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The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradients in upper ocean dissolved organic N and suspended particulate N concentration and/or δ15N were found that would indicate significant N2 fixation fluxes accumulated in these pools, consistent with low Trichodesmium spp. abundance. Finally, comparing the δ15N of sinking particulate N captured within vs. below the euphotic zone indicates that during late spring regenerated N is low in δ15N compared to sinking N.

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol N m−2 d−1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2 – 3 orders of magnitude higher (i.e. 462 – 1144 µmol N m−2 d−1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts., The middle of the Gulf of Mexico is stratified and highly oligotrophic, yet there are anomalously high fluxes of sinking particulate matter from the euphotic zone. Here the authors show that lateral advection of organic matter supports nitrogen export in the Gulf of Mexico’s open ocean.

Biomass and composition of the phytoplankton community were investigated in the deep-water Gulf of Mexico (GoM) at the edges of Loop Current anticyclonic eddies during May 2017 and May 2018. Using flow cytometry, high-performance liquid chromatography pigments and microscopy, we found euphotic zone integrated chlorophyll a of ~10 mg m−2 and autotrophic carbon ranging from 463 to 1268 mg m−2, dominated by picoplankton (

Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general introduction to the BLOOFINZ-GoM project (Bluefin tuna Larvae in Oligotrophic Ocean Foodwebs, Investigation of Nitrogen to Zooplankton) and highlight the finding, based on backtracking of experimental waters to their positions weeks earlier, that lateral transport from the continental slope region may be more of a key determinant of available habitat utilized by larvae than eddy edges per se.

Dissolved organic phosphorus (DOP) has a dual role in the surface ocean as both a product of primary production and as an organic nutrient fueling primary production and nitrogen fixation, especially in oligotrophic gyres. Though poorly constrained, understanding the geographic distribution and environmental controls of surface ocean DOP concentration is critical to estimating distributions and rates of primary production and nitrogen fixation in the global ocean. Here we pair DOP concentration measurements with a metric of phosphate (PO43−) stress (P*), and satellite-based chlorophyll a concentrations and iron stress estimates to explore their relationship with upper 50 m DOP stocks. Our results show that PO43− and iron stress work together to control surface DOP concentrations at basin scales. Specifically, upper 50 m DOP stocks decrease with increasing phosphate stress, while alleviated iron stress leads to either surface DOP accumulation or loss depending on PO43− availability. Our work suggests an interdependence between DOP concentration, inorganic nutrient ratios, and iron availability, and establishes a predictive framework for DOP distributions in the global surface ocean.

The highly stratified, oligotrophic regions of the oceans are predominantly nitrogen limited in the surface ocean and light limited at the deep chlorophyll maximum (DCM). Hence, determining light and nitrogen co-limitation patterns for diverse phytoplankton taxa is crucial to understanding marine primary production throughout the euphotic zone. During two cruises in the deep-water Gulf of Mexico, we measured primary productivity (H13CO3−), nitrate uptake (15NO3−), and ammonium uptake (15NH4+) throughout the water column. Primary productivity declined with depth from the mixed-layer to the DCM, averaging 27.1 mmol C m−2 d−1. The fraction of growth supported by NO3− was consistently low, with upper euphotic zone values ranging from 0.01 to 0.14 and lower euphotic zone values ranging from 0.03 to 0.44. Nitrate uptake showed strong diel patterns (maximum during the day), while ammonium uptake exhibited no diel variability. To parameterize taxon-specific phytoplankton nutrient and light utilization, we used a data assimilation approach (Bayesian Markov Chain Monte Carlo) including primary productivity, nutrient uptake, and taxon-specific growth rate measurements. Parameters derived from this analysis define distinct niches for five phytoplankton taxa (Prochlorococcus, Synechococcus, diatoms, dinoflagellates, and prymnesiophytes) and may be useful for constraining biogeochemical models of oligotrophic open-ocean systems.

