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Nitrate elimination and regeneration as evidenced by dissolved inorganic nitrogen isotopes in Saanich Inlet, a seasonally anoxic fjord

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... Deepwater oxygen renewal events occur primarily in later summer and fall, when dense oxygenated water accumulates outside of the fjord and periodically flows over the sill and into the basin (Anderson & Devol, 1973;Manning et al., 2010). These features make Saanich Inlet an ideal natural laboratory to examine the effects of dissolved oxygen availability on nitrogen cycling (Bourbonnais et al., 2013;Grundle & Juniper, 2011;Ward & Kilpatrick, 1990), greenhouse gas fluxes (Capelle et al., 2018(Capelle et al., , 2019, and microbial community dynamics (Torres-Beltrán et al., 2016;Zaikova et al., 2010). The detailed biogeochemical characterizations of Saanich Inlet provide a foundation for further exploration of N 2 O production pathways and the regulating environmental factors, which have not been reported. ...
... It should be noted that at anoxic conditions (DO ¼ 0-2.5 μmol L −1 ), denitrification and sulfate reduction are probably the dominant microbial metabolism, yet oxygen respiration continues to operate even at nanomolar DO level (Zakem & Follows, 2017). And thus, denitrification, sulfate reduction and oxygen respiration coexist at suboxic depths in Saanich Inlet (Bourbonnais et al., 2013;Manning et al., 2010;Torres-Beltrán et al., 2016). Seawater subsamples for nutrient measurements were filtered (0.22-μm Sterivex™ filter, EMD Millipore, Burlington, MA) and stored at −20°C in acid-washed 60-ml high-density polyethylene bottles (20,160,060, Thermo Scientific, Waltham, MA, USA). ...
... fluxes generated by organic matter remineralization from the anoxic depth in Saanich Inlet (Bourbonnais et al., 2013). Using NH 4 + concentration profiles from this study and assuming a diffusion constant of 1.0 m 2 day −1 previously estimated in Saanich Inlet 130-160 m depth (Louca et al., 2016), upward NH 4 + fluxes from the anoxic zone ranged from 0.6 to 3.5 nmol m −2 s −1 . ...
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Nitrous oxide (N2O) is a strong greenhouse gas and an ozone depleting agent. In marine environments, N2O is produced biologically via ammonium oxidation, nitrite and nitrate reduction. The relative importance of these principle production pathways is strongly influenced by oxygen availability. We conducted 15N tracer experiments of N2O production in parallel with measurements of N2O concentration and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how temporal and vertical oxygen gradients regulate N2O production pathways and rates. In April, June and August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen = 2.5 μmol L‐1 isoline) progressively deepened from 110 m to 160 m. Within the oxygenated and suboxic water column, N2O supersaturation coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially favorable to N2O production from nitrate and nitrite reduction, but N2O undersaturation was observed indicating that N2O consumption exceeded rates of production. In October, tidal mixing introduced oxygenated water from outside the inlet, displacing the suboxic and anoxic deep water. This oxygenation event stimulated N2O production from ammonium oxidation, and increased water column N2O supersaturation, while inhibiting nitrate and nitrite reduction to N2O. Results from 15N tracer incubation experiments and natural abundance isotopomer measurements both implicated ammonium oxidation as the dominant N2O production pathway in Saanich Inlet, fueled by high ammonium fluxes (0.6 – 3.5 nmol m‐2 s‐1) from the anoxic depths. Partial denitrification contributed little to water column N2O production because of low availability of nitrate and nitrite.
... isotopic composition can be used to disentangle NO 3 À consumption and production processes in marine environments Sigman et al., 2005;Bourbonnais et al., 2009Bourbonnais et al., , 2013Casciotti, 2009]. NO 3 À consumption by autotrophic uptake or dissimilatory reduction generally fractionates N and O isotopes equally with a 15 ε: 18 ε of 1 [Granger et al., 2004[Granger et al., , 2008. ...
... 3.3.1.2. Evidence From Coupled NO 3 À N and O Isotopes NO 3 À N and O isotopes are useful to separate NO 3 À consumption and production processes in marine environments [Granger et al., 2004[Granger et al., , 2008Lehmann et al., 2005;Sigman et al., 2005;Bourbonnais et al., 2009Bourbonnais et al., , 2013. Assimilative or dissimilative NO 3 À consumption generally fractionates N and O isotopes equally, with a relationship between δ 18 O-NO 3 À and δ 15 N-NO 3 À ( 18 ε: 15 ε) of 1 [Granger et al., 2004[Granger et al., , 2008. ...
Article
Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent “natural tracer experiments” with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−), and biogenic N2 associated with an anticyclonic mode-water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N-loss was calculated to be only ~13–14‰ (as compared to canonical values of ~20–30‰) assuming a closed system and only slightly higher assuming an open system (16–19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε values for NO3− reduction to NO2− and NO2− reduction to N2 of ~16–21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2−, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for N-loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N-loss.
... 受此影响, 龙洞内营 养盐浓度在80~100 m上下也是两种截然不同的状态. 80 m以上, 营养盐水平整体偏低, 与外礁坡对照点 (图3)和南海其他区域水平相当 [19] , 但是要明显低于 其他边缘海, 如渤海、黄海和东海 [20,21] , 也低于一些 图 4 (网络版彩色)三沙永乐龙洞与外礁坡对照点营养盐结构剖面分布 Figure 4 (Color online) Nutrient structure profiles in the Sansha Yongle Blue Hole and a reference site at the outer reef slope 季节性缺氧的河口海域, 如长江口缺氧区、萨尼奇 湾 [22,23] 、一些沿岸低氧海域, 如马尔马拉海和波罗的 海 [24,25] , 及开阔大洋的氧最小区, 如阿拉伯海和东热 带南太平洋等受上升流影响的海域 [26,27] ; 80 m以下, 主要营养盐浓度均开始升高, 其水平远超对照点(图3) 和南海其他区域同等深度 [19] 、其他边缘海 [20,21] 、季节 性缺氧的河口海域 [22,23] 、沿岸低氧海域 [26,27] 及开阔大 洋氧最小区等, 尤其是铵氮和硅酸盐. ...
... 受此影响, 龙洞内营 养盐浓度在80~100 m上下也是两种截然不同的状态. 80 m以上, 营养盐水平整体偏低, 与外礁坡对照点 (图3)和南海其他区域水平相当 [19] , 但是要明显低于 其他边缘海, 如渤海、黄海和东海 [20,21] , 也低于一些 图 4 (网络版彩色)三沙永乐龙洞与外礁坡对照点营养盐结构剖面分布 Figure 4 (Color online) Nutrient structure profiles in the Sansha Yongle Blue Hole and a reference site at the outer reef slope 季节性缺氧的河口海域, 如长江口缺氧区、萨尼奇 湾 [22,23] 、一些沿岸低氧海域, 如马尔马拉海和波罗的 海 [24,25] , 及开阔大洋的氧最小区, 如阿拉伯海和东热 带南太平洋等受上升流影响的海域 [26,27] ; 80 m以下, 主要营养盐浓度均开始升高, 其水平远超对照点(图3) 和南海其他区域同等深度 [19] 、其他边缘海 [20,21] 、季节 性缺氧的河口海域 [22,23] 、沿岸低氧海域 [26,27] 及开阔大 洋氧最小区等, 尤其是铵氮和硅酸盐. ...
Article
南海三沙永乐龙洞水深约300 m, 是世界上已知最深的海洋蓝洞, 100 m以下水体属于无氧环境, 使其成为研究从有氧到无氧的转变及无氧环境下生源要素迁移转化过程的理想场所. 2017年3月在永乐龙洞开展了现场调查和样品采集, 对溶解态无机营养盐进行了分析, 并结合温度、盐度、溶解氧(DO)等环境参数, 讨论了营养盐浓度与结构的垂直分布特征及影响因素, 初步探讨了营养盐循环过程. 结果表明, 不同的营养盐在蓝洞内有迥异的变化规律, 最大转变发生在氧化还原跃层. 表层营养盐浓度均较低, 但随着深度的增加, 各营养盐浓度体现不同的峰值分布. 例如, 硝酸盐浓度峰值(8.59 mol/L)出现在90 m深处, 而亚硝酸盐在40和95 m处出现双峰分布(分别为0.49和0.18 mol/L). 铵氮浓度在95 m之后迅速升高, 磷酸盐和硅酸盐浓度则从70 m开始升高, 150 m后其浓度均不再增加, 分别稳定在85, 4.9和152 mol/L左右. 营养盐浓度的变化直接影响了其结构的分布, N/P比与Si/N比和Si/P比呈现相反的变化趋势. N/P比在表层较高(接近300), 整体随深度而降低; Si/N比在表层和底层都较低, 在95 m出现15的峰值; Si/P比也是表层较高, 但在95 m也出现达70的峰值. 在160 m以下, 各营养盐比例均保持稳定, 并接近Redfield比值. 永乐龙洞营养盐垂直分布的变化表明其循环过程与DO、有机物和微生物等之间存在密切联系. Blue holes are unique geomorphological units characterized by steep redox and biogeochemical gradients. The Yongle Blue Hole (YBH) is located on the largest atoll (Yongle Atoll) of the western Xisha Islands in the South China Sea (SCS). Although its depth was only just determined in July, 2016, the YBH has been known for centuries. The YBH is ca. 300 m in depth and has been recognized as the deepest known marine blue hole in the world. In this work, water column samples were collected from the YBH in March, 2017, and examined for dissolved inorganic nutrients, temperature, salinity, dissolved oxygen (DO), pH, and chlorophyll for the first time in order to better understand the nutrient cycling in this unique marine blue hole. The YBH water column is characterized by well-defined physical and chemical gradients with sharp transitions in salinity, temperature, density, DO and pH occurring at 80 m depth. With the disappearance of DO at 100 m depth, the hydrogeochemistry of the water column in the YBH dramatically changed from oxic to anoxic. Therefore, the YBH water column stratification existed mostly within the depth range of 80–100 m. Most physical and chemical parameters in the YBH remained relatively uniform below the depth of 160 m. Nutrient profiles in the water column varied distinctively in the YBH, with large shifts at the redoxclines. For example, surface waters (< 20 m) had nitrate concentrations that averaged 0.08 μmol/L, but increased to 8.59 μmol/L below 70 m and then decreased rapidly below 100 m. Nitrite were generally low with two peaks in the water column at 40 m (0.49 μmol/L) and 95 m (0.18 μmol/L). Changes in nitrate and nitrite were reflective of nitrification and denitrification within the 100 m depth of the YBH. Concentrations of ammonium was low within the upper 95 m, but increased to ca. 85 μmol/L below 160 m, where nitrate and DO decreased to near zero. The concentrations of phosphate and silicate were also very low within 70 m (0.04 and 1.21 μmol/L in average, respectively), increased rapidly below 70 m, and kept stable below 160 m at 4.9 and 152 μmol/L, respectively. Relatively constant distributions of ammonium, phosphate and silicate below 160 m was likely attributed to the stable conditions of low OM concentration, chlorophyll-a, temperature, salinity and pH. The N/P ratios were high in the surface layer (up to 300) and decreased with depth. The Si/N ratios were low in both surface and bottom layers, and the peak value of 15 appeared at 95 m. The Si/P ratios were high in the surface layer too, but also had peak value at 95 m (up to 70). All nutrient ratios (Si/N, Si/P, and N/P) were also stable below 160 m and were very close to the Redfield ratios. Vertical profiles of nutrients in the YBH were strongly linked to redoxclines and OM concentrations. In general, the YBH is an ideal place for examining the transport and transformation of biogenetic elements across steep redox gradients in the coastal margin.
