BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-14-3207-2017Preface: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, 18 February to 3 April 2015)MoutinThierrythierry.moutin@mio.osupytheas.frhttps://orcid.org/0000-0003-1297-8893DoglioliAndrea Michelangelohttps://orcid.org/0000-0003-1309-9954de VerneilAlainBonnetSophieAix Marseille Université, CNRS, Université de Toulon, IRD,
OSU Pythéas, Mediterranean Institute of Oceanography (MIO), UM 110,
13288, Marseille, FranceAix Marseille Université, CNRS, Université de Toulon, IRD,
OSU Pythéas, Mediterranean Institute of Oceanography (MIO), UM 110,
98848, Nouméa, New CaledoniaThierry Moutin (thierry.moutin@mio.osupytheas.fr)6July201714133207322016February201728February20172June20175June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/14/3207/2017/bg-14-3207-2017.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/14/3207/2017/bg-14-3207-2017.pdf
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.
General context
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). The ocean has acted thus far as a major sink of
anthropogenic CO2 (Sabine et al., 2004), preventing greater CO2
accumulation in the atmosphere and therefore a greater increase in the Earth's
temperature. The biological pump (Fig. 1), the process by which carbon (C)
is transferred from the upper to the deep ocean by biological processes,
provides the main explanation for the vertical gradient of C in the ocean.
Its strength and efficiency depends upon the complex balance between organic
matter production in the photic zone and its remineralization in both the
epipelagic and mesopelagic zones. Before present and for tens of thousands of
years, the biological pump was thought to be in an equilibrium state with an
associated near-zero net exchange of CO2 with the atmosphere (Broecker,
1991; Murnane et al., 1999). Climate alterations then started to disrupt
this equilibrium and the expected modification of the biological pump will
probably considerably influence oceanic C sequestration (and therefore
global warming) in future decades (Sarmiento and Gruber, 2006). The long-term decrease in phosphate availability, and the shift from primary production previously
limited by nitrogen (N) and now by phosphorus (P), associated with
increasing inputs of N by dinitrogen (N2) fixation observed at the
Hawaii Ocean Time-series (HOT) station in the north Pacific gyre (NPG) (Karl
et al., 1997; Karl, 2014), appear as a first example of biological pump
alteration.
Major C fluxes for a biological pump budget and the main role of N2 fixation. Biological pump is the C transfer into the ocean
interior by biological processes. DIC is dissolved inorganic C, POC is particulate organic C, and DOC is
dissolved inorganic C. See Moutin et al. (2012) for a detailed description.
The input of new N to the surface ocean through biological N2 fixation
represents a major link between the C and N biogeochemical cycles, relating
upper-ocean nutrient availability with the biological pump and ultimately
the ocean and climate. This link was recently shown to play a central role
in previous natural climate changes over long timescales (Galbraith et al.,
2013). It is nevertheless clear that expected climate change due to
anthropogenic atmospheric CO2 input may concern shorter timescales:
the increase in atmospheric CO2 over the past 200 years equals the
increase in atmospheric CO2 between glacial and interglacial periods,
which took place over several thousands of years (Sarmiento and Gruber,
2006). Furthermore, enhanced stratification in the tropical and subtropical
ocean resulting from global warming (Polovina et al., 2008) might decrease
nutrient availability for nutrients such as nitrate (NO3-), potentially
favoring N2 fixation in surface waters. It may also decrease phosphate
availability (Moutin et al., 2008), and in turn N2 fixation, net
primary production, and export (Karl, 2014). McMahon et al. (2015), covering
the past 1000 years, argue that N2 fixation has increased since the
industrial revolution and might provide a negative feedback to rising
CO2.
The ecosystem changes due to climate change are complex and it is therefore
necessary to characterize in detail the interactions between N2 fixation
and the C cycle to obtain a precise representation of the N2 fixation process in global biogeochemical models, leading eventually to
predictions. Even if considerable scientific progress has been made over the
last decades (see reviews from Sohm et al., 2011, and Zehr and Kudela, 2011),
many questions remain regarding the impact of this process on biogeochemical
cycles and C export.
The role of N2 fixation in the oligotrophic ocean and an overview of
previous cruises in the western tropical South Pacific Ocean (WTSP)
The efficiency of oceanic C sequestration depends upon many factors, among
which is the availability of nutrients to support phytoplankton growth in
the photic zone (Fig. 1). Large amounts of N are required for phytoplankton
growth, as it is an essential component of proteins, nucleic acids, and other
cellular constituents. Fixed N in the form of NO3- or ammonium
(NH4+) is directly usable for growth, but concentrations are low
(< 0.1 µmol L-1) and often growth limiting in most of
the open-ocean euphotic zone (Falkowski et al., 1998; Moore et al., 2013).
Dissolved N2 gas concentrations in seawater are in contrast very high
in the euphotic zone (ca. 450 µmol L-1) and could constitute a
nearly inexhaustible N source for the marine biota. However, only some
prokaryotic organisms (bacteria, cyanobacteria, archaea), hereafter referred
to as N2-fixing organisms (or diazotrophs), are able to use this
gaseous N source since they possess the nitrogenase enzyme that breaks the
triple bond between the two N atoms of the N2 molecule and converts it
into a usable form (i.e., NH3) (Zehr et al., 1998). On the global scale,
they provide 140 ± 50 Tg N per year to the surface ocean, contributing
more than atmospheric and riverine N inputs (Gruber, 2004). N2-fixing
organisms thus act as natural fertilizers and contribute to sustaining life
and potential C export in coastal and oceanic environments.
Most of the surface ocean (60 %; Longhurst, 1998) is comprised of
low-nutrient, low-biomass oligotrophic areas, which constitute the largest
coherent ecosystems on our planet. They support a large part (40 %) of
the photosynthetic C fixation in the ocean (Antoine et al., 1996). This C
fixation is mainly performed by picoplankton (smaller than 2–3 µm in
diameter) that are generally thought to represent a negligible fraction of
the total particulate organic C (POC) export flux due to their small size,
slow individual sinking rates, and tight grazer control that leads to high
rates of recycling in the euphotic zone. Consequently, the efficiency of the
biological C pump in these oligotrophic systems has long been considered to
be low as the greatest proportion of fixed C is thought to be recycled in
the surface layer and rapidly reexchanged with the atmosphere.