We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic bluefin tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show extensive nutrient regeneration fueling primary productivity (mostly by cyanobacteria and other picophytoplankton) in the upper euphotic zone. The food web is dominated by the microbial loop (>70% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). By contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process

We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic bluefin tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show extensive nutrient regeneration fueling primary productivity (mostly by cyanobacteria and other picophytoplankton) in the upper euphotic zone. The food web is dominated by themicrobial loop (>70% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). By contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process, ECOLATUN, ECOlogía trófica comparativa de LArvas de aTUN rojo atlántico (Thunnus thynnus) de las áreas de puesta del Medterraneo-NO y el Golfo de México., SI

Marine oxygen deficient zones (ODZs) are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of th...

Dinitrogen (N-2) 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 N-2 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 N-2 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 N-2 fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the N-15(2) 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 N-2 fixation rates using N-15(2) tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N-2 fixation rate estimates intercomparable among studies.

In a coastal lagoon with a shallow, 25 m water column off the southwest coast of New Caledonia, large-volume ( ∼ 50 m3) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8 µM dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO3−) assimilated prior to the start of the 23-day experiments to N2 fixation by the end of the experiments. While the δ15N of the sinking particulate N (PNsink) flux changed during the experiments, the δ15N of the suspended PN (PNsusp) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the δ15N of surface ocean N pools is less responsive than that of PNsink to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO3− in the mesocosms, the δ15N of PNsink indicated that NO3− continued to fuel a significant fraction of export production (20 to 60 %) throughout the 23-day experiments, with N2 fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO3−, and phytoplankton abundance data suggest that sinking material primarily comprised large diatoms. Concurrent molecular and taxonomic studies indicate that the diazotroph community was dominated by diatom–diazotroph assemblages (DDAs) at this time. However, these DDAs represented a minor fraction (2 fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a Cyanothece-like UCYN-C, proliferated during the last phase of the experiments when N2 fixation, primary production, and the flux of PNsink increased significantly, and δ15N budgets reflected a predominantly diazotrophic source of N fueling export. At this time, the export flux itself was likely dominated by the non-diazotrophic diatom, Cylindrotheca closterium, along with lesser contributions from other eukaryotic phytoplankton and aggregated UCYN-C cells, as well as fecal pellets from zooplankton. Despite comprising a small fraction of the total biomass, UCYN-C was largely responsible for driving export production during the last ∼ 10 days of the experiments both directly ( ∼ 5 to 22 % of PNsink) and through the rapid transfer of its newly fixed N to other phytoplankton; we infer that this newly fixed N was transferred rapidly through the dissolved N (including DON) and PNsusp pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and δ15N with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON.

Mixed layer (ML) gross (GOP) and net (NOP) oxygen production rates based on in situ mass balances of triple oxygen isotopes (TOI) and O2/Ar are influenced by vertical transport from below, a term traditionally difficult to constrain. Here, we present a new approach to estimate vertical eddy diffusivity (Kz) based on density gradients in the upper thermocline and wind-speed based rates of turbulent shear at the ML depth. As an example, we use this Kz, verified by an independent 7Be-based estimate, in an O2/TOI budget at a site in the oligotrophic South Pacific Gyre (SPG). NOP equaled 0.31 ± 0.16 mmol m-2 d-1 in the ML (~55-65 m depth) and 1.2 ± 0.4 mmol m-2 d-1 (80%) beneath the ML, while GOP equaled 74 ± 27 mmol m-2 d-1 (86%) in the ML and 12 ± 4 mmol m-2 d-1 (14%) below, revealing a vertical gradient in production rates unquantifiable without the Kz estimate.

Constraining the rates and spatial distribution of dinitrogen (N2) fixation fluxes to the ocean informs our understanding of the environmental sensitivities of N2 fixation as well as the timescale over which the fluxes of nitrogen (N) to and from the ocean may respond to each other. Here we quantify rates of N2 fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope (“δ15N”) budgets. Comparing measurements of water column nitrate + nitrite δ15N with the δ15N of sinking particulate N at a western, central, and eastern station, these δ15N budgets indicate high, modest, and low rates of N2 fixation at the respective stations. The results also imply that N2 fixation supports exceptionally high, i.e. ≥ 50 %, of export production at the western and central stations, which are also proximal to the largest iron sources. These geochemically based rates of N2 fixation are equal to or greater than those previously reported in the tropical North Atlantic, indicating that the WTSP Ocean has the capacity to support globally significant rates of N2 fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific.

Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20°S and 10°S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as ∼ 0.26 nmol L−1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10°S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 × 10−15 mol L−1 to 1.34 × 10−13 mol L−1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM—iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation—have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.

[1] The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean’s oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface-accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.

Four stratigraphic sections in the southern part of the Columbia Basin preserve a sequence of aeolian and non-aeolian sediments ranging in age from 9AE43 to >47AE0 14 C ka based on accelerator mass spectrometry radiocarbon dating of fossil molluscs, geochemistry of Cascade Mountain-sourced tephra and association with formally recognized pedostratigraphic units (the Washtucna and Old Maid Coulee soils). Study sections are interpreted as representing concurrent deposition of loess and distal Missoula Flood rhythmites in valleys tributary to main drainages backflooded during the Missoula Floods, and formation of carbonate and iron-rich soils. Sediments belong to the formally recognized L-1 and L-2 loess units established for the Palouse loess, which were deposited in the Columbia Basin subsequent to events of glacial outburst flooding. Sediments associated with the Mount Saint Helens set S and set C tephras in the study sections preserve a fauna of five species of gastropod mollusc which have not been reported previously from sediments of late Pleistocene age in the Palouse region. The fossils comprise two distinct faunules stratigraphically separated by the Mount Saint Helens So tephra. Accelerator mass spectrometry radiocarbon dating of the fossils collected above the tephra in two of the sections yielded ages of 12AE48 ± 0AE06 and 9AE43 ± 0AE05 14 C kyr. These ages suggest that independent determinations of the 13AE35 14 C kyr age of the So tephra in other areas where Missoula Flood sediments are preserved are probably accurate, and help to refine the age of the latest events in the most recent sequence of catastrophic glacial outburst flooding.

An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m−2 d−1 based on sediment trap fluxes. Whereas the fractional importance of N2 fixation did not change, the N2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N2-fixation rates were higher, up to 100 µmol N m−2 d−1, and detected N2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N2 fixation in the ETSP.

The oxygen isotopic composition of seawater sulphate in oxygen-deficient waters records the degree of in situ sulphur cycling. The sulphur cycle has been suggested to play an important part in closing the marine fixed nitrogen budget in oxygen-deficient regions of the ocean. This paper combines oxygen and sulphur isotope data of seawater sulphate and modelling work and finds that previous estimates for sulphur-driven remineralization and fixed nitrogen loss are near the upper limit for what is possible taking into account in situ sulphate isotope data. A quantitative understanding of sources and sinks of fixed nitrogen in low-oxygen waters is required to explain the role of oxygen-minimum zones (OMZs) in controlling the fixed nitrogen inventory of the global ocean. Apparent imbalances in geochemical nitrogen budgets1 have spurred numerous studies to measure the contributions of heterotrophic and autotrophic N2-producing metabolisms (denitrification and anaerobic ammonia oxidation, respectively)2,3. Recently, ‘cryptic’ sulphur cycling was proposed as a partial solution to the fundamental biogeochemical problem of closing marine fixed-nitrogen budgets in intensely oxygen-deficient regions4. The degree to which the cryptic sulphur cycle can fuel a loss of fixed nitrogen in the modern ocean requires the quantification of sulphur recycling in OMZ settings. Here we provide a new constraint for OMZ sulphate reduction based on isotopic profiles of oxygen (18O/16O) and sulphur (33S/32S, 34S/32S) in seawater sulphate through oxygenated open-ocean and OMZ-bearing water columns. When coupled with observations and models of sulphate isotope dynamics and data-constrained model estimates of OMZ water-mass residence time, we find that previous estimates for sulphur-driven remineralization and loss of fixed nitrogen from the oceans are near the upper limit for what is possible given in situ sulphate isotope data.