... N-cycling and its interactions with the other cycles in SI have been previously interrogated using a variety of geochemical and microbiological analyses. Geochemical data indirectly imply that SI supports relatively high rates of both pelagic and benthic N-loss (Manning et al., 2010;Bourbonnais et al., 2013) that vary seasonally, with the highest rates in the winter (December-February, 8.1 mmol m −2 d −1 ) and lowest in the summer (May-August, 1.7 mmol m −2 d −1 ) (Manning et al., 2010). Multi-omic analyses revealed that microbial communities in SI harbor the metabolic potential to catalyze many components of the N-cycle and to link N to cycling of C and S Hawley et al., 2014Hawley et al., , 2017a. ...
... Chlorophyll a samples collected on filters (0.7 µm nominal porosity) were extracted for 24 h with 90% acetone at −20 • C, and the extracted chlorophyll a measured in a Turner Designs 10AU fluorometer, using an acidification method and corrected for phaeopigment interference Parsons et al. (1984). DIN deficit (DIN def ) was calculated according to Bourbonnais et al. (2013) and corrected for the release and dissolution of iron and manganese oxyhydroxide-bound PO 4 ...
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Marine oxygen minimum zones (OMZs) support 30–50% of global fixed-nitrogen (N) loss but comprise only 7% of total ocean volume. This N-loss is driven by canonical denitrification and anaerobic ammonium oxidation (anammox), and the distribution and activity of these two processes vary greatly in space and time. Factors that regulate N-loss processes are complex, including organic matter availability, oxygen concentrations, and NO2− and NH4+ concentrations. While both denitrification and anammox produce N2, the overall geochemical outcome of these processes are different, as incomplete denitrification, for example, produces N2O, which is a potent greenhouse gas. Information on rates of anammox and denitrification and more detailed ecophysiological knowledge of the microorganisms catalyzing these processes are needed to develop more robust models of N-loss in OMZs. To this end, we conducted monthly incubations with 15N-labeled N during under anoxic conditions and during a deep water renewal cycle in Saanich Inlet, British Columbia, a persistently anoxic fjord. Both denitrification and anammox operated throughout the low oxygen water column with depth integrated rates of anammox and denitrification ranging from 0.15 ± 0.03 to 3.4 ± 0.3 and 0.02 ± 0.006 to 14 ± 2 mmol N2 m−2 d−1, respectively. Most N2 production in Saanich Inlet was driven by denitrification, with high rates developing in response to enhanced substrate supply from deep water renewal. Dynamics in rates of denitrification were linked to shifts in microbial community composition. Notably, periods of intense denitrification were accompanied by blooms in an Arcobacter population against a background community dominated by SUP05 and Marinimicrobia. Rates of N2 production through denitrification and anammox, and their dynamics, were then explored through flux-balance modeling with higher rates of denitrification linked to the physiology of substrate uptake. Overall, both denitrification and anammox operated throughout the year, contributing to an annual N-loss of 2 × 10−3 Tg N2 yr−1, 37% of which we attribute to anammox and 63% to complete denitrification. Extrapolating these rates from Saanich Inlet to all similar coastal inlets in BC (2478 km2), we estimate that these inlets contribute 0.1% to global pelagic N-loss.
... This is often done using the Redfield stoichiometry (Gruber and Sarmiento, 1997). However, it is often questioned whether the Redfield ratio strictly holds in shallow coastal systems due to the anoxic sediments acting as a phosphate source to the overlying water-column (Bourbonnais et al., 2013). The simple relationship used by Naqvi et al. (2006) is: ...
... In all these studies, the low ɛ 15 values have been attributed to contribution from sedimentary denitrification which involves minimal discrimination between 15 N and 14 N (0-3‰, Brandes and Devol, 2002). Sedimentary denitrification has been estimated to account for 60% of total nitrogen loss in the Saanich Inlet (Bourbonnais et al., 2013) and over 75% in SBB (Sigman et al., 2003). As all stations experiencing denitrification over the WCSI are very shallow (depths ≤50 m), the role of sedimentary denitrification is expected to be very significant here. ...
... Existing classifications make no distinction between depositional systems in which redox conditions are more-or-less constant, and those in which redox conditions are ceaselessly changing. High-frequency redox variation can have measurable effects on watermass chemistry, e.g., the speciation of nitrogen (Bourbonnais et al., 2013) and trace metals (Canfield et al., 1993;Holmden et al., 2015), and it is the most likely cause of conflicting signals yielded by two or more independent redox proxies in a paleodepositional system Jin et al., 2018). One common pattern is sedimentological evidence for at least weak oxygenation in the form of metazoan fossils and bioturbated sediment fabric in conjunction with geochemical proxies indicating euxinic conditions (e.g., Kauffman and Sageman, 1990;Kenig et al., 2004;Li et al., 2015b;Balestra et al., 2018). ...
Article
Existing redox classifications and the calibrations of elemental proxies to modern environmental redox scales are in need of re-evaluation. Here, we review environmental redox classifications, commonly used elemental redox proxies, and their intercalibration, and we propose a novel approach to improve the calibration of such proxies, using datasets from the modern Black Sea, Saanich Inlet, and California Margin as examples. Our approach is based on recognition of compound covariation patterns among pairs of elemental redox proxies within a redox framework based on three key thresholds: (1) the Re⁴⁺/Re³⁻ couple near the suboxidized/subreduced boundary of the suboxic zone, (2) the U⁶⁺/U⁴⁺ couple in the middle of the subreduced zone, and (3) the SO4²⁻/H2S couple at the suboxic/euxinic boundary. Within this framework, it is possible to determine the relative timing of onset and the degree of enrichment of other elemental redox proxies. Our analysis demonstrates that, even though some elements exhibit limited enrichment within the suboxic zone, the bulk of authigenic enrichment of the redox-sensitive elements considered in this study occurs within the euxinic zone. One important finding of our study is that the threshold value associated with a given elemental proxy can vary considerably between depositional systems. For this reason, it is inadvisable to transfer published threshold values (i.e., from earlier paleoredox studies) to completely different formations, and redox proxies must be internally calibrated for each individual paleodepositional system under investigation.
... To estimate the supply of newly fixed N to the nitrate pool within the IOSG, we can calculate the fraction of nitrate coming from atmospheric N 2 fixation and the fraction that is added from the underlying source water using the observed δ 15 N-NO − 3 within the upper 200 m and the following equation modified after Bourbonnais et al. (2009Bourbonnais et al. ( , 2013: ...
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The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of
... Due to the use of different initial electron donor and acceptor concentrations among the tested stages, the ratio of cellular NO 3 − uptake and efflux before the enzymatic reaction and the NO 3 − reduction rate was expected to play a role in the variability in the ε 15 N NO3/N2 and ε 18 O NO3/N2 results. Furthermore, a shift in the ε 15 N/ε 18 O ratio with respect to 1, the typical recognized value for denitrification, can be attributed to (I) NO 2 − reoxidation to NO 3 − (Buchwald and Casciotti, 2010;Granger and Wankel, 2016;Wunderlich et al., 2013); (II) NH 4 + oxidation to NO 3 − (Bourbonnais et al., 2013;Dähnke and Thamdrup, 2016;Granger and Wankel, 2016) and (III) major activity of bacteria containing the periplasmic NO 3 − reductase (NAP) instead of the membrane-bound NO 3 − reductase (NAR) (Granger et al., 2008). For this reason, ε 15 N/ε 18 O values close to 2 are usually found in field-scale freshwater denitrification studies (Critchley et al., 2014;Otero et al., 2009), while values remain close to 1 in laboratory experiments performed under controlled closed conditions (Carrey et al., 2013;Grau-Martínez et al., 2017). ...
