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Objective 1

lundi 12 mai 2014



One of the main objectives of the OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) cruise is to characterize the biogeochemical parameters and biological diversity/function along a W-E gradient from the North of New Caledonia, an area exhibiting high N2 fixation rates, to the central South Pacific gyre, which has been characterized as the most oligotrophic oceanic area of the world ocean (Claustre et al., 2008), and exhibiting low N2 fixation rates (e.g. Moutin et al., 2008) (Fig. 2).


Fig. 2. Transect of the OUTPACE cruise superimposed on several maps (A) Absolute geostrophic currents (from CARS climatology ref to 1000 m, plus currents 0-200 m deduced from Argo floats drift, Kessler and Cravatte, 2013, JGR, accepted. Colorbar: zonal currents; Arrows: horizontal currents), (B) CARS2009 SST climatology (Ridgway and Dunn, 2003) (Dec-March), and (C) Surface chlorophyll concentrations (MODIS, composite Dec-March 2011-2012).

Objective 1.1. : Biogeochemical characterization

The classic variables temperature, salinity, dissolved oxygen concentration, along with alkalinity, pigments, organic and mineral C and other biogenic elements (N, P, Si) pools, and trace metals of biogeochemical importance (Fe, Zn, Cu, Co, Ni), will be measured, as well as other specific variables at selected depths (N2 fixation, diazotroph diversity, recycling times for dissolved phosphate...) in 18 short duration (SD, 8h) stations across the 3000 nm transect. The aim is to describe the variations in the biogeochemical characteristics (nutrient and trace metals availability in the photic layer, depth of the nutricline, ratios of dissolved/particulate and organic/mineral in the biogenic elements pools, drawdown of CO2 of anthropogenic origin, phytoplankton biomass and diversity, N2 fixation rates...). This biogeochemical description will extend to the gyre in the central South Pacific where oligotrophic conditions are the most extreme (Claustre and Maritorena, 2003). Our recently acquired techniques for biogeochemical measurements in ultra-oligotrophic waters should enable us to significantly improve our understanding of the biogeochemistry of the SW Pacific. We do not have a good insight into the spatial distribution of certain biogeochemical variables, such as N2 fixation, or absolute nutrients concentrations in surface waters, which have a predominant role in controlling planktonic production. Recent studies infer the importance of oligotrophic areas in the drawdown of anthropogenic C (e.g. Close et al., 2013). It is necessary to quantify the carbon fluxes in the upper water of the SW Pacific and their potential links with dinitrogen fixation.

Specific question: What is the actual zonal distribution of main C, nutrients stocks/fluxes and trace metals in the SW Pacific?

