Special issue  

The goal of this special issue is to present the knowledge obtained concerning the functioning of the western tropical South Pacific (WTSP) ecosystems and associated biogeochemical cycles based on the datasets acquired during the OUTPACE experiment. The physical variability and the hydrodynamic context of biogeochemical sampling are described, as well as the impacts of mesoscale circulation and forcing on biogeographic gradients, along with the longitudinal contrast in small- scale turbulence along ~20° S. An optimum multi-parameter analysis of the water mass structure is also provided.

The focus on diazotrophs and dinitrogen fixation concerns the distribution and drivers of symbiotic and free-living diazotrophic cyanobacteria, the contribution of particulate and dissolved (N release) fractions to the hotspot of N₂ fixation found during the cruise, zonal gradient in N₂ fixation rates, the partition of N₂ fixation rates between the particulate and dissolved fractions, the diazotroph-derived N transfer to the planktonic food web, the N₂ fixation contribution to export production, and finally the N budget at the long-duration stations studied using a Lagrangian adaptative strategy. Aphotic N₂ fixation, heterotrophic diazotrophs and their relationship with labile organic matter, as well as programmed cell death in diazotrophs, are also investigated.

The dynamics of phytoplankton, heterotrophic bacterioplankton, and zooplankton along the gradient of diazotroph diversity and activity are described together with the composition and distribution of dissolved organic carbon and the changes in inorganic carbon content along the longitudinal transect. Net community production, the assimilation of organic carbon and nutrient substrates by unicellular cyanobacteria, and microbial response to the N and P compounds excreted by copepods are quantified. A specific focus on siliceous plankton and Si biogeochemical cycling is also presented.

Optical properties of the WTSP waters are presented, with a focus on the cyanobacterial (diazotroph) impact on bio-optical properties, UV–visible light attenuation and Chl a algorithms for WTSP oligotrophic waters.

Finally, the main processes controlling the biological carbon pump in the WTSP are investigated using a 1DV biogeochemical–physical coupled model, and the new knowledge gained on the interactions between planktonic organisms and the cycle of biogenic elements is used to propose a new scheme for this functioning and role, at the present time and in the near future, in the oligotrophic Pacific Ocean.