Article
Improving the effectiveness and economics of strategies to remediate groundwater nitrate pollution is a matter of concern. In this context, the addition of whey into aquifers could provide a feasible solution to attenuate nitrate contamination by inducing heterotrophic denitrification, while recycling an industry residue. Before its application, the efficacy of the treatment must be studied at laboratory-scale to optimize the application strategy in order to avoid the generation of harmful intermediate compounds. To do this, a flow-through denitrification experiment using whey as organic C source was performed, and different C/N ratios and injection periodicities were tested. The collected samples were analyzed to determine the chemical and isotopic composition of N and C compounds. The results proved that whey could promote denitrification. Nitrate was completely removed when using either a 3.0 or 2.0 C/N ratio. However, daily injection with C/N ratios from 1.25 to 1.5 seemed advantageous, since this strategy decreased nitrate concentration to values below the threshold for water consumption while avoiding nitrite accumulation and whey release with the outflow. The isotopic results confirmed that nitrate attenuation was due to denitrification and that the production of DIC was related to bacterial whey oxidation. Furthermore, the isotopic data suggested that when denitrification was not complete, the outflow could present a mix of denitrified and nondenitrified water. The calculated isotopic fractionation values might be applied in the future to quantify the efficiency of the bioremediation treatments by whey application at field-scale. Postprint version available at: http://hdl.handle.net/2445/161620
... However, one limitation of this calculation is the use of PO 4 3− concentrations in the basin to estimate initial NO 3 − . In low-oxygen environments, respiration is not the only source of PO 4 3− as PO 4 3− bound to iron particles can be released from reducing sediments into the water column (Bourbonnais et al., 2013;King & Barbeau, 2011;Peters et al., 2018;Reimers et al., 1990). Thus, it is possible that the N:P approach taken here overestimated [NO 3 − ] initial at times, as has been concluded elsewhere (Hu et al., 2016;Peters et al., 2018). ...
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Plain Language Summary The current chemical environment of the deep Santa Barbara Basin is unprecedented in the 30 years since measurements began, suggesting that a regional change in ocean chemistry has occurred over this time period. Here we use stable isotope measurements of nitrate to examine how the nitrogen cycle in the deep part of the basin has changed, documenting a recent increase in the amount of nitrate being removed from the water column at this location. Nitrogen removal in the dark ocean, known as denitrification, occurs when microbes utilize nitrate for respiration in the absence of oxygen. Such a process is consistent with the extremely low or absent oxygen concentrations that accompany the altered nitrate isotope signatures we measured in this study. Denitrification in sediments is common in the Santa Barbara Basin, but extensive water column denitrification has not been previously documented. Loss of nitrate from the water column can have important consequences for the balance of nutrients that support primary production in the ocean. These changes appear to be an effect of decreasing oxygen concentrations observed on a regional scale in the North Pacific Ocean, a trend which is likely to continue as the oceans warm.
... Studies on 15 N and 18 O fractionation factors revealed that -NO 3 consumption (the assimilatory and dissimilatory reduction of NO 3 -) generally induced a 1:1 increase in the δ 18 O and δ 15 N of NO 3 - (Granger et al., 2008;Granger et al., 2010;Karsh et al., 2012;Rohde et al., 2015;Osaka et al., 2018). This finding prompted the use of the δ 18 O and δ 15 N of NO 3 -to detect NO 3 -consumption in the actual ecosystem as well as investigations on NO 3 isotope anomalies, specifically isotopic deviations from a slope of 1 in the δ 18 O vs δ 15 N of NO 3 (Δ[15, 18]; Sigman et al., 2005), in order to deepen insights into NO 3 -dynamics (Casciotti et al., 2008;Casciotti and Buchwald, 2012;Bourbonnais et al., 2013;Peters et al., 2018;White et al., 2019). Granger and Wankel (2016) proposed that widely observed deviations in the δ 18 O vs δ 15 N of NO 3 -from the denitrification slope of 1 in freshwater systems (Sigman et al., 2005;Granger et al., 2008; must result from concurrent NO 3 production (nitrification or anammox) in the denitrifying system that has been largely overlooked. ...
Article
Isotopic fractionation factors against ¹⁵N and ¹⁸O during anammox (anaerobic ammonia oxidization by nitrite) are critical for evaluating the importance of this process in natural environments. We performed batch incubation experiments with an anammox-dominated biomass to investigate nitrogen (N) and oxygen (O) isotopic fractionation factors during anammox and also examined apparent isotope fractionation factors during anammox in an actual wastewater treatment plant. We conducted one incubation experiment with high δ¹⁸O of water to investigate the effects of water δ¹⁸O. The N isotopic fractionation factors estimated from incubation experiments and the wastewater treatment plant were similar to previous values. We also found that the N isotopic effect (¹⁵εNXR of –77.8 to –65.9‰ and ¹⁵ΔNXR of –31.3 to –30.4‰) and possibly O isotopic effect (¹⁸εNXR of –20.6‰) for anaerobic nitrite oxidation to nitrate were inverse. We applied the estimated isotopic fractionation factors to the ordinary differential equation model to clarify whether anammox induces deviations in the δ¹⁸O vs δ¹⁵N of nitrate from a linear trajectory of 1, similar to heterotrophic denitrification. Although this deviation has been attributed to nitrite oxidation, the O isotopic fractionation factor for anammox is crucial for obtaining a more detailed understanding of the mechanisms controlling this deviation. In our model, anammox induced the trajectory of the δ¹⁸O vs δ¹⁵N of nitrate during denitrification to less than one, which strongly indicates that this deviation is evidence of nitrite oxidation by anammox under denitrifying conditions.
... The influence of vertical mixing was also likely significant in the QCS region, where we observed the largest N 2 /Ar disequilibria and negative surface O 2 ( Figure 4A). In this region, sedimentary or deep-water denitrification, which has been observed in coastal fjords and inland channels throughout British Columbia (Manning et al., 2010;Bourbonnais et al., 2013) may have elevated N 2 /Ar deep , resulting in the vertical supply of microbially N 2 -enriched water into surface waters. This hypothesis is supported by observations of lower N * , an indicator of nitrate loss via denitrification (Gruber and Sarmiento, 1997), in the QCS region (range ∼−2 to −6 µmol L −1 in the upper 500 m), relative to offshore waters of the NEP (Supplementary Figure 5C). ...
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We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship’s seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N2 and Ar, deriving the tracer ΔO2/N2′. Over most of our survey regions, ΔO2/N2′ provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2′ performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2′ agreed to within ∼2 mmol O2 m–2 d–1, with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O2/N2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.
... From these numbers, we estimated that the proportion of Nloss due to sedimentary N-loss could be up to ∼ 60 % (48 to 64 %) at our coastal stations, which is in the same range than previously reported for other marine coastal environments, e.g. Saanich Inlet (also up to 60 %; Bourbonnais et al., 2013). Our estimate is higher than the 25 % of benthic vs. total Nloss from a reaction-diffusion model and direct flux measurements for the same coastal region off Peru . ...
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O2 deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO3−) reduction and subsequent loss of combined nitrogen (N) to biogenic N2 gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ N-cycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO3− and nitrite (NO2−) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30 m depth with concentrations less than 10 µM, while NO2− concentrations were high, ranging from 6 to 10 µM, and NO3− was in places strongly depleted to near 0 µM. We observed for the first time a positive linear relationship between NO2−δ15N and δ18O at our coastal stations, analogous to that of NO3− N and O isotopes during NO3− uptake and dissimilatory reduction. This relationship is likely the result of rapid NO2− turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO2− facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ15N of NO3−, dissolved inorganic N, or biogenic N2, values for ε are generally much lower than previously reported, reaching as low as 6.5 ‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.
... From these numbers, we estimated that the proportion of N-loss due to sedimentary N-loss could be up to 60 % at our coastal stations, which is in the same range than previously reported for other marine coastal environments, e.g. Saanich Inlet (also up to 60 %; Bourbonnais et al., 2013). Our estimate is however higher than the 25 % of benthic 10 vs. total N-loss from a reaction-diffusion model and direct flux measurements for the same coastal region off Peru (Kalvelage et al., 2013). ...
Article
O2 minimum zones (OMZ) of the world's oceans are important locations for microbial dissimilatory NO3- reduction and subsequent loss of combined nitrogen (N) to biogenic N2 gas. This is particularly so when the OMZ is coupled to a region of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N isotope ratios (and O in the case of NO3- and NO2-) can be used as natural tracers of OMZ N-cycling because of distinct kinetic isotope effects associated with microbially-mediated N-cycle transformations. Here we present NO2- and NO3- stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface O2 was generally depleted below about 30 m depth with O2 less than 10 μM, while NO2- concentrations were high, ranging from 6 to 10 μM and NO3- was in places strongly depleted to near 0 μM. We observed for the first time, a positive linear relationship between NO2- δ15N and δ18O at our coastal stations, analogous to that of NO3- N and O isotopes during assimilatory and dissimilatory reduction. This relationship is likely the result of rapid NO2- turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO2- facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ15N of [NO3-], DIN, or biogenic N2, values for ε are generally much lower than previously reported, reaching as low as 6.5‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.
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The coastal upwelling has profound influence on the surrounding ecosystem by supplying the nutrient-replete water to the euphotic zone. Nutrient biogeochemistry was investigated in coastal waters of the eastern Hainan Island in summer 2015 and autumn 2016. From perspectives of nutrient dynamics and physical transport, the nutrient fluxes entered the upper 50 m water depth (between the mixed layer and the euphotic zone) arisen from the upwelling were estimated to be 2.5–5.4 mmol/(m2·d), 0.15–0.28 mmol/(m2·d), and 2.2–7.2 mmol/(m2·d) for dissolved inorganic nitrogen (DIN), phosphate (DIP), and dissolved silicate (DSi), respectively, which were around 6- to 12-fold those in the background area. The upwelled nutrients supported an additional plankton growth of (14.70±8.95) mg/m2 for chlorophyll a (Chl a). The distributions of nitrate δ15N and δ18O above the 300 m water depth (top of the North Pacific Intermediate Water) were different among the upwelling area, background area in summer, and the stations in autumn, and the difference of environmental and biogeochemical conditions between seasons should be the reason. The higher DIN/DIP concentration ratio, nitrate concentration anomaly, and lower nitrate isotope anomaly (Δ(15, 18)) in the upper ocean in summer than in autumn indicated the stronger nitrogen fixation and atmospheric deposition, and the following fixed nitrogen regeneration in summer. The higher values of Chl a and nitrate δ15N and δ18O within the euphotic zone in autumn than the background area in summer suggested the stronger nitrate assimilation in autumn. The differences in relatively strength of the assimilation, nitrogen fixation and atmospheric deposition, and the following remineralization and nitrification between the two seasons made the higher δ18O:δ15N and larger difference of enzymatic isotope fractionation factors 15ε and 18ε for nitrate assimilation in summer than in autumn above the North Pacific Tropical Water.