The dominant process adding nitrogen (N) to the ocean, di-nitrogen (N(2)) fixation, is mediated by prokaryotes (diazotrophs) sensitive to a variety of environmental factors. In particular, it is often assumed that consequential rates of marine N(2) fixation do not occur where concentrations of nitrate (NO(-) (3)) and/or ammonium (NH(+) (4)) exceed 1μM because of the additional energetic cost associated with assimilating N(2) gas relative to NO(-) (3) or NH(+) (4). However, an examination of culturing studies and in situ N(2) fixation rate measurements from marine euphotic, mesopelagic, and benthic environments indicates that while elevated concentrations of NO(-) (3) and/or NH(+) (4) can depress N(2) fixation rates, the process can continue at substantial rates in the presence of as much as 30μM NO(-) (3) and/or 200μM NH(+) (4). These findings challenge expectations of the degree to which inorganic N inhibits this process. The high rates of N(2) fixation measured in some benthic environments suggest that certain benthic diazotrophs may be less sensitive to prolonged exposure to NO(-) (3) and/or NH(+) (4) than cyanobacterial diazotrophs. Additionally, recent work indicates that cyanobacterial diazotrophs may have mechanisms for mitigating NO(-) (3) inhibition of N(2) fixation. In particular, it has been recently shown that increasing phosphorus (P) availability increases diazotroph abundance, thus compensating for lower per-cell rates of N(2) fixation that result from NO(-) (3) inhibition. Consequently, low ambient surface ocean N:P ratios such as those generated by the increasing rates of N loss thought to occur during the last glacial to interglacial transition may create conditions favorable for N(2) fixation and thus help to stabilize the marine N inventory on relevant time scales. These findings suggest that restricting measurements of marine N(2) fixation to oligotrophic surface waters may underestimate global rates of this process and contribute to uncertainties in the marine N budget.

[1] Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/14N that is substantially higher than the 15N/14N of suspended particulate organic nitrogen (PNsusp) found in the same surface waters (with δ15N of ∼4 to 5‰ and ∼−1 to 1‰, respectively). Moreover, the concentration and 15N/14N of DON are much less dynamic than those of PNsusp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and 15N/14N of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 μM, but the average δ15N of DON is significantly different, 3.9‰ versus air in the North Atlantic and 4.7‰ in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the δ15N of nitrate is 2.6‰, while it is 4.0‰ in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the 15N/14N of DON relative to surface ocean PNsusp as well as the interbasin difference in the 15N/14N of DON. In this model, DON is produced from PNsusp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PNsusp pool, as an integral part of the ammonium-centered cycle that lowers the 15N/14N of PNsusp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the 15N/14N elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N2 fixation rates and DON concentration and 15N/14N on short time scales.

[1] N and O isotope analyses of water column nitrate between Bermuda and Puerto Rico document a bolus of low-δ15N 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 δ15N is apparent. In the upper 200 m, the algal uptake-driven increase in nitrate δ18O is greater than in δ15N, 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-δ15N N in the Sargasso Sea thermocline, in which case the relatively low rate we estimate for N2 fixation would still be too high.

[1] To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time-series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000 through May 2001. The annually averaged TN δ15N in the upper 100 m is 3.9‰, which is greater than thermocline nitrate (2–3‰ at 250 m) and similar to literature values for shallow sinking nitrogen at BATS (3.7‰). We discern no seasonal variation in TN δ15N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a δ15N for the sinking flux that is similar to previous measurements, suggesting that N2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and 15N/14N of total organic nitrogen (TON) between the surface and subsurface (∼250 m) suggest that subsurface remineralization of ∼0.25 μM of the surface TON acts to lower the 15N/14N of nitrate in the thermocline at BATS.

Some of our suppositions about the marine nitrogen cycle may be wrong. An innovative analysis of nutrients at the ocean's surface reveals a feedback mechanism that might hold the whole cycle in balance.