Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

Fumenia, Alain; Moutin, Thierry; Bonnet, Sophie; Benavides, Mar; Petrenko, Anne; Nunige, Sandra; Maes, Christophe

doi:10.5194/bg-2017-557

Cited by 14 publications

(27 citation statements)

References 44 publications

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“…3a. According to the model, N 2 fixation may explain the N excess observed in situ around the main thermocline in the WTSP, as reported by Fumenia et al (2018).…”

Section: N From N 2 Fixation Accumulates In the Main Thermoclinesupporting

confidence: 63%

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Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

et al. 2018

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The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the western tropical South Pacific (WTSP) during the austral summer (March-April 2015). The aim of the OUTPACE project was to investigate a longitudinal gradient of biological and biogeochemical features in the WTSP, and especially the role of N 2 fixation in the C, N, and P cycles. Two contrasted regions were considered in this study: the Western Melanesian Archipelago (WMA), characterized by high N 2 fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the South Pacific Gyre (WGY), characterized by very low N 2 fixation rates, surface production and high DIP concentrations. Since physical forcings and mixed layer dynamics in both regions were similar, it was considered that the gradient of oligotrophy observed in situ between the WMA and WGY was not explained by differences in physical processes, but rather by differences in biogeochemical processes. A one-dimensional physical-biogeochemical coupled model was used to investigate the role of N 2 fixation in the WTSP by running two identical simulations, only differing by the presence (sim WMA ) or absence (sim WGY ) of diazotrophs. We showed that the nitracline and the phosphacline had to be, respectively, deeper and shallower than the mixed layer depth (MLD) to bring Ndepleted and P-repleted waters to the surface during winter mixing, thereby creating favorable conditions for the development of diazotrophs. We also concluded that a preferential regeneration of the detrital phosphorus (P) matter was necessary to obtain this gap between the nitracline and phosphacline depths, as the nutricline depths significantly depend on the regeneration of organic matter in the water column. Moreover, the model enabled us to highlight the presence of seasonal variations in primary production and P availability in the upper surface waters in simWMA, where diazotrophs provided a new source of nitrogen (N) to the ecosystem, whereas no seasonal variations were obtained in simWGY, in the absence of diazotrophs. These main results emphasized the fact that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on the one hand on physical processes (vertical mixing, sinking of detrital particles), and on the other hand on biogeochemical processes (N 2 fixation, remineralization).

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“…3a. According to the model, N 2 fixation may explain the N excess observed in situ around the main thermocline in the WTSP, as reported by Fumenia et al (2018).…”

Section: N From N 2 Fixation Accumulates In the Main Thermoclinesupporting

confidence: 63%

“…The biogeochemical model has been coupled with the onedimensional physical model described in Gaspar (1988). This model solves the conservation equations for heat, salinity, momentum and kinetic energy.…”

Section: One-dimensional Physical Model and Forcingsmentioning

confidence: 99%

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

et al. 2018

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“…This atlas collects and processes all the profiles provided by the Argo floats from 2004 to 2012 in order to calculate a monthly global climatology over the entire depth of the oceans. More details are provided by Gaillard et al (2016). We apply the same methods to calculate the density as those used for the HYCOM re-analysis.…”

Section: Ancillary Datamentioning

confidence: 99%

Intermediate water flows in the western South Pacific: as revealed by individual Argo floats trajectories and a model re-analysis

2018

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Thanks to the autonomous Argo floats of the OUT-PACE cruise (Oligotrophy to UlTra-oligotrophy PACific Experiment) and of the THOT (TaHitian Ocean Time series) project, some features of intermediate-flow dynamics, at around 1000 m depth, within the central and western South Pacific Ocean (around 19 • S, 156 • E-150 • W) are described.In the Coral Sea, we highlight minima in dissolved oxygen of 140 µmol kg −1 that are associated with the signature of a southward transport of waters between two zonal jets: from the North Vanuatu Jet to the North Caledonia Jet. This transport takes place in the core of a cyclonic eddy or via the path between a cyclonic eddy and an anticyclonic one, highlighting the importance of mesoscale dynamics in upper thermocline and surface layers. Further east, we observe a strong meridional velocity shear with long-term float trajectories going either eastward or westward in the lower thermocline. More interestingly, these trajectories also exhibit some oscillatory features. Those trajectories can be explained by a single Rossby wave of 160-day duration and 855 km wavelength. Considering the thermohaline context, we confirm the meridional shear of zonal velocity and highlight a permanent density front that corresponds to the interface between Antarctic intermediate waters and North Pacific deep waters. Hence both circulation and thermohaline contexts are highly prone to instabilities and wave propagation.Published by Copernicus Publications on behalf of the European Geosciences Union.

