Nitrogen biogeochemical cycling in the northwestern Indian Ocean

Fauzi, Riza Sukma; Mantoura, C.; Law, Cliff S.; Owens, Nick; Burkill, Peter H.; Woodward, E Malcolm S; Howland, R.J.M.; Llewellyn, Carole A.

doi:10.1016/0967-0645(93)90050-w

Cited by 94 publications

(5 citation statements)

References 28 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%

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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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“…those arising from either AO or nitrifier-denitrification) are small, however, under low DO concentrations nitrification-N 2 O yields may increase substantially 4,5 . As DO concentrations approach anoxic conditions, denitrification can also be ‘turned on’, and although it can both produce and consume N 2 O, net denitrification yields up to 2% have been observed 6,7 .…”

Section: Introductionmentioning

confidence: 99%

Low oxygen eddies in the eastern tropical North Atlantic: Implications for N2O cycling

Grundle

Löscher

Krahmann

et al. 2017

Sci Rep

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Nitrous oxide (N2O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N2O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N2O concentrations much higher (up to 115 nmol L−1) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N2O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N2O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N2O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N2O at the regional scale, and point out the possible need for a reevaluation of how we view N2O cycling in the ETNA.

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“…To date, modeling and field observations approaches for computing the global ocean N 2 O source and distribution are widely uncertain due to insufficient observations and to incomplete systematic testing of N 2 O production and consumption rates, their dependence on O 2 concentrations [Zamora and Oschlies, 2014], and the potential impact of ocean circulation and mixing on oceanic N 2 O distribution [Nevison et al, 2003;Martinez-Rey et al, 2015]. Furthermore, studies on the oceanic N 2 O cycle have been focused on only a few regions, such as tropical and subtropical gyres of the Pacific Ocean, the Arabian Sea, the Black Sea, the Bedford Basin, and the Scheldt estuary, from which information has been used to derive and test model parameterizations [Elkins et al, 1978;Yoshida et al, 1989;Fauzi et al, 1993;Bange et al, 2000;de Wilde and de Bie, 2000;Punshon and Moore, 2004;Westley et al, 2006;Farías et al, 2007;Yamagishi et al, 2007;Frame and Casciotti, 2010]. Many of these low-oxygenated regions are hot spots of oceanic N 2 O production and consumption but may not be representative of other ocean environments.…”

Section: Introductionmentioning

confidence: 99%

Ventilation versus biology: What is the controlling mechanism of nitrous oxide distribution in the North Atlantic?

Paz

García-Ibáñez

Steinfeldt

et al. 2017

Global Biogeochemical Cycles

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The extent to which water mass mixing and ocean ventilation contribute to nitrous oxide (N2O) distribution at the scale of oceanic basins is poorly constrained. We used novel N2O and chlorofluorocarbon measurements along with multiparameter water mass analysis to evaluate the impact of water mass mixing and Atlantic Meridional Overturning Circulation (AMOC) on N2O distribution along the Observatoire de la variabilité interannuelle et décennale en Atlantique Nord (OVIDE) section, extending from Portugal to Greenland. The biological N2O production has a stronger impact on the observed N2O concentrations in the water masses traveling northward in the upper limb of the AMOC than those in recently ventilated cold water masses in the lower limb, where N2O concentrations reflect the colder temperatures. The high N2O tongue, with concentrations as high as 16 nmol kg−1, propagates above the isopycnal surface delimiting the upper and lower AMOC limbs, which extends from the eastern North Atlantic Basin to the Iceland Basin and coincides with the maximum N2O production rates. Water mixing and basin‐scale remineralization account for 72% of variation in the observed distribution of N2O. The mixing‐corrected stoichiometric ratio N2O:O2 for the North Atlantic Basin of 0.06 nmol/μmol is in agreement with ratios of N2O:O2 for local N2O anomalies, suggesting than up to 28% of N2O production occurs in the temperate and subpolar Atlantic, an overlooked region for N2O cycling. Overall, our results highlight the importance of taking into account mixing, O2 undersaturation when water masses are formed and the increasing atmospheric N2O concentrations when parameterizing N2O:O2 and biological N2O production in the global oceans.

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Nitrogen biogeochemical cycling in the northwestern Indian Ocean

Cited by 94 publications

References 28 publications

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

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

Low oxygen eddies in the eastern tropical North Atlantic: Implications for N2O cycling

Ventilation versus biology: What is the controlling mechanism of nitrous oxide distribution in the North Atlantic?

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