
Bacterial Diversity ◽  
Bay Of Bengal ◽  
High Throughput Screening ◽  
Arabian Sea ◽  
Northern Indian Ocean ◽  
Oxygen Minimum Zones ◽  
Sediment Biogeochemistry ◽  
Generation Sequencing ◽  
Oxygen Minimum

Abstract. The Northern Indian Ocean host two recognized Oxygen Minimum Zones (OMZ): one in the Arabian Sea and the other in the Bay of Bengal region. The next-generation sequencing technique was used to understand the total bacterial diversity from the surface sediment of off Goa within the OMZ of Arabian Sea, and from off Paradip within the OMZ of Bay of Bengal. The dominant phyla identified include Firmicutes (33.06 %) and Proteobacteria (32.44 %) from the Arabian Sea, and Proteobacteria (52.51 %) and Planctomycetes (8.63 %) from the Bay of Bengal. Statistical analysis indicates that bacterial diversity from sediments of the Bay of Bengal OMZ is ~ 48 % higher than the Arabian Sea OMZ. Diverse candidate bacterial clades were also detected, whose function is unknown, but many of these were reported from other OMZs as well, suggesting their putative role in sediment biogeochemistry. Bacterial diversity from the present study reveals that the off Paradip site of Bay of Bengal OMZ is highly diverse and unexplored in comparison to the off Goa site of the Arabian Sea OMZ. Functional diversity analysis indicates that the relative percentage distribution of genes involved in methane, nitrogen, sulfur and many unclassified energy metabolisms is almost the same in both sites, reflecting a similar ecological role, irrespective of the differences in phylotypic diversity.

scholarly journals Oxygen at Nanomolar Levels Reversibly Suppresses Process Rates and Gene Expression in Anammox and Denitrification in the Oxygen Minimum Zone off Northern Chile

mBio ◽  
2014 ◽  
Vol 5 (6) ◽  
Author(s):  
Tage Dalsgaard ◽  
Frank J. Stewart ◽  
Bo Thamdrup ◽  
Loreto De Brabandere ◽  
Niels Peter Revsbech ◽  
...  
Keyword(s):  
Nitrogen Removal ◽  
Nitrogen Cycle ◽  
Nitrite Reductase ◽  
Carbon Fixation ◽  
Nitrogen Loss ◽  
Enzyme Level ◽  
Taxonomic Composition ◽  
Fixed Nitrogen ◽  
Oxygen Minimum Zones ◽  
Oxygen Minimum

ABSTRACTA major percentage (20 to 40%) of global marine fixed-nitrogen loss occurs in oxygen minimum zones (OMZs). Concentrations of O2and the sensitivity of the anaerobic N2-producing processes of anammox and denitrification determine where this loss occurs. We studied experimentally how O2at nanomolar levels affects anammox and denitrification rates and the transcription of nitrogen cycle genes in the anoxic OMZ off Chile. Rates of anammox and denitrification were reversibly suppressed, most likely at the enzyme level. Fifty percent inhibition of N2and N2O production by denitrification was achieved at 205 and 297 nM O2, respectively, whereas anammox was 50% inhibited at 886 nM O2. Coupled metatranscriptomic analysis revealed that transcripts encoding nitrous oxide reductase (nosZ), nitrite reductase (nirS), and nitric oxide reductase (norB) decreased in relative abundance above 200 nM O2. This O2concentration did not suppress the transcription of other dissimilatory nitrogen cycle genes, including nitrate reductase (narG), hydrazine oxidoreductase (hzo), and nitrite reductase (nirK). However, taxonomic characterization of transcripts suggested inhibition ofnarGtranscription in gammaproteobacteria, whereas the transcription of anammoxnarG, whose gene product is likely used to oxidatively replenish electrons for carbon fixation, was not inhibited. The taxonomic composition of transcripts differed among denitrification enzymes, suggesting that distinct groups of microorganisms mediate different steps of denitrification. Sulfide addition (1 µM) did not affect anammox or O2inhibition kinetics but strongly stimulated N2O production by denitrification. These results identify new O2thresholds for delimiting marine nitrogen loss and highlight the utility of integrating biogeochemical and metatranscriptomic analyses.IMPORTANCEThe removal of fixed nitrogen via anammox and denitrification associated with low O2concentrations in oceanic oxygen minimum zones (OMZ) is a major sink in oceanic N budgets, yet the sensitivity and dynamics of these processes with respect to O2are poorly known. The present study elucidated how nanomolar O2concentrations affected nitrogen removal rates and expression of key nitrogen cycle genes in water from the eastern South Pacific OMZ, applying state-of-the-art15N techniques and metatranscriptomics. Rates of both denitrification and anammox responded rapidly and reversibly to changes in O2, but denitrification was more O2sensitive than anammox. The transcription of key nitrogen cycle genes did not respond as clearly to O2, although expression of some of these genes decreased. Quantifying O2sensitivity of these processes is essential for predicting through which pathways and in which environments, from wastewater treatment to the open oceans, nitrogen removal may occur.

scholarly journals Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters

2016 ◽  
Vol 113 (38) ◽  
pp. 10601-10606 ◽  
Author(s):  
Laura A. Bristow ◽  
Tage Dalsgaard ◽  
Laura Tiano ◽  
Daniel B. Mills ◽  
Anthony D. Bertagnolli ◽  
...  
Keyword(s):  
Nitrogen Loss ◽  
Strong Dependence ◽  
Ammonium Oxidation ◽  
Nitrite Oxidation ◽  
Fixed Nitrogen ◽  
Oxygen Minimum Zones ◽  
Oxidation Rates ◽  
Minimum Zone ◽  
Nanomolar Range ◽  
Oxygen Minimum