Recent studies have challenged this view and indicate that all primary
producers, including picoplanktonic cells, contribute to export from the
surface layer of the ocean at rates proportional to their production rates
(Richardson and Jackson, 2007). Export mechanisms differ compared to larger
cells as export of picoplankton is mainly due to packaging into larger
particles via grazing and/or aggregation processes (Jackson, 2001; Lomas et
al., 2010). More recently, Close et al. (2013) pointed out that 40–70 %
of picoplanktonic cells are small enough to escape detection under the most
common definition of suspended particulate organic matter (POM).
In addition to submicron POM export, low δ15N signatures in particles from
sediment traps at the HOT station suggest that at least part of the
production sustained by N2 fixation is ultimately exported out of the
photic zone (Karl et al., 1997, 2012; Scharek et al., 1999a;
Sharek et al., 1999b). This may either be direct through sinking of
diazotrophs, or indirect, through the transfer of diazotroph-derived N to
non-diazotrophic plankton, which are subsequently exported.
Karl et al. (2012) reported an efficient summer export flux of C (3 times
greater than the mean wintertime particle flux) at the HOT station that was
attributed to the direct export of symbiotic N2-fixing cyanobacteria
associated with diatoms (hereafter referred to as diatom–diazotroph
associations or DDAs), which have high sinking and low remineralization rates
during downward transit. This result is in accordance with high export fluxes
measured in the tropical North Atlantic when the diazotroph community is
dominated by DDAs (Subramaniam et al., 2008; White et al., 2012). More
recently, Berthelot et al. (2015) and Bonnet et al. (2016a) studied the fate of
a bloom of unicellular diazotrophs from Group C (UCYN-C) during a mesocosm
experiment in the New Caledonia lagoon after a phosphate enrichment and
revealed that ∼ 10 % of UCYN-C from the water column was exported
to the particle traps daily, representing as much as 22.4±5.5 % of
the total POC exported at the height of the UCYN-C bloom. A δ15N
budget performed in the mesocosms confirmed the high contribution of N2 fixation (56 %; Knapp et al., 2016) to export compared to other tropical
and subtropical regions where active N2 fixation contributes 10 to
25 % to export production (e.g., Altabet, 1988; Knapp et al., 2005) and
exceptionally up to 92 % in the Arabian Sea (Gandhi et al., 2011; Kumar et al., 2017). Mechanistically, the vertical downward flux was enabled by the
aggregation of the small (5.7 ± 0.8 µm) UCYN-C cells into large
(100–500 µm) aggregates. 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 non-diazotrophic plankton
growth in the water column, suggesting an indirect export pathway of organic
matter sustained by N2 fixation in the oligotrophic ocean. Most of the
abovementioned studies were performed in microcosms and mesocosms and
further open-ocean studies combining the set of complementary approaches
described above are needed to better assess the fate of N2 fixation and
its role in C export.
The western tropical South Pacific (WTSP) is an ideal location to study the
fate of N fixed by N2 fixation, as it is considered a hot spot of
N2 fixation in the world ocean (Bonnet et al., 2017a). While average
N2 fixation rates range from
20 to 200 µmol N m-2 d-1 in the tropical North Atlantic
(Benavides and Voss, 2015) and Pacific (Böttjer et al., 2017; Dore et
al., 2002), they reach 30–5400 µmol N m-2 d-1
(average ∼ 800 µmol N m-2 d-1) in the Solomon
Sea (western part of the WTSP) (Bonnet et al.,
2009, 2015), which is in the upper range of rates reported in
the global N2 fixation MAREDAT database and even surpassed its upper
rates (100–1000 µmol N m-2 d-1) (Luo et al., 2012).
Very high rates have also been recently reported in the Arabian Sea (Gandhi
et al., 2011; Kumar et al., 2017). High rates ranging from 151 to
703 µmol N m-2 d-1 have also been reported off New
Caledonia (Garcia et al., 2007), with seasonal variations closely linked with
phosphate availability (Moutin et al., 2005; Van Den Broeck et al., 2004).
The seasonal distribution of N2 fixation is corroborated by in situ and
satellite observations (Dupouy et al., 2011) of recurrent large
Trichodesmium blooms during austral summer conditions
(January–March) over the 1998–2010 period in the Melanesian archipelago
(MA: New Caledonia, Vanuatu, Fiji Islands). In addition to
Trichodesmium, which dominates the diazotroph community in the WTSP
(Moutin et al., 2005; Bonnet et al., 2015), very
high abundances of UCYN-B (up to 106–107 nifH copies L-1) have been
reported (Bonnet et al., 2015; Campbell et al., 2005; Moisander et al.,
2010). The uncultivated UCYN-A (Zehr et al., 2008) also displays high
abundances (105–106 nifH copies L-1; Bonnet et al., 2015; Moisander et
al., 2010) around the MA, but they seem to have different ecological niches
compared to Trichodesmium and UCYN-B (Bonnet et al., 2015; Moisander
et al., 2010).
When going eastward towards the South Pacific gyre (GY), Halm et al. (2012)
reported rates of 12–190 µmol N m-2 d-1 on the
western border of the gyre, and Raimbault and Garcia (2008) and Moutin et al. (2008) reported rates of 60 ± 30 µmol N m-2 d-1 in
the central gyre, indicating a decreasing gradient of N2 fixation from
west to east and low N2 fixation rates relative to other ocean gyre
ecosystems. The organisms responsible for these fluxes are different from
common autotrophic diazotrophs such as Trichodesmium or UCYN-B, and
are mainly affiliated with heterotrophic proteobacteria and low abundances of
UCYN-A (Bonnet et al., 2008; Halm et al., 2012).
The west to east zonal gradient of N2 fixation and the distinct
diversity of N2-fixing organisms along this gradient together provide a
unique opportunity to study how production, mineralization, and export of
organic matter depends upon N2 fixation in contrasting oligotrophic
regimes (from oligotrophy to ultra-oligotrophy). 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.
Objectives of OUTPACE
The overall goal of OUTPACE was to obtain a precise representation of the
complex interactions between planktonic organisms and the cycle of biogenic
elements (C, N, P, Si, Fe), considering a variety of scales, from
single-cell processes to the whole WTSP Ocean. To meet this goal, the three
specific objectives of OUTPACE were
to perform a zonal characterization of the biogeochemistry and biological
diversity of the WTSP during the strongest stratified period (austral
summer).
to study the production and fate of organic matter (including C export)
of three contrasting environments (from oligotrophy to ultra-oligotrophy)
with a particular emphasis on N2 fixation.
to obtain a representation of the main biogeochemical fluxes (C, N, P,
Si, Fe) and the dynamics of the planktonic trophic network, both in situ and by
using microcosm experiments.
