Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

Johnson, Kenneth S.; Riser, Stephen C.; Ravichandran, M.

doi:10.1029/2018gl079881

Cited by 46 publications

(45 citation statements)

References 37 publications

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“…In the Bay of Bengal, strong interannual variations in the intensity of the eddy activity have been reported (Chen et al, 2012). These are expected to cause strong variations in the subsurface ventilation that may eventually lead to deoxygenation and onset of denitrification at the core of the OMZ (Johnson et al, 2019).…”

Section: Discussionmentioning

confidence: 99%

“…However, even a small decline in oxygen concentrations has significant biogeochemical implications. For example, Bristow et al (2017) showed that the low oxygen concentration enables anaerobic microbial processes within the OMZ of the Bay of Bengal even though a constant physical oxygen supply (Johnson et al, 2019) maintained a oxygen levels of 0.01 -0.2 µM. This suffices to support nitrite oxidization and the resulting lack of nitrite strongly reduced, but probably did not prevent, denitrification.…”

Section: Recent Trends In the Bay Of Bengal And The Arabian Seamentioning

confidence: 99%

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Present past and future of the OMZ in the northern Indian Ocean

Rixen¹,

Cowie²,

Gaye³

et al. 2020

Preprint

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Abstract. Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~ 59 % of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21 % of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N2) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.

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Section: Discussionmentioning

confidence: 99%

Section: Recent Trends In the Bay Of Bengal And The Arabian Seamentioning

confidence: 99%

Present past and future of the OMZ in the northern Indian Ocean

Rixen¹,

Cowie²,

Gaye³

et al. 2020

Preprint

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show abstract

“…Given the ability of a float array to make observations on seasonal to interannual timescales at basin to global scales, significant effort has been devoted to integrating biogeochemical sensors onto profiling floats, including oxygen [78,79], nitrate [80], and bio-optical measurements for chlorophyll a fluorescence [81] and particle backscatter [82]. These biogeochemical profiling floats have been used to study production dynamics [83][84][85][86][87], nutrient delivery to oligotrophic waters [88], oxygen minimum zone dynamics [89], regional air-sea fluxes [90][91][92], and elemental ratios [93]. A pCO 2 sensor has been integrated onto a profiling float [94], but has not left the prototyping phase due to issues such as long response time, need for frequent recalibration, and high power requirements.…”

Section: Global Ocean Observations From Profiling Floatsmentioning

confidence: 99%

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

Bushinsky

Takeshita

Williams

2019

Curr Clim Change Rep

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Purpose of Review We summarize recent progress on autonomous observations of ocean carbonate chemistry and the development of a network of sensors capable of observing carbonate processes at multiple temporal and spatial scales. Recent Findings The development of versatile pH sensors suitable for both deployment on autonomous vehicles and in compact, fixed ecosystem observatories has been a major development in the field. The initial large-scale deployment of profiling floats equipped with these new pH sensors in the Southern Ocean has demonstrated the feasibility of a global autonomous open-ocean carbonate observing system. Summary Our developing network of autonomous carbonate observations is currently targeted at surface ocean CO 2 fluxes and compact ecosystem observatories. New integration of developed sensors on gliders and surface vehicles will increase our coastal and regional observational capability. Most autonomous platforms observe a single carbonate parameter, which leaves us reliant on the use of empirical relationships to constrain the rest of the carbonate system. Sensors now in development promise the ability to observe multiple carbonate system parameters from a range of vehicles in the near future.

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“…This points towards a production of a considerable part of organic matter produced by diatoms in symbiotic association with or in close proximity to diazotrophs (Subramaniam et al, 2008). Only few studies report the presence of diazotrophs including Trichodesmium in the BoB (Wu et al, 2019;Shetye et al, 2013;Sahu et al, 2017;Jyothibabu et al, 2006;Mulholland and Capone, 2009), with only one of them using a functional gene approach.…”

Section: Introductionmentioning

confidence: 99%

No nitrogen fixation in the Bay of Bengal?

et al. 2020

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The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which -for some, yet unconstrained, reason -are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key in understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N 2 ) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB's nitrogen input, suggesting a potential role of N 2 fixation in those waters.Here, we present a survey of N 2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N 2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N 2 fixers present throughout the water column (samples collected between 10 and 560 m water depth). While similar communities of N 2 fixers were shown to actively fix N 2 in other OMZs, N 2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ. Consistent with this, no N 2 fixation signal was visible in δ 15 N signatures. We sug-gest that the absence of N 2 fixation may be a consequence of a micronutrient limitation or of an O 2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N 2 fixation by cyanobacteria compared to non-phototrophic N 2 fixers would impact on OMZ O 2 concentrations, a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N 2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O 2 traces in the BoB.

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Oxygen Variability Controls Denitrification in the Bay of Bengal Oxygen Minimum Zone

Cited by 46 publications

References 37 publications

Present past and future of the OMZ in the northern Indian Ocean

Present past and future of the OMZ in the northern Indian Ocean

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

No nitrogen fixation in the Bay of Bengal?

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