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The global marine fixed nitrogen budget acts as a strong control on oceanic primary productivity, and the Arctic plays a disproportionately large role in the sink terms for this budget. This paper aims to quantify the impact of nitrogen cycling on the Canada Basin, utilizing two tracers of denitrification: N2/Ar, a dissolved gas tracer, and N*, a nutrient ratio tracer. In the Pacific Winter Water (PWW), which forms in the Chukchi Sea, we observe a disconnect between N2/Ar and N*, where the excess N2 expected from N* observations is far larger than the N2 excess we measure. We show that loss of N2 to the atmosphere through ventilation on the Chukchi Shelf likely accounts for this disparity, highlighting the importance of using N2/Ar as a denitrification tracer only in isolated water masses. We additionally observe increasing N2/Ar and decreasing N* in the old deep waters of the Canada Basin, suggesting benthic denitrification has been operating in the deep sediments over the 500-year age of this water mass. We use a one-dimensional vertical reaction-diffusion model to estimate denitrification rates of 0.0053–0.0130 mmol-N m⁻² d⁻¹, or 0.04–0.1 Tg N y⁻¹ integrated over the whole basin, which is about half the rates estimated for other deep basins, in-line with lower remineralization rates in the deep Canada Basin. Further measurements of these tracers in the Arctic, particularly directly in the Chukchi Sea, will help constrain the relative importance of physical vs. biological processes on N2 in this region.
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We present an 8-yr time-series of monthly water column N2O measurements from Saanich Inlet, a seasonally anoxic fjord in southern British Columbia. We document seasonal and inter-annual variability in N2O concentrations driven by physical and biological forcing, and examine the relationship between N2O and O2 concentrations across the redoxcline in this system. Near-surface N2O concentrations were typically supersaturated, and increased with depth to a maximum near the oxic-anoxic transition. Genes associated with both the nitrification and denitrification pathways were widely distributed throughout the water column, suggesting the potential for simultaneous N2O production from nitrification and incomplete denitrification. Maximum N2O concentrations in Saanich Inlet were similar to other anoxic basins, but lower than those previously documented in coastal upwelling and open ocean oxygen minimum zones. N2O and O2 were inversely correlated throughout the water column, except below 10 μmol O2 L−1 where net N2O consumption appears to occur. In surface waters, maximum N2O concentrations and sea–air fluxes occurred during late summer/early fall, when O2 levels in the oxycline were low. Annual deep basin renewals often persisted over multiple months, supplying O2, NO3−, and N2O to the deep basin, with N2O subsequently declining to undetectable levels (< 0.4 nmol N2O L−1) by late spring. An unusually weak renewal was observed during 2009, possibly linked to reduced upwelling off the coast of British Columbia during El Niño. Our results can be used as a baseline to identify the effect of longer-term changes in Saanich Inlet, such as deoxygenation and its effect on N2O concentrations.
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Nitrogen (N) is one of the primary nutrients required to build biomass and is therefore in high demand in aquatic ecosystems. Estuaries, however, are frequently inundated with high concentrations of anthropogenic nitrogen, which can lead to substantially degraded water quality. Understanding drivers of biogeochemical N cycling rates and the microbial communities responsible for these processes is critical for understanding how estuaries are responding to human development. Estuaries are notoriously complex ecosystems: not only do individual estuaries by definition encompass gradients of salinity and other changing environmental conditions, but differences in physical parameters (e.g., bathymetry, hydrodynamics, tidal flushing) lead to a tremendous amount of variability in estuarine processes between ecosystems, as well. Here, we review the current knowledge of N cycling processes in estuaries carried out by bacteria and archaea, including both biogeochemical rate measurements and molecular characterizations of N cycling microbial communities. Particular attention is focused on identifying key environmental factors associated with distinct biogeochemical or microbial regimes across numerous estuaries. Additionally, we describe novel metabolisms or organisms that have recently been discovered but have not yet been fully explored in estuaries to date. While the majority of research has been conducted in the benthos, we also describe data from estuarine water columns. Understanding both the common patterns and the differences between estuaries has important implications for how these critical ecosystems respond to changing environmental conditions.
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Isotopic enrichment factors are key to using stable isotope signatures in biogeochemical studies. However, these are typically determined in laboratory experiments and their applicability to environmental conditions is difficult to test. Here, we analyzed nitrogen stable isotope changes associated with nitrification in a coastal basin using weekly time‐series measurements of δ15N in particulate nitrogen, ammonium, nitrite, and nitrate. Two year‐long time series were selected as contrasting natural experiments in the ammonium‐rich, aphotic bottom water of Bedford Basin, Nova Scotia, Canada. In 2014, ammonia oxidation (AO) was associated with Thaumarchaeota and nitrite concentrations remained low (< 0.5 μmol kg−1). In contrast, transient nitrite accumulation (~ 8 μmol kg−1) and a more rapid δ15NNH4 increase in the fall of 2017 were likely caused by ammonia‐oxidizing bacteria, associated with higher AO rates and, possibly, stronger nitrogen‐isotope enrichment (15εAO). Estimates of 15εAO (21.8 ± 2.2‰, 24.1 ± 1.1‰) were derived empirically using Rayleigh models applied to field data from restricted periods during which the bottom waters approximated a closed system and influence on 15εAO from other processes was demonstrably insignificant. Using a numerical reactive‐transport model, we found that the best fit for the δ15N data was obtained with 15εAO values (18.9‰, 25.1‰) close to those determined by the Rayleigh models. The time series also revealed substantial (~ 7‰) 15N‐enrichment of the particulate nitrogen due to light‐independent assimilation of partially nitrified ammonium. Consistent with previous studies, these field‐based nitrogen isotope fractionation experiments suggest that the range of 15εAO values relevant for marine systems may be narrower than determined in laboratory studies.
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Nitrogen stable-isotope compositions ([delta]15N) can help track denitrification and N2O production in the environment, as can knowledge of the isotopic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of dissolved-oxygen concentration and their influence on the isotopic composition of N2O are not known. We developed a simple steady-state reactor to allow the measurement of denitrification isotope effects in Paracoccus denitrificans. With [dO2] between 0 and 1.2 {micro}M, the N stable-isotope effects of NO3[-] and N2O reduction were constant at 28.6[per thousand] {+/-} 1.9[per thousand] and 12.9[per thousand] {+/-} 2.6[per thousand], respectively (mean {+/-} standard error, n = 5). This estimate of the isotope effect of N2O reduction is the first in an axenic denitrifying culture and places the [delta]15N of denitrification-produced N2O midway between those of the nitrogenous oxide substrates and the product N2 in steady-state systems. Application of both isotope effects to N2O cycling studies is discussed.
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The δ 18O value of nitrate produced during nitrification (δ 18O NO3,nit) was measured in experiments designed to mimic oceanic conditions, involving cocultures of ammonia-oxidizing bacteria or ammonia-oxidizing archaea and nitrite-oxidizing bacteria, as well as natural marine assemblages. The estimates of ranged from -1.5‰ ± 0.1‰ to + 1.3‰ ± 1.4‰ at δ 18O values of water (H 2O) and dissolved oxygen (O 2) of 0‰ and 24.2‰ vs. Vienna Standard Mean Ocean Water, respectively. Additions of 18O-enriched H 2O allowed us to evaluate the effects of oxygen (O) isotope fractionation and exchange on. Kinetic isotope effects for the incorporation of O atoms were the most important factors for setting overall values relative to the substrates (O 2 and H 2O). These isotope effects ranged from +10‰ to + 22‰ for ammonia oxidation (O 2 plus H 2O incorporation) and from +1‰ to +27‰ for incorporation of H 2O during nitrite oxidation. values were also affected by the amount and duration of nitrite accumulation, which permitted abiotic O atom exchange between nitrite and H 2O. Coculture incubations where ammonia oxidation and nitrite oxidation were tightly coupled showed low levels of nitrite accumulation and exchange (3% ± 4%). These experiments had δ 18O NO3values of -1.5‰ to +0.7‰. Field experiments had greater accumulation of nitrite and a higher amount of exchange (22% to 100%), yielding an average δ 18O NO3value of +1.9‰ ± 3.0‰. Low levels of biologically catalyzed exchange in coculture experiments may be representative of nitrification in much of the ocean where nitrite accumulation is low. Abiotic oxygen isotope exchange may be important where nitrite does accumulate, such as oceanic primary and secondary nitrite maxima. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.
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Water column depth profiles along the eastern North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate N-15/N-14 and O-18/O-16 associated with denitrification in the oxygen-deficient waters of the eastern tropical North Pacific. The coupled variations in N-15/N-14 and O-18/O-16 suggest that the O and N isotope effects for denitrification are roughly equivalent at 20-25 permil. This is consistent with our culture study of denitrifiers in seawater medium but stands in contrast to the results of published freshwater studies, which suggest that the isotope effect for N is greater than that for O. The maximum in nitrate O-18/O-16 is somewhat shallower than the maxima in both nitrate N-15/N-14 and the nitrate deficit (as reconstructed from nitrate and phosphate concentrations). This reflects a depth-variant deviation toward higher nitrate O-18/O-16 (or lower N-15/N-14) than can be explained by denitrification if the N and O isotope effects of denitrification are indeed equal. We tentatively interpret the apparent deviation to result from the addition of low-N-15/N-14 nitrate to the shallow thermocline by the remineralization of newly fixed N. A simple model indicates that this remineralization has erased roughly half the nitrate deficit that would otherwise be present at 200 m depth in this region.