The detailed study of biological production and its subsequent fate at a
given site implied a combination of adaptive and Lagrangian strategies.
Indeed, as pointed out by d'Ovidio et al. (2015), the spatiotemporal domain
of an oceanographic cruise is also one in which horizontal stirring
generated by ocean circulation at the mesoscale induces strong variability
in biogeochemical tracer distributions. Consequently, ephemeral and local
gradients due to mesoscale activity can easily mask the large-scale
gradients. Following d'Ovidio et al. (2015), this problem can be overcome by
adopting an adaptive sampling strategy (described in Sect. 4.1.) based on
information on sea surface temperature (SST), chlorophyll a (chl a) concentrations, and currents provided by satellite products analyzed in
real time during the entire cruise.
Implementation of the OUTPACE cruise
The OUTPACE cruise was conducted during austral summer conditions from 18
February to 3 April 2015 aboard the RV L'Atalante. We performed a ∼ 4000 km zonal transect from the north of New Caledonia to the western part
of the GY, finally reaching French Polynesia (Fig. 2). Along this transect,
two types of stations were sampled: 15 short-duration (SD, 8 h) stations
dedicated to a large-scale description of biogeochemical and biodiversity
gradients, and three long-duration (LD, 7 days) stations for Lagrangian
process studies.
Transect of the OUTPACE cruise superimposed on (a)
arithmetic mean surface chl a and (b) quasi-Lagrangian weighted
mean chl a of the WTSP during OUTPACE. The two types of station, short
duration (X) and long (+) duration, investigated for a period longer than
7 days, are indicated. The satellite data are weighted in time by each
pixel's distance from the ship's average daily position for the entire
cruise. The white line shows the vessel route (data from the hull-mounted
acoustic Doppler current profiler (ADCP) positioning system). Coral reefs and
coastlines are shown in black, land is grey, and areas of no data are left
white. The ocean color satellite products are produced by CLS with support
from CNES.
Adaptive strategy
Following the planned adaptive strategy, the initial transect designed to
approximatively follow 19∘ S was modified along-route thanks to
the information coming from satellite images. The regions along the vessel
route were first characterized on a large scale through the analysis of
satellite (altimetry, SST, ocean color) data. These data were automatically
retrieved and processed to derive Eulerian and Lagrangian diagnostics of
ocean circulation and biogeochemistry: Okubo–Weiss parameter maps, Lagrangian
coherent structures (LCSs), and chl a maps (d'Ovidio et al.,
2015).
The satellite data were treated in near real time on land and the obtained
data were transmitted onboard together with a daily bulletin containing the
analysis of remote sensing information, along with suggestions for LD
station positions (the complete series of the 42 bulletins is available on
the OUTPACE website at https://outpace.mio.univ-amu.fr, section
“Adaptive Strategy”).
Two main criteria were adopted in suggesting LD station positions:
The areas for the LD A and LD B (LD C) stations were characterized by
local maxima (minima) of sea surface chl a concentration to
sample MA (GY) conditions.
Local minima of surface current intensity for all LD stations were considered to
increase the chance of sampling a homogeneous water mass.
Once the suggested positions for LD stations were relayed via the daily
bulletin, at one of these locations real-time analysis of water samples using
quantitative polymerase chain reaction (qPCR) was conducted to measure
the abundances of six groups of diazotrophs (Stenengren et al., 2017). In this
way, we located regions with diazotrophs, while also resolving the
contrasting role Trichodesmium spp. or UCYN-dominated diazotroph
communities have on biogeochemical cycles.
Finally, the exact locations of the three LD stations were then determined
onboard in real time from a rapid survey using a moving vessel profiler
(MVP) equipped with conductivity–temperature–depth (CTD) and fluorimeter
sensors, accompanied by the hull-mounted thermosalinograph and acoustic
Doppler current profiler.
During this rapid survey, it was planned to follow two different sampling
routes: a cross of about 40 km each side, followed by a zigzag route
covering an area of 25 km each side at the center of the cross. This
strategy was applied as planned for the LD A and LD C stations.
The LD A station was performed east of the northern extremity of New
Caledonia in an anticyclonic recirculation characterized by a relatively high
surface chl a concentration. The LD C station was performed in
a cyclonic eddy in the most oligotrophic part of the OUTPACE transect (GY)
close to the Cook Islands.
The severe meteorological conditions due to the development of tropical
cyclone Pam (a category 5 storm) that hit Vanuatu, forced us to
perform the LD B station at a more easterly location than initially planned.
Satellite imagery allowed for the targeting of a large filament of high
surface chl a concentration close to Niue. Due to the
circulation patterns associated with the bloom, the rapid survey strategy
was adapted in order to perform four sections across the structure (see
details in de Verneil et al., 2017a).
SD station positions were chosen in relation to the LD stations, so that
they were roughly equidistant from each other, respected territorial waters,
and incorporated the changing conditions during the cruise.
General program at each station
Every station (Table 1) was investigated from the surface to 2000 m.
SD stations
Each of the 15 SD stations was investigated for 8 h. Specific operations
during the SD station occupation consisted of
two 0–200 m CTD casts and Niskin bottle sampling and one 200–2000 m CTD
cast and Niskin bottle sampling using the classical SBE 9plus CTD rosette
(C-R) for measurements of core parameters (dissolved oxygen; dissolved
inorganic carbon; total alkalinity; nutrients; chl a;
particulate and dissolved organic C, N, P, and Si) and some more specific
ones (for example primary production rates, N2 fixation rates, and
diazotroph abundance). An underwater vision profiler (UVP) was attached to
the CTD rosette to quantify and visualize suspended particulate material;
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;
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;
hauls for phytoplankton and zooplankton sampling with specific nets; and
a profile of turbulence measurements using a VMP1000 equipped with
microsensors for temperature and shear that enable accurate estimates of the
eddy diffusion coefficient Kz.
The specific configurations of the two CTD rosettes are available here:
https://outpace.mio.univ-amu.fr/spip.php?article137.
Date, location, and general characteristics of the stations
investigated along the OUTPACE transect. Distance in kilometers from the first SD
station (SD1).
Each of the three LD stations was investigated with a drifting array (see
below) that was deployed for 7 days. A series of CTD (C-R) casts (0–500 m)
were performed every 3 h near the actual position of the drifting array
while numerous specific operations (see below) were carried out in between
CTD casts.
