Marine N₂ fixation supports a significant portion of oceanic primary production by making N₂ bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N₂ fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N₂ fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N₂ fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N₂ fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N₂ fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N₂ fixation flux to be 16.7 Tg N. This study broadens the biogeography of N₂ fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles.

The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ¹⁵N and δ¹⁸O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ¹⁸O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ¹⁵N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the ¹⁵N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual ¹⁵N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay in May 2014. Substantial N₂ fixation activity was observed at 8 of the 10 stations sampled, ranging overall from 81 to 384 µmol N m⁻² d⁻¹ (0.7 to 8.2 nmol N L⁻¹ d⁻¹), with two sites close to the Iberian Margin situated between 38.8 and 40.7^∘ N yielding rates reaching up to 1355 and 1533 µmol N m⁻² d⁻¹. Primary production was relatively lower along the Iberian Margin, with rates ranging from 33 to 59 mmol C m⁻² d⁻¹, while it increased towards the northwest away from the peninsula, reaching as high as 135 mmol C m⁻² d⁻¹. In agreement with the area-averaged Chl a satellite data contemporaneous with our study period, our results revealed that post-bloom conditions prevailed at most sites, while at the northwesternmost station the bloom was still ongoing. When converted to carbon uptake using Redfield stoichiometry, N₂ fixation could support 1 % to 3 % of daily PP in the euphotic layer at most sites, except at the two most active sites where this contribution to daily PP could reach up to 25 %. At the two sites where N₂ fixation activity was the highest, the prymnesiophyte–symbiont Candidatus Atelocyanobacterium thalassa (UCYN-A) dominated the nifH sequence pool, while the remaining recovered sequences belonged to non-cyanobacterial phylotypes. At all the other sites, however, the recovered nifH sequences were exclusively assigned phylogenetically to non-cyanobacterial phylotypes. The intense N₂ fixation activities recorded at the time of our study were likely promoted by the availability of phytoplankton-derived organic matter produced during the spring bloom, as evidenced by the significant surface particulate organic carbon concentrations. Also, the presence of excess phosphorus signature in surface waters seemed to contribute to sustaining N₂ fixation, particularly at the sites with extreme activities. These results provide a mechanistic understanding of the unexpectedly high N₂ fixation in productive waters of the temperate North Atlantic and highlight the importance of N₂ fixation for future assessment of the global N inventory.

Euphotic layer dinitrogen (N₂) fixation and primary production (PP) were measured in the eastern Atlantic Ocean (38°N–21°S) using ¹⁵N₂ and ¹³C bicarbonate tracer incubations. This region is influenced by Saharan dust deposition and waters with low nitrogen to phosphorus (N/P) ratios originating from the Subantarctic and the Benguela upwelling system. Depth‐integrated rates of N₂ fixation in the north (0°N–38°N) ranged from 59 to 370 µmol N m⁻² d⁻¹, with the maximal value at 19°N under the influence of the northwest African upwelling. Diazotrophic activity in the south (0°S–21°S), though slightly lower, was surprisingly close to observations in the north, with values ranging from 47 to 119 µmol N m⁻² d⁻¹. Our North Atlantic N₂ fixation rates correlate well with dust deposition, while those in the South Atlantic correlate strongly with excess phosphate relative to nitrate. There, the necessary iron is assumed to be supplied from the Benguela upwelling system. When converting N₂ fixation to carbon uptake using a Redfield ratio (6.6), we find that N₂ fixation may support up to 9% of PP in the subtropical North Atlantic (20°N–38°N), 5% in the tropical North Atlantic (0°N–20°N), and 1% of PP in the South Atlantic (0°S–21°S). Combining our data with published data sets, we estimate an annual N input of 27.6 ± 10 Tg N yr⁻¹ over the open Atlantic Ocean, 11% of which enters the region between 20°N and 50°N, 71% between 20°N and 10°S, and 18% between 10°N and 45°S.

Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show that nitrification is significant (~40–80% of the seasonal nitrate uptake) in the naturally iron‐fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate‐based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light dependent) and nitrification (partly light inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.