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Oxygen minimum zones (OMZs) are major sites of fixed nitrogen removal from the open ocean. However, commonly used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids where O2 falls in the suboxic range (O2 < 2-10 mmol m-3), thereby underestimating the extent of O2 depletion in OMZs. We evaluate the distribution of the OMZs by (1) mapping high-quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. The resulting suboxic volumes are a factor 3 larger than in the uncorrected gridded WOA. We combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. We obtain a removal of fixed nitrogen of 70 ± 50 Tg year-1 in the open ocean and 198 ± 64 Tg year-1 in the sediments, and a global N2O production of 6.2 ± 3.2 Tg year-1. Our results (1) reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes, (2) revise existing estimates of sediment denitrification down by 1/3d through the use of spatially explicit fluxes, and (3) provide independent evidence supporting the idea of a historically balanced oceanic nitrogen cycle. These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg year-1 of nitrogen per 1 mmol m-3 of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.
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A major impediment to understanding long-term changes in the marine nitrogen (N) cycle is the persistent uncertainty about the rates, distribution, and sensitivity of its largest fluxes in the modern ocean. We use a global 3-dimensional ocean circulation model to obtain the first estimate of marine denitrification rates that is maximally consistent with available observations of nitrate deficits and the nitrogen isotopic ratio of ocean nitrate. We find a global rate of marine denitrification in suboxic waters and sediments of 120-240 Tg N yr-1, which is lower than most other recent estimates. The difference stems from the ability to represent the 3-D spatial structure of suboxic zones, where denitrification rates of 50-77 Tg N yr-1 result in up to 50% depletion of nitrate. This depletion reduces the effect of local isotopic enrichment on the rest of the ocean, allowing the N isotope ratio of oceanic nitrate to be achieved with a sedimentary denitrification rate about 1.3-2.3 times that of suboxic zones. This balance of N losses between sediments and suboxic zones is shown to obey a simple relationship between isotope fractionation and the degree of nitrate consumption in the core of the suboxic zones. The global denitrification rates derived here suggest that the marine nitrogen budget is likely close to balanced.
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N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low-delta 15N nitrate throughout the Sargasso Sea thermocline, which we attribute primarily to the input of recently fixed N. Although previous work suggests southward increases in N2 fixation and ventilation age, no meridional trend in nitrate delta 15N is apparent. In the upper 200 m, the algal uptake-driven increase in nitrate delta 18O is greater than in delta 15N, because of (1) a higher fraction of nitrate from N2 fixation at shallower depths and/or (2) cycling of N between nitrate assimilation and nitrification. A mean depth profile of newly fixed nitrate estimated from the nitrate isotope data is compared with results from an ocean circulation model forced with different Atlantic fields of N2 fixation. The nitrate from N2 fixation is communicated between the model's North and South Atlantic and suggests a whole Atlantic N2 fixation rate between 15 and 24 Tg N a-1. One important caveat is that fixed N in atmospheric deposition may contribute a significant proportion of the low-delta 15N N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N2 fixation would still be too high.
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The δ¹⁵N of dissolved ammonium was determined in three anoxic marine basins: Black Sea, Saanich Inlet, B.C., Canada, and Framvaren Fjord, Norway. In each basin, the δ¹⁵N-NH{sub 4{sup +}} was greatest near the Oâ/HâS interface, with δ¹⁵N as high as +21{per thousand}. The depth distributions of NH{sub 4{sup +}} and δ¹⁵N-NH{sub 4{sup +}} for Black Sea and Framvaren Fjord were examined with a one-dimensional, steady-state, vertical advection-diffusion model to calculate the isotope fractionation during the consumption of NH{sub 4{sup +}} by bacteria. Isotope enrichments, {var epsilon}, for Black Sea were between 5 and 15{per thousand}, whereas in Framvaren Fjord {var epsilon} ranged from 20 to 30{per thousand}. These differences are related mainly to the ambient concentration of NH{sub 4{sup +}}. Biosynthetic uptake of NH{sub 4{sup +}} rather than nitrification was responsible for the fractionation. The δ¹⁵N-NH{sub 4{sup +}} in Saanich Inlet appears related to in-situ regeneration of NH{sub 4{sup +}} with little isotopic fractionation between dissolved and particulate nitrogen (PN).
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1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O 2 ] $ 3 mM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the ''redoxcline'' (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15 N/ 14 N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15 N/ 14 N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18 O/ 16 O suggest that the lower-than-expected 15 N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low-15 N/ 14 N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.
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A new quasi-conservative tracer N*, defined as a linear combination of nitrate and phosphate, is proposed to investigate the distribution of nitrogen fixation and denitrification in the world oceans. Spatial patterns of N* are determined in the different ocean basins using data from the Geochemical Ocean Sections Study (GEOSECS) cruises (1972-1978) and from eight additional cruises in the Atlantic Ocean. N* is low (2.0mumolkg-1) indicative of prevailing nitrogen fixation are found in the thermocline of the tropical and subtropical North Atlantic and in the Mediterranean. This suggests that on a global scale these basins are acting as sources of fixed nitrogen, while the Indian Ocean and parts of the Pacific Ocean are acting as sinks. Nitrogen fixation is estimated in the North Atlantic Ocean (10°N-50°N) using the N* distribution along isopycnal surfaces and information about the water age. We calculate a fixation rate of 28TgNyr-1 which is about 3 times larger than the most recent global estimate. Our result is in line, however, with some recent suggestions that pelagic nitrogen fixation may be seriously underestimated. The implied flux of 0.072molNm-2yr-1 is sufficient to meet all the nitrogen requirement of the estimated net community production in the mixed layer during summer at the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea. Extrapolation of our North Atlantic estimate to the global ocean suggests that the present-day budget of nitrogen in the ocean may be in approximate balance.
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We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48+/-5TgNyr-1 is calculated for the Eastern Tropical Pacific using N* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15+/-3TgNyr-1. We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4+/-12TgNyr-1. Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59+/-14TgNyr-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron suppy.
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Heterocystous, planktonic cyanobacteria capable of fixing atmospheric N2 into available nitrogen (N) are common and critically important to nutrient cycling in many lakes, yet they are rarely observed in estuaries at salinities >10 ppt, even when strongly N limited. In a series of meso- cosm experiments using water from Narragansett Bay (Rhode Island), we manipulated top-down (grazing) and bottom-up (nutrient) factors hypothesized to exclude heterocystous cyanobacteria from estuaries. We previously reported that planktonic, heterocystous cyanobacteria grew and fixed N in the absence of grazers. Here, we focus on responses to phosphorus (P) additions and grazer manipu- lations. Zooplankton (Acartia sp.) populations typical of temperate zone estuaries suppressed cyanobacteria, and their influence was direct through grazing rather than indirect on nutrient stoi- chiometry. Cyanobacterial abundance and heterocysts were low in treatments with no external P inputs. Concentrations of dissolved inorganic P comparable to those in Narragansett Bay were obtained only in P-fertilized mesocosms. Unlike previous estuarine mesocosm experiments with P fertilization, planktonic cyanobacteria grew and fixed N in our experimental systems. However, mean cell and heterocyst abundances under the most favorable conditions (high P, low N:P, and low grazers) were much lower than in comparable freshwater experiments, with N limitation maintained. These results support the hypothesis that intrinsic growth of heterocystous cyanobacteria in saline estuaries is slower than in freshwater, and that slower growth is unlikely to be due to systematic dif- ferences in P availability. Slow growth, combined with grazing, can severely limit development of planktonic, N-fixing cyanobacterial blooms in estuaries.
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We report the first measurements of coupled nitrogen (N) and oxygen (O) isotopic variations of nitrate (NO ) 2 3 during its assimilation by laboratory cultures of marine phytoplankton and derive the N and O kinetic isotope effects for nitrate assimilation by three species of diatoms ( Thalassiosira weissflogii, Thalassiosira oceanica,and Thalas- siosira pseudonana) and a coccolithophorid (Emiliana huxleyi). Large interspecies and intraspecies variations in the N isotope effects were observed. The O isotope effect associated with nitrate consumption was consistently close to the N isotope effect, such that the 18 O/ 16 O and 15 N/ 14 N of nitrate varied in a ratio of ;1 : 1, regardless of species or of the magnitude of the isotope effect. In addition, the 18 O/ 16 O and 15 N/ 14 N of internal nitrate of T. weissflogii grown under various environmental conditions were elevated relative to the medium nitrate by a proportion of ; 1: 1. These findings are consistent with a nitrate isotopic fractionation mechanism that involves nitrate reduction as the chief fractionating step. The observed N : O isotopic coupling during nitrate assimilation suggests that combined N and O isotopic measurements of water column nitrate can provide new constraints on the ocean N cycle.
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The eastern Mediterranean Sea is an unusually nutrient-poor ocean basin where the N-15/N-14 isotope ratios in many compartments of reactive nitrogen are lower than in comparable oceanic settings. To elucidate possible reasons, we determined stable isotope ratios in nitrate, suspended particulate nitrogen (PN), and total dissolved reduced nitrogen for stations across the eastern Mediterranean Sea occupied in January and February 2007; sinking PN was collected at one of the stations in the period from February to September 2007. The delta N-15 levels of all reactive N compartments in waters of the basin are very low (grand average 2.6 parts per thousand) compared to other oceanic environments. Deep water nitrate below 500 m water depth (delta N-15 = 2.2 +/- 0.3 parts per thousand) was more depleted in N-15 than nitrate generally found in deep water nitrate pools of other oceans (delta N-15 ranges from 4.7 to 5.4 parts per thousand), whereas N-15 was enriched in suspended particulate N (delta N-15 = 7.3 +/- 0.8 parts per thousand) and reduced dissolved N (delta N-15 = 5.7 +/- 3.8 parts per thousand) compared to nitrate and sinking particulate N intercepted in sediment traps (delta N-15 = 0.9 +/- 0.8 parts per thousand). We infer that extensive mineralization is the cause of the isotopic makeup of reactive N in deep water, in concert with the lack of water column denitrification. Nitrogen and oxygen isotope ratios in nitrate of the mixed layer suggest an external source of nitrate depleted in N-15, probably anthropogenic NOx rather than fixed nitrogen. To explain the observed isotope anomaly in the mixed layer, either the ammonium formed by the breakdown of organic matter must be predominantly nitrified, or atmospheric NOx characteristically enriched in O-18 was present.
