N-loss stoichiometry in a Peru ODZ eddy

Assuming heterotrophic denitrification as the dominant microbial process, Richards (1965) formulated a stoichiometry governing nitrogen loss in open-ocean oxygen deficient zones (ODZs). It prescribes the quantitative coupling between the oxidation of organic matter by NO–3 in the absence of O2 and the corresponding production of CO2, N2, and PO–34. Applied globally, this relationship defines key linkages between the C, N, and P cycles. However, the validity of Richards\u27s stoichiometry is challenged by recognition of complex microbial N processing in ODZs including anammox as an important pathway and nitrite reoxidation. Whereas Richards\u27s stoichiometry would result in N2-N production to NO–3 removal rates of 1.17, dominance by anammox with respect to biogenic N2 production could in theory result in a ratio as high as 2. Ratios with PO–34 production provide an additional constraint on the quantity and composition of respired organic matter. Here we use a mesoscale eddy with extreme N-loss in the Peru ODZ as a natural laboratory to examine N-loss stoichiometry. Its intense biogeochemical signatures, relatively well-defined timescales, and simplified hydrography allowed for the development of strong co-occurring gradients in NO–3, NO–2, biogenic N2, and PO–34. The production of biogenic N2 as compared with the removal of NO–3 (analyzed either directly or as N deficits) was slightly less than predicted by Richards\u27s stoichiometry and did not at all support any excess biogenic N2. PO–34 production, however, was twice the expectation from Richards\u27s stoichiometry suggesting that respired organic matter was P-rich as compared with C:N:P Redfield composition. These results suggest major gaps remain between current understanding of microbial N pathways in ODZs and their net biogeochemical output

Nitrate isotope anomalies as indicator of N2 fixation in the Azores Front region (subtropical N-E Atlantic)

Afin de mieux comprendre les sources et flux de nitrate dans la zone euphotique et le rôle potentiel de la fixation de l'azote atmosphérique dans la région du front des Açores (Atlantique nord-est subtropical), nous avons étudié la distribution du δ15N et δ18O du nitrate à six stations. Nous avons également mesuré le δ15N de l'azote organique dissous (AOD) de l'eau de surface, ainsi que la composition isotopique de l'azote particulaire (AP) exporté de 2003 à 2005 à la station KIEL276 (2000 m de profondeur). Nous observons une diminution du δ15N-NO3 et une augmentation du δ18O-NO3- à mesure que la concentration de nitrate diminue dans l'eau de surface pour la plupart des stations. Étant donné que l'assimilation du nitrate par le phytoplancton produit un enrichissement égal du δ15N et δ18O du nitrate, les anomalies des isotopes du nitrate dans l'eau de surface (Δ(15;18) jusqu'à ~-6%o) indiquent que l'assimilation du nitrate n'est pas le seul processus qui contrôle la composition isotopique du nitrate dans la zone photique et suggère qu'il y a reminéralisation d'azote atmosphérique nouvellement fixé dans les eaux de surface et sous surface. La concentration et le δ15N de l'AOD de l'eau de surface sont spatialement invariables avec des valeurs moyennes respectives de 4,7±0,5 f-µmol/L et 2,6±0,4%o (n=35), ce qui est en accord avec l'idée d'un réservoir océanique d'AOD majoritairement récalcitrant. La moyenne pondérée du δ15N de l'AP exporté (1,8±0,8%o, n=33) pendant les deux années d'échantillonnage est faible par rapport à la valeur du δ15N du nitrate dans la thermocline. Le faible δ15N de l'AP exporté, avec les anomalies isotopiques du N et O du nitrate et l'observation de ratios élevés des concentrations de nitrate par rapport au phosphate dans les eaux de surface et sous surface suggèrent fortement que la fixation de l'azote atmosphérique, étant un apport de matière organique ayant un faible δ15N, représente une composante majeure du cycle de l'azote dans l'Atlantique nord-est subtropical. Des bilans isotopiques simplifiés (pour le N et O du nitrate) indiquent que la fixation de l'azote, à un taux de 56-75 mmol N m-2 an-1, pourrait représenter jusqu'à ~40% de la production primaire exportée et être ainsi aussi importante que la diffusion verticale du nitrate à travers la thermocline pour soutenir la production nouvelle.\ud ______________________________________________________________________________ \ud MOTS-CLÉS DE L’AUTEUR : Fixation de l'azote, Front des Açores, Atlantique nord-est subtropical, δ15N et δ18O du nitrate, δ15N de l'azote organique dissous, δ15N de l'azote particulaire exporté

N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

Mesoscale eddies in Oxygen Minimum Zones (OMZ's) 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 the 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 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 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 NO3− and NO2− reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss

Protocols for Assessing Transformation Rates of Nitrous Oxide in the Water Column

Nitrous oxide (N2O) is a potent greenhouse gas and an ozone destroying substance. Yet, clear step-by-step protocols to measure N2O transformation rates in freshwater and marine environments are still lacking, challenging inter-comparability efforts. Here we present detailed protocols currently used by leading experts in the field to measure water-column N2O production and consumption rates in both marine and other aquatic environments. We present example 15N-tracer incubation experiments in marine environments as well as templates to calculate both N2O production and consumption rates. We discuss important considerations and recommendations regarding (1) precautions to prevent oxygen (O2) contamination during low-oxygen and anoxic incubations, (2) preferred bottles and stoppers, (3) procedures for 15N-tracer addition, and (4) the choice of a fixative. We finally discuss data reporting and archiving. We expect these protocols will make 15N-labeled N2O transformation rate measurements more accessible to the wider community and facilitate future inter-comparison between different laboratories

Global reconstruction reduces the uncertainty of oceanic nitrous oxide emissions and reveals a vigorous seasonal cycle

Assessment of the global budget of the greenhouse gas nitrous oxide ([Formula: see text]O) is limited by poor knowledge of the oceanic [Formula: see text]O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological [Formula: see text]O emissions from the ocean by training a supervised learning algorithm with over 158,000 [Formula: see text]O measurements from the surface ocean-the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of [Formula: see text]O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean [Formula: see text]O flux of 4.2 ± 1.0 Tg N[Formula: see text], 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This [Formula: see text]O flux ranges from a low of 3.3 ± 1.3 Tg N[Formula: see text] in the boreal spring to a high of 5.5 ± 2.0 Tg N[Formula: see text] in the boreal summer. Much of the seasonal variations in global [Formula: see text]O emissions can be traced to seasonal upwelling in the tropical ocean and winter mixing in the Southern Ocean. The dominant contribution to seasonality by productive, low-oxygen tropical upwelling systems (>75%) suggests a sensitivity of the global [Formula: see text]O flux to El Niño-Southern Oscillation and anthropogenic stratification of the low latitude ocean. This ocean flux estimate is consistent with the range adopted by the Intergovernmental Panel on Climate Change, but reduces its uncertainty by more than fivefold, enabling more precise determination of other terms in the atmospheric [Formula: see text]O budget

A Critical Review of the \u3csup\u3e15\u3c/sup\u3eN\u3csub\u3e2\u3c/sub\u3e Tracer Method to Measure Diazotrophic Production in Pelagic Ecosystems

Dinitrogen (N2) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N2 fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported—confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N2 fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N2 fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the 15N2 tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N2 fixation rates using 15N2 tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N2 fixation rate estimates intercomparable among studies

OOI Biogeochemical Sensor Data: Best Practices and User Guide. Version 1.0.0.

The OOI Biogeochemical Sensor Data Best Practices and User Guide is intended to provide current and prospective users of data generated by biogeochemical sensors deployed on the Ocean Observatories Initiative (OOI) arrays with the information and guidance needed for them to ensure that the data is science-ready. This guide is aimed at researchers with an interest or some experience in ocean biogeochemical processes. We expect that users of this guide will have some background in oceanography, however we do not assume any prior experience working with biogeochemical sensors or their data. While initially envisioned as a “cookbook” for end users seeking to work with OOI biogeochemical (BGC) sensor data, our Working Group and Beta Testers realized that the processing required to meet the specific needs of all end users across a wide range of potential scientific applications and combinations of OOI BGC data from different sensors and platforms couldn’t be synthesized into a single “recipe”. We therefore provide here the background information and principles needed for the end user to successfully identify and understand all the available “ingredients” (data), the types of “cooking” (end user processing) that are recommended to prepare them, and a few sample “recipes” (worked examples) to support end users in developing their own “recipes” consistent with the best practices presented here. This is not intended to be an exhaustive guide to each of these sensors, but rather a synthesis of the key information to support OOI BGC sensor data users in preparing science-ready data products. In instances when more in-depth information might be helpful, references and links have been provided both within each chapter and in the Appendix

Simulating the global distribution of nitrogen isotopes in the ocean

We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes prognostic tracers for the two stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3−), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3− uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3− by sedimentary denitrification. A global database of δ15NO3− observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones and the meridional and vertical gradients in the Southern Ocean. The observed pronounced subsurface minimum in the Atlantic is underestimated by the model presumably owing to too little simulated nitrogen fixation there. Sensitivity experiments reveal that algal NO3− uptake, nitrogen fixation, and water column denitrification have the strongest effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 fixation in the model. Important model deficiencies are identified, and strategies for future improvement and possibilities for model application are outlined