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
The efficiency of the oceanic carbon (C) sequestration depends upon a complex
balance between the organic matter production in the euphotic zone and its
remineralization in both the epipelagic and mesopelagic zones. The growth of
autotroph organisms, and therefore the assimilation of
Furthermore, in the current context of climate change,
The OUTPACE cruise was carried out between 18 February and 3 April 2015 from
Noumea (New Caledonia) to Papeete (French Polynesia) in the WTSP
(Fig.
As already mentioned, in order to investigate the role of
Conceptual diagram of the biogeochemical model (Eco3M-Med enhanced with diazotrophs) from the 1 DV coupled physical–biogeochemical used for the OUTPACE project.
Transect of the OUTPACE cruise with the location of the SD and LD stations superimposed on a bathymetry map (GEBCO_2014 grid). The two main regions studied in this work are shown in green for the WMA and in blue for the WGY.
The methods used to measure dissolved inorganic nitrogen (DIN), DIP,
Model outputs were compared to the observations gathered during the OUTPACE cruise at the WMA and WGY. For each profile presented in the Results section, we plotted the discrete values of the data collected at the WMA and WGY (circles), and the average over the respective sampling periods of the WMA and WGY for the model results (from 21 February 2015 to 2 March 2015 for simWMA, and from 21 to 31 March 2015 for simWGY). Both simulations were run over 10 years. Since a cyclic steady state was reached in the near-surface layer after 3 years, the vertical profiles of the third year of simulation (solid line) are presented for both simWMA and simWGY. Moreover, since the outputs of simWMA provided interesting information regarding the role of diazotrophs in fueling the system with new N inputs, the 10 vertical profiles of the 10-year run are all presented in the Results section.
The biogeochemical model implemented in this work is embedded in the Eco3M
modular numerical tool
The model includes eight planktonic functional types (PFTs): four autotrophs
(a large and a small classic phytoplankton and a large and a small nitrogen
fixer), three grazers (zooplankton) and one decomposer (heterotrophic
bacteria). Each of them is represented in terms of several concentrations (C,
N, P and chlorophyll for phytoplankton) and an abundance (cells or
individuals per liter)
In the model,
Since the
While several studies have shown that the intracellular
Moreover, in this oligotrophic to ultra-oligotrophic region, the regeneration
of the organic matter is crucial to maintaining the ecosystem balance, and
certain modifications have been made in this regard: (1) to indirectly take
into account the enhanced consumption of organic P through the activity of
extracellular alkaline phosphatase produced by bacteria in oligotrophic areas
The biogeochemical model has been coupled with the one-dimensional physical
model described in
The atmospheric forcings (i.e., the sensible and latent heat fluxes, the
short- and long-wave radiation and the wind stress) for the physical model
were provided by the Weather Research Forecast (WRF) model
The initial profiles of temperature (
DIN and DIP concentrations are presented in Fig.
The vertical profiles of DIN
Vertical profiles of
The
Figure
The main differences between Chl
The particulate carbon biomass (POC) presented in
Fig.
Unlike the available in situ data, the model can provide the time
variations of all the above-mentioned biogeochemical variables. The seasonal
pattern of the nutrient pools,
The nutrient variations throughout the water column are in part related to
the variations of the mixed layer depth (MLD) during the year. The seasonal
variations of the MLD are plotted in Fig.
Seasonal dynamics of
Unlike DIN, DIP presents significant seasonal variations throughout the year
(Fig.
Accounting for the huge computer memory this would require, the values of the
different biogeochemical fluxes calculated by the model are not
systematically saved. As a result, numerical values of fluxes are saved at a
lower vertical resolution than concentrations (pools). For this reason we
decided to represent the dynamics of
Figure
The production of C biomass in simWMA shows significant seasonal variations
in the photic layer (Fig.
The WTSP has been recently qualified as a hotspot of
While DIN concentration remains below the quantification limit (50 nM)
everywhere in the surface layer, there is a significantly higher DIP
concentration in the photic layer at the WGY than at the WMA in both the data
and the model outputs (Fig.
Studies on the role of
Vertical profiles of
The patterns of surface production calculated by simWMA and simWGY are
consistent (Fig.
As with the DCM, the nutriclines in simWGY are significantly deeper than
those measured in situ in the WGY. We assume that this gap is
because
In a previous study using nearly the same biogeochemical model including TRI
and UCYN state variables in a one-dimensional configuration without physical
coupling,
In such oligotrophic areas, the positions of the nutriclines are crucial in
controlling surface production
During OUTPACE, the phosphacline (above 50 m) appeared shallower than the
nitracline (about 75 m) in the WMA region (Fig.
On the basis of our model, we understood that it was crucial to take into
account the nutricline depths, and that a shallower phosphacline than
nitracline was needed to observe
To obtain with the model a phosphacline shallower than the nitracline, and
thereby decrease DIP depletion in the surface layer, we had to decouple the
regeneration of the detrital organic particulate N (DET-N) and the detrital
particulate organic P (DET-P) by significantly increasing the
remineralization rate of DET-P compared to that of DET-N and DET-C. The use
of extracellular phosphoenzymes (e.g., alkaline phosphatase, nucleotidase,
polyphosphatase or phosphodiesterase) by microorganisms to regenerate DIP
from dissolved organic P when DIP is depleted is well known
By running the model simulations over 10 years, we observed the storage of
the N input by diazotrophy. The nitrate accumulation observed in simWMA from
70 to 500 m (Fig.
To date, the WTSP, and more generally the South Pacific Ocean, has been much
less studied than the North Pacific Ocean, which has been sampled since the
late 1980s within the framework of the Long-term Oligotrophic Habitat
Assessment (ALOHA) near the Hawaii islands. The South Pacific has been
sampled from west to east during the BIOSOPE
Seasonal variations obtained in sim
The purpose of this study was to investigate the direct and/or indirect role
of
Winter mixing allows the annual replenishment of the surface layer in excess
P, creating ideal conditions for diazotroph growth and intensive
All data and metadata are available at the French INSU/CNRS
LEFE CYBER database (scientific coordinator: Hervé Claustre; data
manager, webmaster: Catherine Schmechtig) at the following web address:
The supplement related to this article is available online at:
The authors declare that they have no conflict of interest.
This article is part of the special issue “Interactions between
planktonic organisms and biogeochemical cycles across trophic and
This is a contribution of the OUTPACE (Oligotrophy from Ultra-oligoTrophy
PACific Experiment) project (