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“…2. A detailed description of the water masses encountered during the OUTPACE cruise can be found in Fumenia et al (2018). Briefly, from the surface to 2000 dbar, the following features are distinguished: the surface waters (σθ < 23.5) were characterized by temperatures over 25 °C with increasing temperature and salinity towards the east and AOU close to zero.…”

Section: Hydrological Context Along the Outpace Transectmentioning

confidence: 99%

“…Orthophosphate and silicate concentration were considered in the calculation. Methods for nutrients measurement are presented in details in Fumenia et al (2018). When nutrient data were not available (Station SD-8), silicate and orthophosphate were estimated from the nutrient profile measured on cast out_c_163 (interpolated values).…”

Section: Derived Parametersmentioning

confidence: 99%

Carbonate system distribution, anthropogenic carbon and acidification in the Western Tropical South Pacific (OUTPACE 2015 transect)

Wagener¹,

Metzl²,

Caffin³

et al. 2018

Preprint

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Abstract.The western tropical South Pacific was sampled along a longitudinal 4000 km transect (OUTPACE cruise, 18 Feb., 3 Apr. 2015) for measurement of carbonates parameters (total alkalinity and total inorganic carbon) between the Melanesian Archipelago (MA) and the western part of the South Pacific gyre (WGY). This manuscript reports this new dataset and derived properties: pH on the total scale (pHT) and the CaCO3 saturation state with respect to calcite (Ωcal) and aragonite (Ωara). We also estimate anthropogenic carbon (CANT) distribution in the water column using the TrOCA method (Tracer combining Oxygen, inorganic Carbon and total Alkalinity). Along the OUTPACE transect, CANT inventories of 37 -43 mol m -2 were estimated with higher CANT inventories in MA waters (due to a deeper penetration of CANT in the intermediate waters) than in the WGY waters although highest CANT concentrations were detected in the sub-surface waters of WGY. By combining our OUTPACE dataset with data available in GLODAPv2 (1974GLODAPv2 ( -2009, temporal changes in oceanic inorganic carbon were evaluated. An increase of 1.3 to 1.6 µmol kg IntroductionHuman activities inject about 10 13 kg of carbon per year to the atmosphere which might have major consequences on climate. It is recognized that the ocean plays a key role in the control of atmospheric CO2 through uptake by the so called "oceanic carbon pump". Through this "pump", the ocean sequesters ca. 30% of the CO2 injected annually in the atmosphere by human activities (Le Quéré et al., 2018). A consequence of the ocean carbon uptake is a decrease of the oceanic pH (Feely et al, 2004) which is described as ocean acidification (the so-called "other" CO2 problem). Effects of ocean acidification have been observed on marine organisms and could affect the marine ecosystems (Riebesell et al., 2000). Improving our understanding of the oceanic CO2 uptake relies primarily on observations of the marine carbonate cycle. Studies on the oceanic carbonate cycle have been mostly conducted in the frame of international programs. The World Ocean Circulation Experiment (WOCE) and the Joint Global Flux Study (JGOFS) in the 90's have coordinated oceanographic cruises along large sections in the ocean to collect samples through the water column and to perform accurate measurements of carbonate parameters and ancillary parameters (temperature, salinity, dissolved oxygen, nutrients,...). Since 2000, efforts have been made to "revisit" oceanic sections according to the WOCE strategy in order to assess oceanic changes at the scale of a decade. These programs have generated important databases for oceanic carbonate chemistry (e.g. GLODAP_V2 -Olsen et al., 2016).In order to better assess the role of the ocean on the global carbon cycle, the concept of oceanic anthropogenic carbon (hereafter named CANT) has been introduced and refers to the fraction of dissolved inorganic carbon (CT) in the ocean that originates from carbon injected by human activities in the atmosphere since the industrial revolutio...

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Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

Cited by 14 publications

References 44 publications

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

Intermediate water flows in the western South Pacific: as revealed by individual Argo floats trajectories and a model re-analysis

Carbonate system distribution, anthropogenic carbon and acidification in the Western Tropical South Pacific (OUTPACE 2015 transect)

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