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Rainwater collected on the island of Bermuda between January 2000 and January 2001 shows pronounced seasonal variation in the nitrogen and oxygen isotopic composition of nitrate. Higher 15N/14N and lower 18O/16O ratios are observed in the warm season (April-September) in comparison to the cool season (October-March): The mean δ15N of nitrate for the warm and cool seasons is - 2.1‰ and - 5.9‰ (versus air N2), respectively, while the mean δ18O is 68.6‰ and 76.9‰ (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high 15N/14N and low 18O/16O exhibited trajectory paths originating from the south, similar to those of warm season samples. Accordingly, the region from which air is transported to the island determines the 15N/14N and 18O/16O of the nitrate. The source region provides precursor nitrogen oxides (NOx), influencing the 15N/14N of nitrate, and contributes to the chemistry that produces nitrate from NOx, which determines the 18O/16O of nitrate. While the range in nitrate 15N/14N observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season 15N/14N suggests that lightning is a significant source of nitrate to Bermuda. The isotopic evidence for a significant southern source of nitrate to Bermuda helps to explain the previous observation of unexpectedly high nitrate concentrations in warm season rain. The 18O/16O of nitrate in rain at Bermuda is high throughout the year (δ18O = 60.3 to 86.5‰) as a result of interactions of precursor NOx with ozone, which has a high 18O/16O ratio. The lower nitrate 18O/16O in the warm season and in cool season air masses from the south is consistent with elevated concentrations of hydroxyl radical (OH), which dilutes the isotopic signal of ozone. Our limited data set suggests that the relative importance of the OH sink for NOx during the cool season varies spatially over as large a range as is observed between the warm and cool seasons.
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We report the first measurements of coupled nitrogen (N) and oxygen (O) isotope fractionation of nitrate by laboratory cultures of denitrifying bacteria. Two seawater strains (Pseudomonas stutzeri, Ochrobactrum sp.) and three freshwater strains (Paracoccus denitrificans, Pseudomonas chlororaphis, Rhodobacter sphaeroides) were examined. Among four strains of facultative anaerobic denitrifiers, N and O isotope effects were variable, ranging from 5% to 25%, with evidence for a drop in the isotope effects as nitrate concentrations approached the half-saturation constant for nitrate transport. O isotope effects were similar to their corresponding N isotope effect, such that the progressive increase in nitrate δ18O, when plotted against that in δ15N (where δ18 Osample = [(18O : 16O)sample/(18O : 16O)reference - 1] x 1000, and δ15N sample = [(15N : 14N)sample/( 15N : 14N)reference - 1] x 1000), yielded slopes of 0.86 to 1.02, with a mean value of 0.96. R. sphaeroides, a photo-heterotroph that possesses only a periplasmic (nonrespiring) dissimilatory nitrate reductase, showed less variability in nitrate N isotope effects, between 13% and 20%, with a modal value of ∼15%. In contrast to the respiratory denitrifiers, R. sphaeroides consistently showed a distinct ratio of δ18O to δ15N change of ∼0.62. We hypothesize that heavy N and O isotope discrimination during respiratory denitrification occurs during the intracellular reduction of nitrate by the respiratory nitrate reductase, and the observed magnitude of fractionation is likely regulated by the ratio of cellular nitrate efflux relative to uptake. The data for R. sphaeroides are consistent with isotope discrimination directly reflecting the N and O isotope effects of the periplasmic nitrate reductase NAP, without modification by nitrate uptake and efflux. © 2008, by the American Society of Limnology and Oceanography, Inc.
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Little is known about dissolved inorganic nitrogen (DIN) transformations in hydrothermal vent (HV) fluids. Here, we present the first isotopic measurements of nitrate (δ15N and δ18O) and ammonium (δ15N) from three HV fields on the Juan de Fuca ridge (NE-Pacific). The dominant process that drives DIN concentration variations in low-T diffuse fluids is water mass mixing below the seafloor, with no effect on the DIN isotope ratios. Strong inter-site variations in the concentration and δ15N of NH4+ in high-T fluids suggest different subsurface nitrogen (N) sources (deep-sea nitrate versus organic sediments) for hydrothermally discharged ammonium. Low NH4+ community N isotope effects (<3‰) for net NH 4+ consumption suggest an important contribution from gross ammonium regeneration in low-T fluids. Elevation of HV nitrate 15N/14N and 18O/16O over deep-sea mean isotope values at some sites, concomitant with decreased nitrate concentrations, indicate assimilatory or dissimilatory nitrate consumption by bacteria in the subsurface, with relatively low community N isotope effects (15εk < 3‰). The low N isotope effects suggest that nitrate assimilation or denitrification occur in bacterial mats, and/or in situ production of low δ15N nitrate. A significantly stronger relative increase for nitrate δ18O than for δ15N was observed at many sites, resulting in marked deviations from the 1:1 relationship for nitrate δ15N versus δ18O that is expected for nitrate reduction in marine settings. Simple box-model calculation show that the observed un-coupling of N and O nitrate isotope ratios is consistent with nitrate regeneration by either nitrite reoxidation and/or partial nitrification of hydrothermal ammonium (possibly originating from N2 fixation). Our isotope data confirm the role of subsurface microbial communities in modulating hydrothermal fluxes to the deep ocean.
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The nitrogen (N) stable isotopic composition of pore water nitrate and total dissolved N (TDN) was measured in sediments of the St. Lawrence Estuary and the Gulf of St. Lawrence. The study area is characterized by gradients in organic matter reactivity, bottom water oxygen concentrations, as well as benthic respiration rates. N isotope effects on the water column associated with the benthic exchange of nitrate (εapp) and TDN (εsed) during benthic nitrification-denitrification coupling were investigated. The sediments were a major sink for nitrate and a source of reduced dissolved N (RDN = DON + NH4+). We observed that both the pore water nitrate and RDN pools were enriched in 15N relative to the water column, with increasing δ15N downcore in the sediments. As in other marine environments, the biological nitrate isotope fractionation of net fixed N loss was barely expressed at the scale of sediment-water exchange, with &varepsilon;app values
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The accurate measurement of ammonium concentrations is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yields inconsistent results, particularly when ammonium concentrations are low. Here, we present a fluorometric method that gives precise measurements of ammonium over a wide range of concentrations and salinities emphasizing submicromolar levels. The procedure not only solves analytical problems but also substantially simplifies sample collection and preservation. It uses a single working reagent (consisting of orthophthaldialdehyde, sodium sulfite, and sodium borate) that is stable for months when stored in the dark. The working reagent and sample can be mixed immediately after sample collection and the reaction proceeds to completion within 3 h at room temperature. Matrix effects and background fluorescence can be corrected without introducing substantial error. This simple method produces highly reproducible results even at very low ammonium concentrations.
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1] We report 15 N/ 14 N measurements of water column nitrate and ammonium, sinking particles, and sediments from the Cariaco Basin, an anoxic marine basin off the coast of Venezuela. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic/anoxic interface ($275 m). Being free of a large denitrification signal, the d 15 N of shallow thermocline nitrate is $3.5%, significantly lower than the mean deep ocean nitrate d 15 N of 5%. This may be due to the nitrification of newly fixed N, whether it occurs within the basin or in open Atlantic waters that flow into the Cariaco over the sill. The 15 N/ 14 N of the sinking flux in the deepest trap ($1250 m) is similar to that of thermocline nitrate, as expected given the complete consumption of nitrate in the surface layer. Moreover, the 15 N/ 14 N of the seafloor sediment is similar to that of the sinking flux, as is common in environments of high export production, low O 2 , and good organic matter preservation. Thus the modern Cariaco Basin records the 15 N/ 14 N of the thermocline nitrate, which, in turn, may record the input of newly fixed N to the upper ocean, be it local or more regional in origin.
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The bioavailability of nutrients represents one of the most important factors controlling the strength of the biological carbon pump and ultimately the impact of ocean biology on atmospheric CO2. Among those nutrients, the macro-nutrients nitrate (NO 2-) and phosphate (PO 4-3) play a particularly important role in limiting biological productivity as evidenced by their often near complete exhaustion in surface waters. As near surface NO 2- concentrations are generally somewhat lower than those of PO 4-3 relative to the demand by phytoplankton, biological oceanographers have argued historically that NO 2- rather than PO 4-3 is the primary macro-nutrient controlling phytoplankton productivity[Smith, 1984; Codispoti, 1989; Tyrrell, 1999] . Geologists, in contrast, regarded PO 4-3 as the primary controlling macronutrient[Codispoti, 1989]. They argued that while NO 2- may indeed be the limiting factor at any given location and time, PO 4-3 is truly the limiting factor on geological time-scales, because the biologically mediated fixation of the much more abundant dinitrogen gas (N2) into organic nitrogen is alleviating the scarcity of bioavailable nitrogen (Figure 1). Phosphate on the other hand, does not have such a biologically mediated source (Figure 1). It is therefore the geologically controlled balance between the riverine (and atmospheric) input of PO 4-3 and its burial on the sea-floor that ultimately controls marine biological productivity. Tyrrell [ 1999] provided a synthesis of these two views by identifying NO 2- as the proximate nutrient, while giving PO 4-3 the role of being the ultimate nutrient.
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1] A nitrogen stable isotopic model was constructed in order to constrain the Holocene marine-fixed nitrogen budget. The primary sources and sinks considered were riverine and atmospheric sources, nitrogen fixation, sedimentary and water column denitrification, and sediment burial. The source budget was found to be insensitive to changes in nitrogen fixation rates, and thus could not be used to constrain this term. However, the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification. If the isotopic value of marine-fixed nitrogen has not changed during the Holocene, as supported by sedimentary records, then in order to balance the isotopic value of sinks and sources, approximately 280 Tg N yr À1 of sedimentary denitrification is required. If such a high rate of denitrification has been sustained throughout the Holocene, it implies that present-day estimates of marine nitrogen fixation are grossly underestimated. It also implies that the marine nitrogen budget has a residence time of less than 2000 years. Citation: Brandes, J. A., and A. H. Devol, A global marine-fixed nitrogen isotopic budget: Implications for Holocene nitrogen cycling, Global Biogeochem. Cycles, 16(4), 1120, doi:10.1029/2001GB001856, 2002.
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1] The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of N 2 fixation in providing new nitrogen (N), we investigated the depth distribution of nitrate d 15 N and d 18 O at six stations across the Azores Front in the NE Atlantic. In addition, we measured the d 15 N of dissolved organic N (DON) in surface waters and of sinking particulate N collected in sediment traps at 2000 m depth between 2003 and 2005 at Station KIEL276. The nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the d 15 N from depth toward low-nitrate surface waters, concomitant with an increase in d 18 O. Given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate d 15 N and d 18 O, the observed surface water nitrate isotope anomalies (D(15;18) up to À6%) indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed N that is remineralized in surface and subsurface waters. Both the concentration of DON and its d 15 N in surface water were spatially invariant, showing mean values of 4.7 ± 0.5 mmol L À1 and 2.6 ± 0.4% (n = 35), respectively, supporting the conjecture of a mostly recalcitrant DON pool. The weighted biannual mean d 15 N of sinking particulate N (1.8 ± 0.8%, n = 33) was low with respect to thermocline nitrate. The anomalous dual nitrate isotope signatures together with the low d 15 N of export production and elevated nitrate-to-phosphate ratios in surface and subsurface waters strongly suggest that N 2 fixation represents a substantive source of N in this part of the subtropical northeast Atlantic. Simple isotope mass balance suggests that, locally, N 2 fixation supplies between 56 and 259 mmol N m À2 a À1 for phytoplankton growth in the photic zone, accounting for up to $40% of the estimated export production.
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Samples were collected from the coastal waters off the states of Washington and California, from the central North Atlantic and northeast Pacific Oceans, and from Chesapeake Bay. Total abundances of ammonium oxidizers ranged from 107 cells 1-1 in Chesapeake Bay and 105 cells 1-1 in inshore ocean waters to 103 to 104 cells 1-1 in the open ocean. A potential nitrification rate was calculated. Microbial nitrification rates in the water column can be substantial and may be sufficient to balance the annual new production demand for nitrate.-from Author
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Primary production in the oceans results from allochthonous nutrient inputs to the euphotic zone (new production) and from nutrient recycling in the surface waters (regenerated production). Global new production is of the order of 3.4-4.7 × 109 tons of carbon per year and approximates the sinking flux of paniculate organic matter to the deep ocean.
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The present study examines oxygen and phosphorus dynamics at a seasonally hypoxic site in the Arkona basin of the Baltic Sea. A coupled benthic–pelagic reactive-transport model is used to describe the evolution of bottom-water solute concentrations, as well as pore-water and sediment profiles. Aerobic respiration dominates remineralization, with iron reduction, denitrification, and sulphate reduction playing secondary roles, while other pathways are negligible. Sediments represent a significant oxygen sink chiefly due to the aerobic degradation of organic matter, as well as nitrification and iron oxyhydroxide precipitation. Most phosphorus deposited in sediments is in organic matter, yet cycling is dominated by iron-bound phosphorus due to rapid dissimilatory iron reduction coupled with aerobic iron oxyhydroxide formation. Sustained hypoxia results in an initial decrease in sediment phosphorus content due to dissolution of phosphorus-bearing iron oxyhydroxides, resulting in a pulse of phosphate to overlying waters. Although an organic-rich layer is formed under low-oxygen conditions, enhanced remineralization of organic phosphorus relative to organic carbon tempers sedimentary phosphorus accumulation. Upon reoxygenation of bottom waters after a decade of sustained hypoxia, oxygen concentrations do not immediately achieve values observed prior to hypoxia because the organic-rich layer creates a higher benthic oxygen demand. Artificial reoxygenation of bottom waters leads to a substantial increase in the iron-bound phosphorus pool&semi; the total phosphorus content of the sediment, however, is unaffected. A relapse into hypoxia would consequently produce a large pulse of phosphate to the overlying waters potentially exacerbating the situation.
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Abstract During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (d,3 undercurrent chemistry. The equatorial Pacific is one of the Pacific Ocean’s three major high nutrient‐low chlorophyll (HNLC) regions. It is distinguished by persistent equatorial upwelling, and this dominant source of new macronutrients (nitrogen [N], phos- phorus [P], and silicon [Si]) to the euphotic zone (e.g., Wyrt-
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Denitrification rates in sediments within the oxygen deficient waters off Mexico and from the Gulf of Maine were investigated on the basis of interstitial nutrient profiles. Nitrate fluxes into the sediments were calculated from gradients across the sediment-water interface and vertical molecular diffusion coefficients and averaged 0.151 (Mexico) and 0.0920 (Gulf of Maine) pmol NO−3 cm−2 s−1. These are minimum values, since these gradients may have been underestimated. In the Gulf of Maine, bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate. In addition, only 15-22% of the expected ammonium is present in Gulf of Maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification. This large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water. The global denitrification rate in continental shelf sediments was reassessed to be >50 Tg N yr−1 (1 Tg = 1012 g), demonstrating that sediments are an important sink for oceanic nitrogen. Globally, current nitrogen losses from the oceans may exceed inputs by 60-90 Tg N yr−1. Over the glacial-interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area. An oscillating oceanic nitrogen budget over these time scales could occur given the sequence of (1) scouring and dumping of terrestrial nitrogen into the oceans during glacial advance, (2) removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat, and (3) reincorporation of this N into terrestrial biomass during the interglacial period.
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Over geological time, photosynthetic carbon fixation in the oceans has exceeded respiratory oxidation of organic carbon. The imbalance between the two processes has resulted in the simultaneous accumulation of oxygen in, and drawdown of carbon dioxide from, the Earth's atmosphere, and the burial of organic carbon in marine sediments1-3. It is generally assumed that these processes are limited by the availability of phosphorus4,5, which is supplied by continental weathering and fluvial discharge5-7. Over the past two million years, decreases in atmospheric carbon dioxide concentrations during glacial periods correlate with increases in the export of organic carbon from surface waters to the marine sediments8-11, but variations in phosphorus fluxes appear to have been too small to account for these changes12,13. Consequently, it has been assumed that total oceanic primary productivity remained relatively constant during glacial-to-interglacial transitions, although the fraction of this productivity exported to the sediments somehow increased during glacial periods12,14. Here I present an analysis of the evolution of biogeochemical cycles which suggests that fixed nitrogen, not phosphorus, limits primary productivity on geological timescales. Small variations in the ratio of nitrogen fixation to denitrification can significantly change atmospheric carbon dioxide concentrations on glacial-to-interglacial timescales. The ratio of these two processes appears to be determined by the oxidation state of the ocean and the supply of trace elements, especially iron.
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We examined the oxygen isotopic systematics for ammonia oxidation, the first step in the regeneration of nitrate from ammonium. In particular, oxygen isotopic fractionation and exchange with water were evaluated for their roles in determining the delta(18)O of nitrite produced by four species of ammonia-oxidizing bacteria (AOB). Microbially catalyzed oxygen isotopic exchange between nitrite and water was less than 25% at low cell densities (106 cells mL(-1)) and ammonium concentrations (less than 50 mu mol L(-1)). The amount of exchange was relatively constant for a given species of ammonia oxidizer but varied between 1% and 25% among the four species tested. The delta(18)O value of nitrite produced at a given water delta(18)O value also varied by nearly 10% among the different species. Isotopic fractionation, either during oxygen (O(2)) incorporation by ammonia monooxygenase and/or water incorporation by hydroxylamine oxidoreductase plays an important role in setting the delta(18)O of nitrite produced by AOB. This work provides a detailed characterization of the oxygen isotopic systematics of ammonia oxidation by AOB, which will help us better interpret the oxygen isotopic distributions of nitrite, nitrate, and nitrous oxide in terrestrial and aquatic environments.
