The spatial distribution of biological N₂ fixation fluxes to the ocean remains poorly constrained. Here we use nitrogen isotope budgets to identify significant N₂ fixation inputs to the western tropical South Pacific (WTSP), where N₂ fixation supports > 50 % of export production at stations proximal to iron sources. The significant N₂ fixation inputs in the WTSP may offset nitrogen loss in the oxygen-deficient zones of the eastern tropical South Pacific.

We performed N budgets to assess the role of N₂ fixation on production and export in the western tropical South Pacific Ocean. We deployed a combination of techniques including high-sensitivity measurements of N input and sediment traps deployment. We demonstrated that N₂ fixation was the major source of new N before atmospheric deposition and upward nitrate fluxes. It contributed significantly to organic matter export, indicating a high efficiency of this region to export carbon.

Oceanographic campaigns to measure biogeochemical processes popularly deploy drifters with onboard incubations to stay in a single body of water. Here, we aggregate physical data taken during such a cruise, OUTPACE, to independently test in a new approach whether the drifter really stayed in what can be considered a single biological or chemical environment. This study concludes that future campaigns would benefit from similar data collection and analysis to validate their sampling strategy.

The Melanesian archipelago waters between 160° E and 170° W are characterized by a significant N₂ fixation rates and an excess of particulate organic nitrogen compared to the canonical ratio of Redfield and a positive N^*. We hypothesize that the southern branch of the subtropical gyre is probably the main vector of excess nitrogen transport in the thermocline waters showing an influence of nitrogen fixation occurring in the western tropical in a large part of the South Pacific.

A surface summer plankton bloom in the western tropical South Pacific was sampled during the Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise. We characterize the bloom's properties and the circulation responsible for its evolution. Nitrogen fixation helped sustain the bloom, and larger-scale flows, rather than the smaller ones, explain its movements. Future studies of blooms in this region can make use of these findings to track the horizontal export of plankton production.

The overall goal of OUTPACE was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N₂ fixation gradients. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L’Atalante and involved 60 scientists. The transect covered ~4 000 km from the western part of the Melanesian archipelago to the western boundary of the gyre.

In the context of the VAHINE mesocosm experiment in the Nouméa lagoon (New Caledonia), a 1-D vertical biogeochemical mechanistic model was used together with the in situ experiment to complement our comprehension of the planktonic ecosystem dynamics and the main biogeochemical carbon, nitrogen and phosphate fluxes. The model also showed the fate of fixed N₂ by providing, over time, the proportion of diazotroph-derived nitrogen (DDN) in each compartment (mineral and organic) of the model.

The goal of this manuscript was to track the fate of newly fixed nitrogen (N) in large volume mesocosms in the coastal waters of New Caledonia. We used a N isotope ("δ¹⁵N") budget and found a shift in the δ¹⁵N of sinking particulate N over the 23-day experiment, indicating that nitrate supported export production at the beginning of the experiment, but that nitrogen fixation supported export at the end. We infer that nitrogen fixation supported export production by a release of dissolved N.

In the marine environment, sticky sugar-containing gels, termed transparent exopolymeric particles (TEP), are produced from biological sources and physical and chemical processes. These compounds are essential vectors enhancing downward flow of organic matter and its storage at depth. Spatial and temporal dynamics of TEPs were followed for 23 days during the VAHINE mesocosm experiment that investigated the fate of nitrogen and carbon derived from organisms fixing atmospheric N₂ (diazotrophs).

The phytoplankton is at the base of the plankton food web in large parts of oceanic "deserts" such as the South Pacific Ocean, where nitrogen sources limit activity. Mesocosms were fertilized with phosphorus to stimulate diazotrophy (atmospheric N₂ fixation). Mostly diazotroph-derived nitrogen fuelled the heterotrophic bacterial community through indirect processes generating dissolved organic matter and detritus, such as mortality, lysis and grazing of both diazotrophs and non-diazotrophs.

e main goal of the VAHINE project was to study the fate of N₂ fixation in the ocean. Three large-volume (~ 50 m³) mesocosms were deployed in a tropical oligotrophic ecosystem (the New Caledonia lagoon, south-eastern Pacific). This introductory paper describes the scientific objectives of the project in detail as well as the implementation plan: the mesocosm description and deployment, the selection of the study site, and the logistical and sampling strategy.

Dissolved organic carbon (DOC) has already been identified as a potentially significant source of carbon export in the Mediterranean Sea, though in situ export estimations are scarce. This work provides a thorough analysis at basin scale of carbon export with the coupled model NEMO-MED12/Eco3M-MED model. The seasonality and the processes of particulate and dissolved carbon production are also investigated. DOC export appears to be dominant in most regions, especially in the eastern basin.

Along a transect in the warm nutrient-depleted Mediterranean Sea, we found that microorganisms shared phosphate resources by using different uptake strategies. Heterotrophic prokaryotes dominated phosphate uptake in the upper layers while cyanobacteria dominated uptake fluxes around the deep chlorophyll maximum depth. Synechococcus seemed well equipped to respond to pulses of phosphate, whereas Prochlorococcus and heterotrophs were more adapted to very low concentrations.