Objective 1.2. Biological Diversity

A major objective will be to produce an overall description of the planktonic community composition in the SW Pacific pelagic ecosystems, providing a “functional” structure for specific analysis. A detailed description of the biological diversity is essential as it is hypothesised that biodiversity increases the functional redundancy of marine ecosystems. Such redundancy may play an important role in an ecosystems ability to withstand natural and anthropogenic disturbances (Fonseca and Ganade, 2001). The need to describe a more “operational” functional structure should not replace having a “complete” description of the species composition. Variation in species composition probably remains the most effective tool in identifying natural and / or anthropogenic perturbations. A coupling of complementary techniques will be used along the transect, including surface continuous flow cytometry (CYTOSUB) and discrete flow cytometry for bacteria, pico-, nano-and micro-phytoplankton (Marie et al., 1999), microscope determination of taxonomy for microphyto-, microzooplankton (Hasle et al., 1978) and heterotrophic nanoflagellates, and imagery (zooscan, flowcam) for metazooplankton taxonomy (Grosjean et al., 2004), associated with particle counters (UVP, LISST, LOPC) for spatial distribution and size-strucure of particules and plankton.. The UVP will also assess the Trichodesmium spp. colony abundance in the water column (Guidi et al., 2012).
A special attention will be given to studying the diversity of diazotrophs. The scientific community made intense progress over the last decade thanks to the emergence of molecular methods, which allowed the discovery of an increasing diversity of N2-fixing organisms (Zehr et al., 2001; 2008), active in previously unsuspected ecological niches such as N-rich areas (e.g. Short and Zehr, 2007; Needoba et al., 2007; Moutin et al., 2008; Bonnet et al., 2011; Fernandez et al., 2011; Dekaezemacker et al., 2013). The filamentous photosynthetic cyanobacterium Trichodesmium spp. has long been considered the dominant marine diazotroph. Recent molecular studies have challenged this view as new evidence suggests that photosynthetic unicellular nano-planktonic diazotrophic cyanobacteria (UCYN; belonging to Group B or C, Zehr et al, 2001), photo-heterotrophic unicellular picoplanktonic cyanobacteria from Group A (Zehr et al., 2008) or heterotrophic bacteria are widespread in the oceans and capable of fixing N2 (Halm et al., 2011; Riemann et al., 2010; Zehr et al., 2008; Zehr et al., 1998; Zehr et al., 2001). The discovery of heterotrophic and photo-heterotrophic (UCYN-A) diazotrophs implies that N2 fixation is not necessarily linked with C fixation in the euphotic zone, and N2 fixation has recently been reported in aphotic waters down to 800 m (Hamersley et al., 2011; Rahav et al., 2013) and 2000 m (Bonnet et al., Accepted). However, despite the increasing recognition of their abundance in the global ocean (Luo et al., 2012), very few studies have examined in details the precise role and regulation of N2 fixation in photo-heterotrophic and heterotrophic (cyano)bacteria in marine waters. Their ubiquity has significantly widened the geographical limits where N2 fixation was supposed to occur and, therefore, the potential amount of fixed N entering the oceans. The physiology of these (photo)-heterotrophic diazotrophs as well as the environmental factors controlling their distribution, their activity and their relation with other organisms in the ocean are still very poorly known. However, the little knowledge we have on these diazotrophs suggests that they are very different from Trichodesmium spp and are regulated in the SE Pacific by organic C and N availability (Bonnet et al., Accepted). In the North Pacific Subtropical Gyre (NPSG) at station ALOHA, Dore et al. (2008) estimated that the highest integrated N2 fixation rates occurred during summer while either Trichodesmium sp. or diatom blooms (mainly constituted by Rhizosolenia sp. or Hemiaulus sp. with R. intracellularis) were observed. However, the Southern Pacific exhibits a negative Si* (SiOH4-NO3) whereas it is strongly postive in the Northern Pacific (Sarmiento and Grüber, 2006) and the relative importance of diatoms in N2 fixation remains unknown in the proposed study area. We should not dismiss the role of diazotrophs living in association with diatoms as possible key players in N2 fixation and C export (Karl et al., 2012), and offering a link between the N and silicon biogeochemical cycles. There is a clear need to study all diazotrophs if we want to know how N2 fixation and therefore the oceanic N budget will respond to global change in the ocean.
The cruise transect will cross a gradient of phytoplanktonic biomass identified from satellite data, providing in situ observations to compare with such satellite-based analysis. It will be used to calibrate PHYSAT, which allows determining large organism’s classes from space (Alvain et al. 2005; 2008), toward a better representation of N2 fixing organisms. A comparison with vertical distribution of phytoplankton communities based on surface chlorophyll determination (Uitz et al., 2006) will also be undertaken. Ocean color remote sensing will also give a synoptic overview of the chlorophyll concentration in the South Pacific during the cruise. In parallel, synoptic observations of satellite SST and Sea Level anomaly will provide insights about the physical context of the upper water column at basin scale: i.e. variability of ocean stratification and thermocline depth (which can be used as a proxy of the nutricline depth) as well as horizontal and vertical advection from the surrounding waters of the transect (Wilson and Coles, 2005; Martinez et al., 2009). These physical observations will also be compared to in situ measurments to validate the use of this proxy.

Specific questions: What species currently characterize the SW Pacific’s pelagic ecosystems during austral summer conditions? Which species or group is responsible for specific functions (N2 fixation, dissolved organic phosphate utilization, bacterial production, silification rates)?