Abstract
Abstract. The overall goal of OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) 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. Within the context of climate change, it is necessary to better quantify the ability of the oligotrophic ocean to sequester carbon through biological processes. OUTPACE was organized around three main objectives, which were (1) to perform a zonal characterization of the biogeochemistry and biological diversity of the western tropical South Pacific during austral summer conditions, (2) to study the production and fate of organic matter (including carbon export) in three contrasting trophic regimes (increasing oligotrophy) with a particular emphasis on the role of dinitrogen fixation, and (3) to obtain a representation of the main biogeochemical fluxes and dynamics of the planktonic trophic network. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists (30 onboard). The west–east transect covered ∼ 4000 km from the western part of the Melanesian archipelago (New Caledonia) to the western boundary of the South Pacific gyre (French Polynesia). Following an adaptive strategy, the transect initially designed along the 19° S parallel was adapted along-route to incorporate information coming from satellite measurements of sea surface temperature, chlorophyll a concentration, currents, and diazotroph quantification. After providing a general context and describing previous work done in this area, this introductory paper elucidates the objectives of OUTPACE, the implementation plan of the cruise and water mass and climatological characteristics and concludes with a general overview of the other papers that will be published in this special issue.
Highlights
This reveals that N2 fixation can fuel large-size nondiazotrophic plankton growth in the water column, suggesting an indirect export pathway of organic matter sustained by N2 fixation in the oligotrophic ocean
An underwater vision profiler (UVP) was attached to the CTD rosette to quantify and visualize suspended particulate material; 2. one 0–500 m CTD cast and bottle sampling using the trace metal clean SBE 9plus CTD rosette (TM-R) equipped with 24 teflon-lined GO-FLO bottles to sample for trace metal analyses; 3. optical sensors casts in which integrated measurements of bio-optical properties and pigments were made with instruments measuring hyperspectral radiometry in the UV–visible domain with UV–VIS TriOS spectroradiometers, and a MicroPro free-fall profiler (Satlantic) was used for downward irradiance measurements; 4. hauls for phytoplankton and zooplankton sampling with specific nets; and
The OUTPACE cruise took place in the western tropical South Pacific Ocean (WTSP), a region impacted by the El Niño–Southern Oscillation (ENSO), known to be the most important mode of sea surface temperature (SST) variability on interannual to decadal timescales (Sarmiento and Gruber, 2006)
Summary
The additional carbon dioxide (CO2) in the atmosphere, mainly resulting from fossil fuel emissions linked with human activities (anthropogenic CO2), is the main cause of global warming (Fifth Assessment Report – Climate Change 2013 – IPCC). In addition to direct export of diazotrophs, the use of nanoSIMS (nanoscale secondary ion mass spectrometry) enabled the tracking of the fate of 15N from both Trichodesmium (Bonnet et al, 2016b) and UCYN blooms (Berthelot et al, 2015; Bonnet et al, 2016c) and demonstrated that ∼ 8 % of N originating from N2 fixation is quickly transferred to non-diazotrophic plankton, in particular diatoms, i.e., efficient C exporters to depth (Nelson et al, 1995) during Trichodesmium blooms (Bonnet et al, 2016b) This reveals that N2 fixation can fuel large-size nondiazotrophic plankton growth in the water column, suggesting an indirect export pathway of organic matter sustained by N2 fixation in the oligotrophic ocean. Comparisons between different systems along a zonal gradient of trophic status and N2 fixation will provide new insights for identifying and understanding fundamental interactions between marine biogeochemical C, N, P, silica (Si), and iron (Fe) cycles in oligotrophic ecosystems
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