A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O2) sensitivity of these processes down to the O2concentrations present in the OMZ core (<10 nmol⋅L−1) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O2levels between 5 nmol⋅L−1and 20 μmol⋅L−1. Rates of both processes were detectable in the low nanomolar range (5–33 nmol⋅L−1O2), but demonstrated a strong dependence on O2concentrations with apparent half-saturation constants (Kms) of 333 ± 130 nmol⋅L−1O2for ammonium oxidation and 778 ± 168 nmol⋅L−1O2for nitrite oxidation assuming one-component Michaelis–Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis–Menten model, indicating a high-affinity component with aKmof just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O2affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O2is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.

scholarly journals Microbial diversity of the Arabian Sea in the Oxygen minimum zones by metagenomics approach

2019 ◽  
Author(s):  
Mandar S. Paingankar ◽  
Kedar Ahire ◽  
Pawan Mishra ◽  
Shriram Rajpathak ◽  
Deepti D. Deobagkar
Keyword(s):  
Community Structure ◽  
Microbial Community ◽  
Arabian Sea ◽  
Community Dynamics ◽  
Oxygen Depletion ◽  
Oxygen Minimum Zones ◽  
Microbial Community Dynamics ◽  
Nitrate Reducers ◽  
Insight Into ◽  
Oxygen Minimum

ABSTRACTLarge oxygen depleted areas known as oxygen minimum zones (OMZ) have been observed in the Arabian Sea and recent reports indicate that these areas are expanding at an alarming rate. In marine waters, oxygen depletion may also be related to global warming and the temperature rise, acidification and deoxygenation can lead to major consequences wherein the plants, fish and other biota will struggle to survive in the ecosystem.The current study has identified the microbial community structure using NGS based metagenomics analysis in the water samples collected at different depth from the oxygen depleted and non-OMZ areas of Arabian Sea. Environmental variables such as depth, site of collection and oxygen concentration appeared to influence the species richness and evenness among microbial communities in these locations. Our observations clearly indicate that population dynamics of microbes consisting of nitrate reducers accompanied by sulphate reducers and sulphur oxidizers participate in the interconnected geochemical cycles of the OMZ areas. In addition to providing baseline data related to the diversity and microbial community dynamics in oxygen-depleted water in the OMZ; such analysis can provide insight into processes regulating productivity and ecological community structure of the ocean.

scholarly journals Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

2021 ◽  
Vol 18 (9) ◽  
pp. 2827-2841
Author(s):  
Martin Tetard ◽  
Laetitia Licari ◽  
Ekaterina Ovsepyan ◽  
Kazuyo Tachikawa ◽  
Luc Beaufort
Keyword(s):  
North Pacific ◽  
Transfer Functions ◽  
Late Quaternary ◽  
Global Climate ◽  
Single Species ◽  
Shape Index ◽  
Pore Waters ◽  
Oxygen Minimum Zones ◽  
Global Calibration ◽  
Oxygen Minimum

Abstract. Oxygen minimum zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxon-independent morphometric analysis of benthic foraminiferal tests, developed and calibrated using WNP (western North Pacific, including its marginal seas), ENP (eastern North Pacific), and ESP (eastern South Pacific) OMZ samples. This new approach is based on an average size and shape index for each sample. This method, as well as two already published micropalaeontological techniques based on benthic foraminiferal assemblages' variability and porosity investigation of a single species, was calibrated here based on availability of new data from 45 core tops recovered along an oxygen gradient (from 0.03 to 2.88 mL L−1) from the WNP, ENP, EEP (eastern Equatorial Pacific), ESP, SWACM (southwest African continental margin), and AS (Arabian Sea) OMZs. Global calibrated transfer functions are herein proposed for these methods. These micropalaeontological reconstruction approaches were then applied to a palaeorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblage and morphometric approaches (which are also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) showed ambiguous estimations.

Centennial changes in North Pacific anoxia linked to tropical trade winds

Science ◽  
2014 ◽  
Vol 345 (6197) ◽  
pp. 665-668 ◽  
Author(s):  
Curtis Deutsch ◽  
William Berelson ◽  
Robert Thunell ◽  
Thomas Weber ◽  
Caitlin Tems ◽  
...  
Keyword(s):  
Climate Warming ◽  
North Pacific ◽  
Sediment Cores ◽  
Equatorial Pacific ◽  
Biological Productivity ◽  
Anoxic Zone ◽  
Trade Winds ◽  
Oxygen Minimum Zones ◽  
Oxygen Minimum ◽  
The Tropical Pacific

Climate warming is expected to reduce oxygen (O2) supply to the ocean and expand its oxygen minimum zones (OMZs). We reconstructed variations in the extent of North Pacific anoxia since 1850 using a geochemical proxy for denitrification (δ15N) from multiple sediment cores. Increasing δ15N since ~1990 records an expansion of anoxia, consistent with observed O2 trends. However, this was preceded by a longer declining δ15N trend that implies that the anoxic zone was shrinking for most of the 20th century. Both periods can be explained by changes in winds over the tropical Pacific that drive upwelling, biological productivity, and O2 demand within the OMZ. If equatorial Pacific winds resume their predicted weakening trend, the ocean’s largest anoxic zone will contract despite a global O2 decline.

Sign in / Sign up

Export Citation Format

Share Document