A revised nitrogen budget for the Arabian Sea

Bange, Hermann W.; Rixen, Tim; Johansen, Anne M.; Siefert, Ronald L.; Ramesh, R.; Ittekkot, V.; Hoffmann, Michael R.; Andreae, Meinrat O.

doi:10.1029/1999gb001228

Cited by 119 publications

(107 citation statements)

References 54 publications

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“…The rate of denitrification calculated here is lower than some previous estimates that were based on geochemical methods [9][10][11][12][13][14] . The difference cannot be attributed to any single factor and a detailed comparison of the various methods of determining denitrification rates is beyond the scope of this study.…”

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confidence: 41%

“…The wide range of estimates of water-column denitrification rate [9][10][11][12][13][14] reflects uncertainties in nutrient stoichiometries and the volume of suboxic waters, and is further exacerbated by the limits of simple mixing models and transient age tracers, especially in poorly ventilated waters 14 . To reduce these uncertainties requires robust tracers of net N loss and the ability to accurately account for the physical transport in and around suboxic zones.…”

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confidence: 99%

“…The Arabian Sea, for which the most data are available, shows both the lowest denitrification rates and the smallest uncertainty (12)(13)(14)(15)(16) Tg N yr −1 at a 1-σ confidence level). Previous estimates of water-column denitrification in the Arabian Sea calculated a total N loss of 13- (Fig.…”

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Global rates of water-column denitrification derived from nitrogen gas measurements

et al. 2012

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Biologically available nitrogen (N) limits phytoplankton growth over much of the ocean. The rate at which N is removed from the contemporary ocean by denitrifying bacteria is highly uncertain 1-3 . Some studies suggest that N losses exceed inputs 2,4-6 ; others argue for a balanced budget 3,7,8 . Here, we use a global ocean circulation model to simulate the distribution of N 2 gas produced by denitrifying bacteria in the three main suboxic zones in the open ocean. By fitting the model to measured N 2 gas concentrations, we infer a globally integrated rate of water-column denitrification of 66 ± 6 Tg N yr −1 . Taking into account isotopic constraints on the fraction of denitrification occurring in the water column versus marine sediments, we estimate that the global rate of N loss from marine sediments and the oceanic water column combined amounts to around 230 ± 60 Tg N yr −1 . Given present estimates of N input rates, our findings imply a net loss of around 20 ± 70 Tg of N from the global ocean each year, indistinguishable from a balanced budget. A balanced N budget, in turn, implies that the marine N cycle is governed by strong regulatory feedbacks.The loss of N in the ocean occurs as bacteria reduce N to N 2 gas through a combination of heterotrophic denitrification and anaerobic ammonium oxidation (hereafter collectively termed denitrification). Suboxic conditions in which O 2 is scarce enough to favour these processes are found in three principal locations in the water column: the Arabian Sea, the eastern tropical South Pacific (ETSP) and the eastern tropical North Pacific (ETNP; Fig. 1). Denitrification rates in these areas estimated from nitrate deficits and water-mass age tracers yield a global rate of watercolumn denitrification in the range of 60-90 Tg N yr −1 (refs 9-14), but recently rates as large as 150 Tg N yr −1 have been proposed 2 . Denitrification also occurs in suboxic pore waters of sediments throughout the ocean, but the rates are spatially heterogeneous and have been measured at only a handful of sites 15 , precluding a direct global estimate. However, the global rate of sedimentary N loss (primarily denitrification, but also a small burial of organic N) is constrained by the isotopic ratio of mean ocean nitrate to be one to four times as large as the rate of denitrification in the suboxic water column 6,16,17 . Estimates of the total rate of N loss in the ocean therefore inherit and amplify the uncertainty in the rate of denitrification in suboxic waters, yielding a combined rate of about 270 Tg N yr −1 (ref.3) to more than 400 Tg N yr −1 (refs 2,6). These rates far exceed the estimated global rate of N fixation, which is in the range of 100-160 Tg N yr −1 (refs 7,18). The apparent deficit is partially offset by riverine and atmospheric inputs of about 40-125 Tg N yr −1 (refs 3,15). Still, if the upper end of the denitrification-rate estimates are correct, the ocean must be rapidly losing fixed N.

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Global rates of water-column denitrification derived from nitrogen gas measurements

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“…The deposition rate from the atmosphere is perhaps relatively more important. Bange et al (2000) estimated a total DIN deposition of ∼1.6 Tg y −1 for the region north of 6 • N latitude, and although this is based on observations offshore, the flux density is expected to be higher over the Indian shelf. It would be reasonable to assume that most of the aeolian and riverine inputs are of anthropogenic origin, which could account for a significant fraction of denitrification rate over the shelf estimated above, and the shift from natural suboxic to anthropogenic anoxic conditions.…”

Section: Denitrification Ratesmentioning

confidence: 99%

Coastal versus open-ocean denitrification in the Arabian Sea

Naqvi

Naik²,

Pratihary³

et al. 2006

Biogeosciences

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Abstract. The Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and midshelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N 2 O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO

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“…The oxygen minimum zone (OMZ) of the Arabian Sea represents a globally important site for oceanic fixed nitrogen (N) loss (Bange et al, 2000(Bange et al, , 2005 and references therein). Its vertical distribution extends from approximately 150 to 1000 m below the sea surface with oxygen concentrations at times as low as 0.1 mM (Morrison et al, 1999;Paulmier and Ruiz-Pino, 2009), rendering it one of the most expansive and intense OMZs globally.…”

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confidence: 99%

Niche segregation of ammonia-oxidizing archaea and anammox bacteria in the Arabian Sea oxygen minimum zone

et al. 2011

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Ammonia-oxidizing archaea (AOA) and anaerobic ammonia-oxidizing (anammox) bacteria have emerged as significant factors in the marine nitrogen cycle and are responsible for the oxidation of ammonium to nitrite and dinitrogen gas, respectively. Potential for an interaction between these groups exists; however, their distributions are rarely determined in tandem. Here we have examined the vertical distribution of AOA and anammox bacteria through the Arabian Sea oxygen minimum zone (OMZ), one of the most intense and vertically exaggerated OMZs in the global ocean, using a unique combination of intact polar lipid (IPL) and gene-based analyses, at both DNA and RNA levels. To screen for AOA-specific IPLs, we developed a high-performance liquid chromatography/mass spectrometry/mass spectrometry method targeting hexose-phosphohexose (HPH) crenarchaeol, a common IPL of cultivated AOA. HPH-crenarchaeol showed highest abundances in the upper OMZ transition zone at oxygen concentrations of ca. 5 lM, coincident with peaks in both thaumarchaeotal 16S rDNA and amoA gene abundances and gene expression. In contrast, concentrations of anammox-specific IPLs peaked within the core of the OMZ at 600 m, where oxygen reached the lowest concentrations, and coincided with peak anammox 16S rDNA and the hydrazine oxidoreductase (hzo) gene abundances and their expression. Taken together, the data reveal a unique depth distribution of abundant AOA and anammox bacteria and the segregation of their respective niches by 4400 m, suggesting no direct coupling of their metabolisms at the time and site of sampling in the Arabian Sea OMZ.

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A revised nitrogen budget for the Arabian Sea

Cited by 119 publications

References 54 publications

Global rates of water-column denitrification derived from nitrogen gas measurements

Global rates of water-column denitrification derived from nitrogen gas measurements

Coastal versus open-ocean denitrification in the Arabian Sea

Niche segregation of ammonia-oxidizing archaea and anammox bacteria in the Arabian Sea oxygen minimum zone

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