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Measurements of the N2 produced by denitrifica- tion, a better understanding of non-canonical pathways for N2 production such as the anammox reaction, better appre- ciation of the multiple environments in which denitrification can occur (e.g. brine pockets in ice, within particles outside of suboxic water, etc.) suggest that it is unlikely that the oceanic denitrification rate is less than 400 Tg N a 1 . Be- cause this sink term far exceeds present estimates for nitro- gen fixation, the main source for oceanic fixed-N, there is a large apparent deficit ( 200 Tg N a 1 ) in the oceanic fixed-N budget. The size of the deficit appears to conflict with appar- ent constraints of the atmospheric carbon dioxide and sed- imentary 15 N records that suggest homeostasis during the Holocene. In addition, the oceanic nitrate/phosphate ratio tends to be close to the canonical Redfield biological uptake ratio of 16 (by N and P atoms) which can be interpreted to in- dicate the existence of a powerful feed-back mechanism that forces the system towards a balance. The main point of this paper is that one cannot solve this conundrum by reducing the oceanic sink term. To do so would violate an avalanche of recent data on oceanic denitrification. A solution to this problem may be as simple as an up- wards revision of the oceanic nitrogen fixation rate, and it is noted that most direct estimates for this term have concen- trated on nitrogen fixation by autotrophs in the photic zone, even though nitrogen fixing genes are widespread. Another simple explanation may be that we are simply no longer in the Holocene and one might expect to see temporary imbal- ances in the oceanic fixed-N budget as we transition from the Holocene to the Anthropocene in line with an apparent deni- trification maximum during the Glacial-Holocene transition. Other possible full or partial explanations involve plausible changes in the oceanic nitrate/phosphate and N/C ratios, an oceanic phosphorus budget that may also be in deficit, and
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Vertical profiles of C-14-uptake were acquired monthly from the mouths and landward stations of periodically anoxic Saanich Inlet and oxygenated Jervis Inlet, British Columbia, Canada from August 1985 to October 1989. Saanich Inlet (490 g C m(-2) year(-1)) was 1.7 times more productive than Jervis Met (290 g C m(-2) year(-1)) and primary production toward the mouths of both inlets was 1.4 times higher than at the landward stations. The elevated rates of primary production in Saanich Inlet may have been due to exchange with the nutrient-rich surface waters of the passages leading to the Pacific Ocean and the up-inlet gradients in both fjords also may have reflected relative nutrient supply. Sediment-trap results show enhanced fluxes of biogenic silica to the deep waters of Saanich Inlet; associated organic matter is likely to have caused a large oxygen demand. Combined with the high primary production and export flux, low rates of vertical mixing and particle-entrapment within the fjord, factors associated with weak estuarine circulation as well as weak winds and tides in Saanich Inlet, may also stimulate anoxia. Although in Jervis Inlet there is more stagnant water behind the sill and deep-water renewals appear to be less frequent than in Saanich Inlet, the deep sill allows degradation of a significant fraction of the sinking organic matter before the stagnant waters are reached, reducing the chances of oxygen depletion in the bottom waters. (C) 2001 Elsevier Science B.v. All rights reserved.
Article
Saanich Inlet is a fjord (24km long) having a submerged (75-m) sill at the entrance, behind which there is a deep (234-m) basin. The properties of the water have been observed from time to time from 1927 through 1960. The resulting data are reviewed to provide representative values and structures of temperature, salinity, density ( σt), dissolved oxygen content and sound velocity for each month throughout the year. Above the sill depth the properties of the water are normal and continuous with those in the approaches which connect with the Strait of Georgia. The waters below the sill depth are isolated, oxygen-deficient, and usually contain hydrogen sulphide. There is considerable ambient variation in the structures because the currents are too weak to disperse or mix the locally generated concentrations.The runoff into the head of the inlet is negligible. The major source of fresh water is in the approaches. It intrudes the inlet and provides a weak estuarine flushing mechanism above the sill depth. The waters below the sill are flushed only when the water in the approaches becomes sufficiently dense to cascade over the sill into the deep basin.The sound-ranging conditions are far from ideal. From March through July there is a major sound channel at mid-depths above the sill. This vanishes in August and a sound divergence zone develops and persists until December. In addition, the ambient variations provide anomalous transmission conditions.During the autumn (September through November) high concentrations of fish have been observed at the sill depth, associated with the oxycline. Probably the fish are attracted to the area by very large concentrations of zooplankton (Euphausids) that have been observed there at the same time.
Article
The seasonal renewal of deep water in Saanich Inlet was investigated. Origin and temperature-salinity characteristics of the deep water and of the water that renews the deep water were identified. Renewals equivalent to 64 and 33% of the deep-water volume, calculated from nitrate budgets, occurred in 1962 and in 1969. The time for 37% renewal of the deep water was estimated to be about 12 days. A bolus-type flushing mechanism is postulated.
Article
Marine ammonium-oxidizing bacteria in seawater were enumerated by means of immunofluorescent assays developed for Nitrosococcus oceanus and Nitrosomonas marina. Samples were collected from the coastal waters off the states of Washington and California, from the central North Atlantic and northeast Pacific Oceans, and from Chesapeake Bay. Total abundances of ammonium oxidizers ranged from 10/sup 7/ cells 1/sup -1/ in Chesapeake Bay and 10/sup 5/ cells 1/sup -1/ in inshore ocean waters to 10/sup 3/ to 10/sup 4/ cells 1/sup -1/ in the open ocean. Using mean abundances from surface waters, a potential nitrification rate was calculated. Results imply that microbial nitrification rates in the water column can be substantial and may be sufficient to balance the annual new production demand for nitrate.
Article
The dynamic changes of carbon and nitrogen stable isotopic ratios in suspended and sedimented particulate matter were observed together with many other chemical and biological properties during a phytoplankton bloom induced by nutrient addition in a controlled ecosystem enclosure (CEE, about 70 m3) in Saanich Inlet, British Columbia, Canada. Both of the stable isotopic ratios of carbon and nitrogen in suspended particulate organic matter showed characteristic patterns of variations in surface water during the bloom. The particulate organic matter produced in the nutrient controlled phytoplankton bloom can be classified into three phases from an isotopic viewpoint. -from Authors
Article
Large discrepancies in published neon and nitrogen solubility data limit the interpretation of oceanic measurements of these gases. We present new solubility measurements for neon, nitrogen and argon in distilled water and seawater, over a temperature range of 1-30 � C: Water was equilibrated with air at measured temperatures, salinities and pressures. Dissolved Ne concentrations were then determined by isotope dilution using a quadrupole mass spectrometer. Ratios of O2=N2=Ar were measured ona stable isotope ratio mass spectrometer, from which absolute N 2 and Ar concentrations were calculated using published O2 solubilities. We propose new equations, fitted to the data, for the equilibrium concentrations of Ne, N2 and Ar with estimated errors of 0.30%, 0.14% and 0.13%, respectively. The Ar results matched those of most previous researchers within0.4%. However, the Ne and N 2 results were greater by 1% or more thanthose of Weiss (J. Chem. En g. Data 16(2) (1971b) 235, Deep-Sea Res. 17(4) (1970) 721). r 2004 Elsevier Ltd. All rights reserved.
Article
Recent interest in the role of nitrous oxide in the natural nitrogen cycle has focused on its production through the nitrification process1. Nitrous oxide depleted in 15N has been found in the shallow water column of the eastern tropical Pacific Ocean2, suggesting that 15N is depleted during N2O production. Here we report significant 15N depletion in N2O produced by bacterial nitrification, implying that N2O is produced via NO– 2 rather than directly from NH+ 4 and that N2O should be very depleted in 15N where nitrification is the predominant production process for N2O. The magnitude of the isotope depletion is so large that this work should lead to further use of data on the natural abundance of 15N for assessing the sources and sinks of nitrogenous compounds.
Article
DATA on dissolved nitrous oxide (N2O) in the ocean indicate that its distribution is strongly affected by oxygen-controlled biochemical reactions1–4. N2O has been measured in oxygenated, open ocean waters of the Atlantic and Pacific Oceans1–3, and in the oxygen deficient waters of the eastern tropical North Pacific4 but no measurements of N2O in anoxic marine environments have yet been made. This report presents measurements of N2O from Saanich Inlet, an intermittently anoxic basin on the southeastern side of Vancouver Island, British Columbia, and evidence for its consumption during denitrification.
Article
Nitrification rates were measured monthly from April to October 2008, at depths ranging from the lower euphotic zone to the sub-oxic waters of Saanich Inlet. Ammonium (NH4+) and nitrite (NO2−) oxidation rates ranged from undetectable to 0.319 and 0.478 μmol L− 1 d− 1, respectively. NH4+ oxidation rates and concentrations were positively correlated at substrate concentrations less than 0.8 μmol NH4+ L− 1. Positive correlations between NH4+ oxidation rates, NO2− concentrations, and NO2− oxidation rates were also observed, highlighting the important role that NH4+ oxidation plays in supporting NO2− oxidation in Saanich Inlet. Despite the apparent dependence of NO2− oxidation rates on NH4+ oxidation rates, the former was still 44% higher than the latter and we concluded that Saanich Inlet NO2− oxidation rates were augmented by fortnightly spring-tide nutrient renewal. From May to October, sub-oxic zone waters were isolated from any significant mixing events, and we estimated that nitrification was responsible for approximately 25% of dissolved oxygen consumption. This estimate is in close agreement with that calculated using Redfield stoichiometry, and as such highlights the accuracy with which nitrification rates can be quantified using incubation techniques. Finally, nitrification rates in the euphotic zone were at times substantial, and we suggest that earlier estimates of new production in Saanich Inlet may have been overestimated by approximately 15%.Research highlights► We conducted measurements of nitrification through the water column of Saanich Inlet. ► NH4+ oxidation rates and [NH4+] were positively correlated at [NH4+] less than 0.8 μM. ► NH4+ oxidation rates were positively correlated with [NO2−] and NO2− oxidation rates. ► Nitrification was at times substantial in the euphotic zone. ► New production may have previously been overestimated by approximately 15%.
Article
1] Coupled measurements of nitrate (NO 3 À), nitrogen (N), and oxygen (O) isotopic composition (d 15 N NO3 and d 18 O NO3) were made in surface waters of Monterey Bay to investigate multiple N cycling processes occurring within surface waters. Profiles collected throughout the year at three sites exhibit a wide range of values, suggesting simultaneous and variable influence of both phytoplankton NO 3 À assimilation and nitrification within the euphotic zone. Specifically, increases in d 18 O NO3 were consistently greater than those in d 15 N NO3 . A coupled isotope steady state box model was used to estimate the amount of NO 3 À supplied by nitrification in surface waters relative to that supplied from deeper water. The model highlights the importance of the branching reaction during ammonium (NH 4 +) consumption, in which NH 4 + either serves as a substrate for regenerated production or for nitrification. Our observations indicate that a previously unrecognized proportion of nitrate-based productivity, on average 15 to 27%, is supported by nitrification in surface waters and should not be considered new production. This work also highlights the need for a better understanding of isotope effects of NH 4 + oxidation, NH 4 + assimilation, and NO 3 À assimilation in marine environments.