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 15 N 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 15 N 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.

Dinitrogen (N2) fixation can alleviate N limitation of primary productivity by introducing fixed nitrogen (N) to the world's oceans. Although measurements of pelagic marine N2 fixation are predominantly from oligotrophic oceanic regions, where N limitation is thought to favor growth of diazotrophic microbes, here we report high rates of N2 fixation from seven cruises spanning four seasons in temperate, western North Atlantic coastal waters along the North American continental shelf between Cape Hatteras and Nova Scotia, an area representing 6.4% of the North Atlantic continental shelf area. Integrating average areal rates of N2 fixation during each season and for each domain in the study area, the estimated N input from N2 fixation to this temperate shelf system is 0.02 Tmol N/year, an amount equivalent to that previously estimated for the entire North Atlantic continental shelf. Unicellular group A cyanobacteria (UCYN‐A) were most often the dominant diazotrophic group expressing nifH, a gene encoding the nitrogenase enzyme, throughout the study area during all seasons. This expands the domain of these diazotrophs to include coastal waters where dissolved N concentrations are not always depleted. Further, the high rates of N2 fixation and diazotroph diversity along the western North Atlantic continental shelf underscore the need to reexamine the biogeography and the activity of diazotrophs along continental margins. Accounting for this substantial but previously overlooked source of new N to marine systems necessitates revisions to global marine N budgets.

The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (<20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

The Mid-Atlantic Bight (MAB) region of the Northeast U.S. continental shelf is one of the world's most productive marine ecosystems (O'Reilly & Busch, 1984;O'Reilly et al., 1987) and is critical to regional commercial fisheries (Sherman et al., 1996). Unlike the MAB continental shelf and shelfbreak front (e.g., Ryan et al., 1999; Zhang et al., 2013), the MAB slope sea to the south is generally characterized by lower biomass (e.g., Xu et al., 2011), with summer subsurface chlorophyll (Chl) maximum layers dominated by nanoplankton (O'Reilly & Zetlin, 1998).MAB net community production is highly sensitive to ocean circulation (Friedrichs et al., 2019), but the response of the region's marine ecosystems to recent changes in northwest Atlantic circulation remains poorly constrained. Over the past two decades, the destabilization point of the Gulf Stream (GS) has shifted westward, resulting in more vigorous meandering of the GS south of the MAB (Andres, 2016). Consequently, the influence of the GS on the MAB has increased through both direct intrusion of GS water (Gawarkiewicz et al., 2012) and indirect interactions associated with more frequent GS shedding of anticyclonic warm-core rings (WCRs) (Gangopadhyay et al., 2019;Gawarkiewicz et al., 2018). To first order, increasing intrusions of GS water have been expected to decrease slope sea biological productivity (e.g., Brown et al., 1985;Zhang & Gawarkiewicz, 2015) as surface GS water is more oligotrophic than the slope (Brown et al., 1985;Olson et al., 1994). Here, we show observations from the MAB slope sea suggesting that the opposite can also occur.

In the North Atlantic Ocean, dinitrogen (N 2 ) fixation on the western continental shelf represents a significant fraction of basin-wide nitrogen (N) inputs. However, the factors regulating coastal N 2 fixation remain poorly understood, in part due to sharp physico-chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N 2 fixation on the western North Atlantic shelf, at the confluence of Mid-and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the 15 N 2 bubble release method and used to build empirical models of regional N 2 fixation via a random forest machine learning approach. N 2 fixation rates were then predicted from high-resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N 2 fixation rates (> 100 nmol N L −1 d −1 ), which were likely driven by the haptophyte-symbiont UCYN-A. Above and below this hotspot, N 2 fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N 2 uptake rates, a biomassindependent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N 2 fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

Examination of dinitrogen (N 2 ) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N 2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N 2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N 2 fixation rates were measured using a robust 15 N tracer method (bubble removal) that accounts for the slow dissolution of N 2 gas and calculated using a conservative approach. N 2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20 μmol O 2 kg −1 ) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor noncyanobacterial diazotrophs. Nevertheless, this study indicates that N 2 fixation is neither widespread nor quantitatively important throughout this region.

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The apparently obligate symbiosis between the diazotroph Candidatus Atelocyanobacterium thalassa (UCYN-A) and its haptophyte host, Braarudosphaera bigelowii, has recently been found to fix dinitrogen (N2) in polar waters at rates (per cell) comparable to those observed in the tropical/subtropical oligotrophic ocean basins. This study presents the novel observation that this symbiosis increased in abundance during a wind-driven upwelling event along the Alaskan Beaufort shelfbreak. As upwelling relaxed, the relative abundance of B. bigelowii among eukaryotic phytoplankton increased most significantly in waters over the upper slope. As the host’s nitrogen demands are believed to be supplied primarily by UCYN-A, this response suggests that upwelling may enhance N2 fixation as displaced coastal waters are advected offshore, potentially extending the duration of upwelling-induced phytoplankton blooms. Given that such events are projected to increase in intensity and number with ocean warming, upwelling-driven N2 fixation as a feedback on climate merits investigation.