A total of 13 surface velocity program (SVP) drifters anchored at 15 m depth
were deployed in order to study relative surface dispersion. The drifters
were launched with three at LD A, six at LD B, and four at LD C. A drifting
array (equipped with three PPS5 sediment traps, current meters, specific
oxygen sensors, and specific high-frequency temperature sensors; please
consult https://outpace.mio.univ-amu.fr/spip.php?article75 for
details) was then deployed at the chosen station position to start the
process study. The drifting array was recovered at the end of each LD
station occupation, immediately following the last CTD cast. An additional
CTD cast from the surface to the bottom (5000 m) was undertaken at the LD B station.
Specific operations during LD stations were identical to those performed at
SD stations, along with other operations like in situ production measurements, in situ
particulate material sedimentation measurements, trace metal clean pumping
for process experiments, and profiles of current measurements. Finally, at
each LD station, a drifting PROVOR-type Argo float (ProBio equipped with
sensors to measure chl a (fluorescence), chromophoric dissolved organic matter (CDOM) (fluorescence),
PAR, irradiance at three wavelengths, backscattering, and dissolved oxygen
(optode)) was deployed (Table 2).
PROVOR Argo floats deployed along the OUTPACE transect.
The following color code was proposed to present data from the different LD
stations: LD A is green, LD B is red, and LD C is blue.
All details regarding the OUTPACE cruise are available on the OUTPACE website: https://outpace.mio.univ-amu.fr/.
The general scheduled and realized programs are available here:
http://www.obs-vlfr.fr/proof/php/outpace/outpace_log_and_basic_files.php.
Zonal sections of (a) conservative temperature Θ,
(b) absolute salinity SA, (c) density anomaly, and (d) fluorescence in the upper
700 m from the classic CTD rosette at SD and LD stations along the
OUTPACE transect. The three LD stations are highlighted by their color-coded
letter and corresponding arrow.
General characteristics of the upper water masses in the WTSP
Water characteristics (temperature, salinity, density, chl a
fluorescence) for the upper 700 m as measured by the C-R are presented in
Fig. 3. The deep CTD casts from all SD and LD stations are presented
subsequent to post-cruise processing using Sea-Bird Seasoft software,
adopting the TEOS-10 standard. The upper surface layer (0–30 m) observed
during the OUTPACE transect was characterized by warm waters, with
temperatures between 26.18 and 29.93 ∘C (Fig. 3a), and relatively
low salinity, i.e., absolute salinity between 35.03 and 35.81 g kg-1
(Fig. 3b). Density anomalies of 21.72–22.91 kg m-3 between 0 and 30 m
increased gradually between 30 and 200 m to reach 24.89–25.38 kg m-3
(Fig. 3c). The salinity increase in subsurface waters (100–200 m) from
187∘ W longitude (Fig. 3b) indicated the geographical border
between waters under the influence of the MA (SD1 to SD12) and waters from
the GY (SD13 to SD15, LD C). LD B was not classified here and required a
further analysis (de Verneil et al., 2017a). The classification between MA
and GY waters will be helpful to describe general biogeochemical and
biological features. Between 200 and 700 m, a decreasing gradient of
temperature and salinity indicated the presence of permanent thermocline
waters. Temperature fell from 19.27 to 22.08 ∘C at 200 m to
5.48–6.91 ∘C at 700 m (Fig. 3a). Absolute salinities of
35.76–36.21 g kg-1 at 200 m decreased to 34.49–34.61 g kg-1 (Fig. 3b). The density anomalies of 24.89–25.38 kg m-3 at 200 m
increased largely to 26.97–27.13 kg m-3 at 700 m (Fig. 3c). The maximum
fluorescence depth, considered here as the main indicator of the trophic
state, increased from ∼ 100 m depth in the MA waters to
∼ 115–150 m depth in the GY waters, which allowed us to sample
the oligotrophic to ultra-oligotrophic transition in the WTSP for the purpose
of the OUTPACE project.
The climatological context of the campaign
The OUTPACE cruise took place in the WTSP, a region impacted by the El Niño–Southern Oscillation (ENSO), known to be the most important mode of SST
variability on inter-annual to decadal timescales (Sarmiento and Gruber,
2006). ENSO-related SST anomalies are caused by a combination of changes in
ocean circulation (mainly changes in the strength and source of equatorial
upwelling) and anomalous local air–sea heat exchanges. The most dramatic
effects of ENSO in the surface ocean are well documented in the eastern
tropical Pacific, where seasonal upwelling conditions can be suppressed with
severe economic consequences for fisheries (Chavez et al., 2003). During
El Niño phases (negative Southern Oscillation Index (SOI), defined
below), the warm pool normally positioned in the western Pacific is found
farther east, resulting in the aforementioned suppression of upwelling
conditions off Peru. La Niña, the opposite phase with a positive SOI,
reverses this situation. After decades of intense study, ENSO is still an
active field of research (Takahashi et al., 2011).
Given the known importance of ENSO for the tropical South Pacific, it is
worthwhile to determine in which climatological conditions the cruise was
performed. To achieve this goal, we identified years of opposing ENSO phases
and analyzed the corresponding WTSP conditions with available satellite
data. ENSO phases were identified using the monthly time series of SOI provided by NCEP (http://www.cpc.ncep.noaa.gov/, downloaded 5 December 2016). The SOI metric
uses differences in standardized sea level air pressure between Papeete
(Tahiti) and Darwin (Australia) to represent the ENSO phase, as previously
mentioned. The OUTPACE region lies between these two locations (Fig. 2),
highlighting again ENSO's possible influence on the cruise.
WTSP conditions were estimated with SST and surface chl a
concentration measured by the MODIS Aqua satellite mission and available at
the NASA Ocean Color Data website (https://oceandata.sci.gsfc.nasa.gov/, downloaded 14 December 2016). Global
annual and monthly (March) averages of both SST and chl a were
provided by NASA at level 3 (i.e., mapped) with 4 km satellite pixel
resolution, resulting in four separate datasets from 2003 to present. The
data within the OUTPACE region, defined between 25∘ S,
155∘ E, and 15∘ S, 149∘ W, to envelope the
cruise track as in Fig. 2, were extracted. March was chosen as the month of
study because it was the central month of the cruise. Plots of these four
datasets can be found on the OUTPACE dataset website
(http://www.obs-vlfr.fr/proof/php/outpace/outpace_figures.php). Probability density distributions were generated for each
dataset in its entirety. Additional probability distributions were also
calculated on data subsets, for years 2003 and 2011 as chosen by SOI to
represent opposite ENSO phases, along with 2015, the year of the cruise. In
order to gauge significance between probability distributions of different
years, the temporal standard deviation of each
pixel was calculated for each of the four datasets. This resulted in a distribution of standard
deviations, the median of which was taken to represent inherent interannual
variability.
Time series of the monthly Southern Oscillation Index (SOI) from January
2000 to October 2016. Negative and positive values are shaded red and blue
to signify El Niño and La Niña, respectively. SOI smoothed by
a lowess filter with a five-point window is shown as a solid black line.
Black arrows indicate March for the years when satellite data are
available. Dashed vertical lines indicate March 2003, 2011, and 2015.
Probability density estimates for MODIS Aqua data in the OUTPACE
region, using mean (a) annual SST, (b) annual chl a, (c) March
SST, and (d) March chl a. Probability densities for the
ensemble of all years is shown in white, while densities for 2003, 2011, and
2015 are in red, blue, and green, respectively. Also plotted are 95 %
confidence intervals (2 standard deviations) for each subset, estimated
using the median variance of pixel interannual variability. Note the
logarithmic scale for chl a.
The time series of SOI is presented in Fig. 4, with El Niño (La
Niña) values shaded red (blue). During OUTPACE cruise sampling in
austral summer 2015, a strong El Niño was observed. The other El
Niño year considered here was 2003, chosen because of its relative
strength and duration similar to 2015; for similar reasons of intensity and
duration, 2011 was designated a representative La Niña year.
The probability density distributions for the annual and March means of both
SST and chl a are presented in Fig. 5. In SST, the March mean
showed a larger proportion of warmer temperatures in relation to the annual
mean (white lines in Fig. 5a, c), reflecting the austral summer season. The
2011 La Niña distributions showed a greater proportion of warm
temperatures in both annual and March distributions (blue lines in Fig. 5a, c), which is consistent with the idea of La Niña accentuating the
warm pool present in the WTSP. The probability peak of March 2011 SST was
much more localized, between 26 and 30 ∘C, than 2003, 2015, and
the mean (Fig. 5c). The 2003 March SST (red line in Fig. 5c) was the widest
distribution, with a slight rightward skew. By contrast, both the mean and
2015 March probabilities (white and green lines, respectively) had a left
skew. Considering the median 95 % confidence interval for the March SST
time series, the central peaks for these distributions could not be
considered distinct.
Satellite chl a showed distributions completely different from SST.
For all quantities considered, the annual and March means (Fig. 5b, d), as
well as the years 2003, 2011, and 2015, showed a bimodal distribution, where
the two peaks were distinct enough from each other to be significant from the
median pixel variability. Interestingly, 2003 and 2011 (red and blue lines,
Fig. 5b) annual chl a distributions overlapped considerably more
than 2015 (green line), which had its entire probability distribution shifted
to the right, though this shift may not have been significant. The March mean
for 2003 and 2015 chl a (red and green lines, respectively,
Fig. 5d), however, almost entirely overlapped, possibly signifying that El
Niño chl a distributions were more alike than La Niña
years. The annual 2015 mean of chl a being slightly different
from other years, and yet the March 2015 mean resembling 2003, might indicate
that 2015 was indeed a slightly different year from a surface chlorophyll
perspective, but this slight difference was concentrated in other parts of
the year than late austral summer when OUTPACE took place.
In summary, the SOI metric of ENSO showed that 2015 was an El Niño event,
and both the SST and chl a satellite data in the WTSP partially
reflected this. The March 2003 and 2015 SST distributions resembled each
other, but they also resembled the entire dataset more than the 2011 La
Niña distribution. Looking at SST, perhaps La Niña events
impact the region more than El Niño. Chl a distributions for
the El Niño years also overlapped more than with La Niña, but the
variability inherent in the time series precluded the declaration of significant
differences. Overall, in the WTSP both SST and chl a were not
atypical from what one would expect; thus, we determined that climatological
effects upon the results of OUTPACE were minimized.
Special issue presentation
The goal of this special issue is to present the knowledge obtained
concerning the functioning of WTSP ecosystems and associated biogeochemical
cycles based on the datasets acquired during the OUTPACE experiment. The
cruise strategy was organized to promote collaboration between physicists,
biologists, and biogeochemists with expertise including marine physics,
chemistry, optics, biogeochemistry, microbiology, molecular ecology,
genetics, and modeling. Most of the contributions to this volume have
benefited from this collective effort and are presented according to the
main objectives of the OUTPACE experiment.
The hydrological and dynamical context of biogeochemical sampling is
described for the entire cruise route (Fumenia et al., 2017) and specifically
at the three long-duration stations, where low physical variability validated
the quasi-Lagrangian sampling strategy employed (de Verneil et al., 2017b).
Turbulence measurements revealed an interesting longitudinal gradient with
higher turbulence levels in the west, i.e., the Coral Sea, compared to the
eastern part within the gyre, consistent with the increasing oligotrophy
(Bouruet-Aubertot et al., 2017). The large-scale circulation was dominant
even though the mesoscale and sub-mesoscale circulation can have a strong
influence (Rousselet et al., 2017), in particular on the bloom observed at
station LD B (de Verneil et al., 2017a).
An important focus of OUTPACE was on dinitrogen fixation and its fate in the
ecosystem (Caffin et al., 2017a, b). N2 fixation was
detected at all 18 sampled stations and the transect could be divided into
two main characteristic subareas (Bonnet et al., 2017b): (i) the MA waters
(160∘ W to 170∘ E) exhibiting very high N2 fixation
rates (631 ± 286 µmol N m-2 d-1, i.e., among the
highest reported for the global ocean; Luo et al., 2012) and dominated by
Trichodesmium (Stenengren et al., 2017), and (ii) the GY waters
(170–160∘ E) exhibiting low N2 fixation rates
(average 85 ± 79 µmol N m-2 d-1) dominated by
UCYN-B (Stenengren et al., 2017). The differing δ15N signature of
suspended particles measured over the photic layer of MA (-0.41 ‰)
and GY waters (8.06 ‰) confirms the presence of two contrasting
regions in terms of N2 fixation. Thanks to the Lagrangian strategy
followed at the LD stations and the low dispersion measured, showing that we
sampled the same water masses (de Verneil et al., 2017b), N budgets were
established (Caffin et al., 2017b). N2 fixation was the
major external source of N, representing more than 90 % of new N input
into the photic layer, and the e-ratio quantifying the efficiency of a system
to export particulate organic matter was higher in MA waters than in GY
waters (Caffin et al., 2017b). Caffin et al. (2017a)
revealed that the diazotroph-derived nitrogen (DDN) was efficiently
transferred from diazotrophs (Trichodesmium and UCYN) to
non-diazotrophic phytoplankton, both autotrophs and heterotrophs. Hunt et al. (2017) report an efficient transfer of DDN in zooplankton. The fate of C
and N was under the influence of programmed cell death in diazotrophs
(Berman-Frank et al., 2017) but diazotrophs were poorly exported directly,
and we suspect that this transfer of DDN fueled indirect export associated
with N2 fixation. By using nitrogen isotope budgets, Knapp et al. (2017) confirmed that > 50 % of export production was supported by
N2 fixation in MA waters. Stenegren et al. (2017) identified a clear
niche separation between a subsurface (UCYN-A1 and A2 with their hosts) and a
surface group (Trichodesmium, UCYN-B, and the heterotrophic group, het-group) based on a
temperature–depth gradient. They also found discrepancies between the UCYN-A
and their hosts in both abundance and distribution, which suggests that the
UCYN-A could be living freely or with a wider diversity of hosts than
previously believed. Finally, the assemblage of epibiotic microorganisms
associated with Trichodesmium were characterized in relation with
environmental parameters (Frischkorn et al., 2017). N2 fixation in
the ocean does not only occur in tropical sunlit surface waters but also in
less obvious environments such as temperate latitudes and aphotic waters.
Here, N2 fixation was also measured in the mesopelagic zone along
the OUTPACE transect and the diazotroph community present were identified.
Deep N2 fixation rates were low but measurable and recurrently found
along the transect, with the exception of the easternmost stations located in
the ultra-oligotrophic subtropical Pacific gyre (Benavides et al., 2017).
N2 fixation activity was presumably driven by the dominating
gamma-proteobacterial community and fueled by the presence of labile organic
matter compounds. Benavides et al. (2017) provided further evidence that
N2 fixation in the deep ocean is not negligible and likely impacts
global nitrogen inputs in a significant manner.
The dynamics of phytoplankton (Bock et al., 2017; Leblanc et al., 2017;
Lefevre et al., 2017; Guidi, 2017), heterotrophic bacterioplankton (Van
Wambeke et al., 2017), and zooplankton (Carlotti et al., 2017; Hunt et al.,
2017) along the zonal gradient of diazotroph diversity and activity are
described together with the composition and distribution of dissolved organic
carbon (Panagiotopoulos et al., 2017) and the changes in inorganic carbon
content along the longitudinal transect (Wagener et al., 2017). Stations
sampled during the OUTPACE cruise were characterized by a highly stratified
community structure, with significant contributions of Prochlorococcus and
picophytoeukaryote populations to biomass (Bock et al., 2017).
Size-fractionated results show a non-negligible contribution of the
pico-sized fraction (< 2–3 µm) to both Si biomass and uptake,
which could confirm the previous hypothesis of Si assimilation by
Synechococcus populations or reflect the presence of an overlooked Si group
such as Parmales (Leblanc et al., 2017). Surface DOC concentrations varied
little (50–75 µM) across the transect, with slightly higher values
observed at LD B (78 µM), and labile organic matter (sugars were
used as a good proxy) closely followed DOC patterns ranging from 1.5 to
3 µM with higher values also recorded at LD B (3.5 µM).
Labile organic matter accounted for about 3–5 % of DOC, with glucose being
the dominant sugar (> 60 % of total sugars) (Panagiotopoulos et al.,
2017). Valdes et al. (2017) suggest that copepods can retain N and P
compounds obtained from feeding in the upper layer, preventing the rapid loss
of these nutrients. Copepods were able to sustain and modify the composition
of microbial communities and could provide P for further development of
cyanobacterial blooms.
Optical properties of the WTSP waters are presented, with a focus on the
cyanobacterial (diazotroph) impact upon bio-optical properties, UV–VIS light
attenuation (Dupouy et al., 2017), and chl a algorithms (Frouin et al., 2017).
Operational NASA bio-optical algorithms (OC4v6; color index, CI) substantially
underestimated surface chl a concentration, but a normalized
reflectance difference index, robust to atmospheric correction errors,
performed well over the range of chl a values encountered across
the transect (Frouin et al., 2017). Trichodesmium is considered the
main nitrogen-fixing species, especially in the South Pacific region. Due to
the paucity of in situ observations, alternative methods for estimating the
presence of Trichodesmium must be sought to evaluate the global
impact of these species on primary production. Rousset et al. (2017)
elaborate a new satellite-based algorithm and use that algorithm to estimate
the extent of Trichodesmium surface blooms and their dynamics during
the OUTPACE experiment. Finally, the main processes controlling the biological
carbon pump in the WTSP were investigated using a unidimensional vertical (Gimenez et al., 2017)
and regional (Dutheil et al., 2017) biogeochemical–physical coupled models.
The new knowledge gained on the interactions between planktonic organisms and
the cycle of biogenic elements is then used to propose a new scheme for the
biological carbon pump function and its role, at the present time and in
the near future, in the oligotrophic Pacific Ocean (Moutin et al.,
2017).
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:
http://www.obs-vlfr.fr/proof/php/outpace/outpace.php (INSU/CNRS LEFE
CYBER, 2017).
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 N2 fixation
gradients in the western tropical South Pacific Ocean: a multidisciplinary
approach (OUTPACE experiment)”. It does not belong to a conference.
Acknowledgements
This is a contribution of the OUTPACE (Oligotrophy from Ultra-oligoTrophy
PACific Experiment) project (https://outpace.mio.univ-amu.fr/) funded
by the French research national agency (ANR-14-CE01-0007-01), the LEFE-CYBER
program (CNRS-INSU), the GOPS program (IRD), and the CNES (BC T23, ZBC
4500048836). The OUTPACE cruise (10.17600/15000900) was managed by MIO
(OSU Institut Pythéas, AMU) from Marseilles (France). The authors thank
the crew of the RV L'Atalante for outstanding on-ship operations.
Gilles Rougier and Marc Picheral are warmly thanked for their efficient help
in CTD rosette management and data processing, as well as Catherine
Schmechtig for the LEFE-CYBER database management. The satellite-derived data
of sea surface temperature, chl a concentrations, and currents have been
provided by CLS in the framework of the CNES funding; we warmly thank Marie
Isabelle Pujol and Guillaume Taburet for their support in providing these
data. We acknowledge NOAA, and in particular Rick Lumpkin, for providing the
SVP drifters. Edited by: Emilio
Marañón Reviewed by: two anonymous referees
References
Altabet, M. A.: Variations in Nitrogen Isotopic Composition between Sinking
and Suspended Particles – Implications for Nitrogen Cycling and Particle
Transformation in the Open Ocean, Deep-Sea Res., 35, 535–554, 1988.
Antoine, D., Andre, J. M., and Morel, A.: Oceanic primary production: 2.
Estimation at global scale from satellite (coastal zone color scanner)
chlorophyll, Global Biogeochem. Cy., 10, 57–69, 1996.Benavides, M. and Voss, M.: Five decades of N2 fixation research in the
North Atlantic Ocean, Frontiers in Marine Science, 2, 40, 10.3389/fmars.2015.00040, 2015.
Benavides, M., Moisander, P. H., Dittmar, T., Berthelot, H., Grosso, O., and
Bonnet, S.: Aphotic N2 fixation is related to labile organic matter in the
Western Tropical South Pacific, Biogeosciences, in preparation, 2017.
Berman-Frank, I., Spungin, D., Belkin, N., Van-Wambeke, F., Gimenez, A.,
Caffin, M., Stengren, M., Foster, R., Knapp, A., and Bonnet, S.: Programmed
cell death in diazotrophs and the fate of C and N in the Western Tropical
South Pacific, Biogeosciences, in preparation, 2017.Berthelot, H., Moutin, T., L'Helguen, S., Leblanc, K., Hélias, S.,
Grosso, O., Leblond, N., Charrière, B., and Bonnet, S.: Dinitrogen
fixation and dissolved organic nitrogen fueled primary production and
particulate export during the VAHINE mesocosm experiment (New Caledonia
lagoon), Biogeosciences, 12, 4099–4112, 10.5194/bg-12-4099-2015, 2015.
Bock, N., Dion, M., Van Wambeke, F., and Duhamel, S.: Picophytoplankton
Community Structure in the Western Tropical South Pacific During Austral
Summer, Biogeosciences, in preparation, 2017.Bonnet, S., Guieu, C., Bruyant, F., Prášil, O., Van Wambeke, F.,
Raimbault, P., Moutin, T., Grob, C., Gorbunov, M. Y., Zehr, J. P.,
Masquelier, S. M., Garczarek, L., and Claustre, H.: Nutrient limitation of
primary productivity in the Southeast Pacific (BIOSOPE cruise),
Biogeosciences, 5, 215–225, 10.5194/bg-5-215-2008, 2008.
Bonnet, S., Biegala, I. C., Dutrieux, P., Slemons, L. O., and Capone, D. G.:
Nitrogen fixation in the western equatorial Pacific: Rates, diazotrophic
cyanobacterial size class distribution, and biogeochemical significance,
Global Biogeochem. Cy., 23, 1–13, 2009.Bonnet, S., Rodier, M., Turk-Kubo, K., Germineaud, C., Menkes, C., Ganachaud,
A., Cravatte, S., Raimbault, P., Campbell, E., Quéroué, F., Sarthou,
G., Desnues, A., Maes, C., and Eldin, G.: Contrasted geographical
distribution of N2 fixation rates and nifH phylotypes in the Coral and
Solomon Seas (South-Western Pacific) during austral winter conditions, Global
Biogeochem. Cy., 29, 1874–1892, 10.1002/2015GB005117, 2015.Bonnet, S., Baklouti, M., Gimenez, A., Berthelot, H., and Berman-Frank, I.:
Biogeochemical and biological impacts of diazotroph blooms in a low-nutrient,
low-chlorophyll ecosystem: synthesis from the VAHINE mesocosm experiment (New
Caledonia), Biogeosciences, 13, 4461–4479, 10.5194/bg-13-4461-2016,
2016a.
Bonnet, S., Berthelot, H., Turk-Kubo, K., Cornet-Barthaux, V., Fawcett, S.,
Berman-Frank, I., Barani, A., Gregori, G., Dekaezemacker, J., Benavides, M.,
and Capone, D. G.: Diazotroph derived nitrogen supports diatom growth in the
South West Pacific: A quantitative study using nanoSIMS, Limnol. Oceanogr.,
61, 1549–1562, 2016b.Bonnet, S., Berthelot, H., Turk-Kubo, K., Fawcett, S., Rahav, E., L'Helguen,
S., and Berman-Frank, I.: Dynamics of N2 fixation and fate of
diazotroph-derived nitrogen in a low-nutrient, low-chlorophyll ecosystem:
results from the VAHINE mesocosm experiment (New Caledonia), Biogeosciences,
13, 2653–2673, 10.5194/bg-13-2653-2016, 2016c.Bonnet, S., Berthelot, H., Caffin, M., and Moutin, T.: A hot spot of N2 fixation in the western tropical South Pacific pleads for a spatial
decoupling between N2 fixation and denitrification, P. Natl. Acad. Sci.
USA, 114, E2800–E2801, 10.1073/pnas.1619514114, 2017a.Bonnet S., Caffin, M., Berthelot, H., Grosso, O., Guieu, C., and Moutin, T.:
Contribution of dissolved and particulate fractions to the Hot Spot of N2
fixation in the Western Tropical South Pacific Ocean (OUTPACE cruise),
Biogeosciences, in preparation, 2017b.
Böttjer, D., Dore, J. E., Karl, D. M., Letelier, R. M., Mahaffey, C.,
Wilson, S. T., Zehr, J., and Church, M. J.: Temporal variability of nitrogen
fixation and particulate nitrogen export at Station ALOHA, Limnol. Oceanogr.,
62, 200–216, 2017.Bouruet-Aubertot, P., Cuypers, Y., Le Goff, H., Rougier, G., de Verneuil, A.,
Doglioli, A. M., Picheral, M., Yohia, C., Caffin, M., Lefèvre, D., Petrenko,
A., and Moutin, T.: Longitudinal contrast in small scale turbulence along
20∘ S in the Pacific Ocean: origin and impact on biogeochemical
fluxes, Biogeosciences, in preparation, 2017.
Broecker, W. S.: Keeping global change honest, Global Biogeochem. Cy., 1,
15–29, 1991.Caffin, M., Foster, R., Berthelot, H., Stenegren, M., Caputo, A., Berntzo,
L., and Bonnet, S.: Fate of N2 fixation in the Western Tropical South
Pacific Ocean: Transfert of diazotroph-derived nitrogen to non-diazotrophic
communities and export of diazotrophs, Biogeosciences, in preparation, 2017a.
Caffin, M., Moutin, T., Bouruet-Aubertot, P., Doglioli, A. M., Berthelot, H.,
Grosso, O., Helias-Nunige, S., Leblond, N., Gimenez, A., de Verneil, A., and
Bonnet, S.: Nitrogen budget in the photic layer of the Western Tropical
South Pacific (WTSP) Ocean: evidence of high nitrogen fixation rates,
Biogeosciences, in preparation, 2017b.
Campbell, L., Carpenter, E. J., Montoya, J. P., Kustka, A. B., and Capone, D.
G.: Picoplankton community structure within and outside a Trichodesmium bloom
in the southwestern Pacific Ocean, Vie Milieu., 55, 185–195, 2005.
Carlotti,F., Pagano, M., Guilloux, L., Donoso, K., and Valdes, V.:
Mesozooplankton structure and functioning across the Western Tropical South
Pacific Ocean, Biogeosciences, in preparation, 2017.
Chavez, F. P., Ryan, J., Lluch-Cota, S. E., and Niquen, M.: From anchovies to
sardines and back: Multidecadal change in the Pacific Ocean, Science, 299,
217–221, 2003.
Close, H. G., Shah, S. R., Ingalls, A. E., Diefendorf, A. F., Brodie, E. L.,
Hansman, R. L., Freeman, K. H., Aluwihare, L. I., and Pearson, A.: Export of
submicron particulate organic matter to mesopelagic depth in an oligotrophic
gyre, P. Natl. Acad. Sci. USA, 110, 12565–12570, 2013.de Verneil, A., Rousselet, L., Doglioli, A. M., Petrenko, A. A., and Moutin,
T.: The Fate of a Southwest Pacific Bloom: Gauging the impact of submesoscale
vs. mesoscale circulation on biological gradients in the subtropics,
Biogeosciences Discuss., 10.5194/bg-2017-84, in review,
2017a.
de Verneil, A., Rousselet, L., Doglioli, A. M., Petrenko, A. A.,
Bouruet-Aubertot,
P., Maes, C., and Moutin, T.: OUTPACE Long Duration Stations: Physical
variability and context of biogeochemical sampling, Biogeosciences, in preparation, 2017b.
Dore, J., Brium, J. R., Tupas, L. M., and Karl, D. M.: Seasonal and
interannual variability in sources of nitrogen supporting export in the
oligotrophic subtropical North Pacific Ocean, Limnol. Oceanogr., 47,
1595–1607, 2002.d'Ovidio, F., Della Penna, A., Trull, T. W., Nencioli, F., Pujol, M.-I., Rio,
M.-H., Park, Y.-H., Cotté, C., Zhou, M., and Blain, S.: The
biogeochemical structuring role of horizontal stirring: Lagrangian
perspectives on iron delivery downstream of the Kerguelen Plateau,
Biogeosciences, 12, 5567–5581, 10.5194/bg-12-5567-2015, 2015.Dupouy, C., Benielli-Gary, D., Neveux, J., Dandonneau, Y., and Westberry, T.
K.: An algorithm for detecting Trichodesmium surface blooms in the
South Western Tropical Pacific, Biogeosciences, 8, 3631–3647,
10.5194/bg-8-3631-2011, 2011.
Dupouy, C., Frouin, R., Maillard, M., Tedetti, M., Charriere, B., Roettgers, R.,
Martias, C., Rodier, M., Pujo-Pay, M., Duhamel, S., Guidi, L., and Sempere, R.:
Influence of cyanobacteria on Longitudinal and short-term variations in
UV-Vis optical properties in the Western Tropical South Pacific Ocean,
Biogeosciences, in preparation, 2017.Dutheil, C., Menkes, C., Aumont, O., Lorrain, A., Bonnet, S., Rodier, M.,
Shiozaki, T., and Dupouy, C.: Impact of Trichodesmium sp. on Pacific
primary production, Biogeosciences, in preparation, 2017.
Falkowski, P. G., Barber, R. T., and Smetacek, V.: Biogeochemical Controls
and Feedbacks on Ocean Primary Production, Science, 281, 200–206, 1998.
Frischkorn, K. R., Krupke A., Dyhrman, S. T., and Van Mooy, B. A. S.: Quorum
sensing influences Trichodesmium consortia physiology in the oligotrophic
South Pacific, Biogeosciences, in preparation, 2017.
Frouin, R., Dupouy, C., and Tan, J.: Chlorophyll-a algorithms for South
Pacific oligotrophic waters, Biogeosciences, in preparation, 2017.
Fumenia, A., Moutin, T., Petrenko, A., Doglioli, A. M., de Verneil, A., and
Maes, C.: Optimum multiparameter analysis of the water mass structure in the
western Tropical South Pacific Ocean during OUTPACE, Biogeosciences, in preparation, 2017.
Galbraith, E. D., Kienast, M., Albuquerque, A. L., Altabet, M. A., Batista,
F., Bianchi, D., Calvert, S. E., Contreras, S., Crosta, X., De Pol-Holz, R.,
Dubois, N., Etourneau, J., Francois, R., Hsu, T. C., Ivanochko, T., Jaccard,
S. L., Kao, S. J., Kiefer, T., Kienast, S., Lehmann, M. F., Martinez, P.,
McCarthy, M., Meckler, A. N., Mix, A., Mobius, J., Pedersen, T. F., Pichevin,
L., Quan, T. M., Robinson, R. S., Ryabenko, E., Schmittner, A., Schneider,
R., Schneider-Mor, A., Shigemitsu, M., Sinclair, D., Somes, C., Studer, A.
S., Tesdal, J. E., Thunell, R., Yang, J. Y. T., and Members, N. W. G.: The
acceleration of oceanic denitrification during deglacial warming, Nat.
Geosci., 6, 579–584, 2013.Gandhi, N., Singh, A., Prakash, Ramesh, R., Raman, M., Sheshshayee, M. S.,
and Shetye, S.: First direct measurements of N2 fixation during a
Trichodesmium bloom in the eastern Arabian Sea, Global Biogeochem. