Significance: Global models indicate that the human-derived nitrogen emissions that reach the ocean through atmospheric transport and deposition directly impact biology and the oceanic carbon dioxide (CO ₂ ) sink. Here, we find that the organic nitrogen in marine aerosols derives predominantly from biological production in the surface ocean rather than from pollution on land. Our previous work has shown significant anthropogenic influence on North Atlantic nitrate deposition, whereas ammonium cycles dynamically between the upper ocean and lower atmosphere. Collectively, these findings indicate that the ocean is not a passive recipient of anthropogenic nitrogen deposition, as it has previously been considered. This implies that the contribution of atmospheric nitrogen deposition to ocean fertility, oceanic CO ₂ removal, and nitrous oxide emissions has been overestimated.

Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph‐derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto‐ and heterotrophic plankton has not been measured. Here, we use high‐resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with ¹⁵N₂ isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during three diazotroph blooms dominated by the cyanobacterium Trichodesmium spp. in the South West Pacific. During these experiments, 13% ± 2% to 48% ± 5% of the fixed ¹⁵N₂ was released into the dissolved pool and 6% ± 1% to 8% ± 2% of this DDN was transferred to non‐diazotrophic plankton after 48 h. The primary beneficiaries of this DDN were diatoms (45% ± 4% to 61% ± 38%) and bacteria (22% ± 27% to 38% ± 12%), followed by pico‐phytoplankton (3% ± 1% to 21% ± 14%). The DDN was quickly converted to non‐diazotrophic plankton biomass, in particular that of diatoms, which increased in abundance by a factor of 1.4–15 over the course of the three experiments. The single‐cell approach we used enabled quantification of the actual transfer of DDN to specific groups of autotrophic and heterotrophic plankton in the surface ocean, revealing a previously unseen level of complexity in the pathways that occur between N₂ fixation and the eventual export of DDN from the photic zone.

The Southern Ocean is disproportionately important in its effect on the Earth system, impacting climatic, biogeochemical, and ecological systems, which makes recent observed changes to this system cause for global concern. The enhanced understanding and improvements in predictive skill needed for understanding and projecting future states of the Southern Ocean require sustained observations. Over the last decade, the Southern Ocean Observing System (SOOS) has established networks for enhancing regional coordination and research community groups to advance development of observing system capabilities. These networks support delivery of the SOOS 20-year vision, which is to develop a circumpolar system that ensures time series of key variables, and delivers the greatest impact from data to all key end-users. Although the Southern Ocean remains one of the least-observed ocean regions, enhanced international coordination and advances in autonomous platforms have resulted in progress toward sustained observations of this region. Since 2009, the Southern Ocean community has deployed over 5700 observational platforms south of 40°S. Large-scale, multi-year or sustained, multidisciplinary efforts have been supported and are now delivering observations of essential variables at space and time scales that enable assessment of changes being observed in Southern Ocean systems. The improved observational coverage, however, is predominantly for the open ocean, encompasses the summer, consists of primarily physical oceanographic variables, and covers surface to 2000 m. Significant gaps remain in observations of the ice-impacted ocean, the sea ice, depths >2000 m, the air-ocean-ice interface, biogeochemical and biological variables, and for seasons other than summer. Addressing these data gaps in a sustained way requires parallel advances in coordination networks, cyberinfrastructure and data management tools, observational platform and sensor technology, two-way platform interrogation and data-transmission technologies, modeling frameworks, intercalibration experiments, and development of internationally agreed sampling standards and requirements of key variables. This paper presents a community statement on the major scientific and observational progress of the last decade, and importantly, an assessment of key priorities for the coming decade, toward achieving the SOOS vision and delivering essential data to all end-users.

N₂ fixation rates were measured daily in large (∼ 50 m³) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N₂ fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N₂ fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L⁻¹ d⁻¹ over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L⁻¹ d⁻¹. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N₂ fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10 % of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 µm) UCYN-C cells into large (100–500 µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with ¹⁵N₂ isotopic labeling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.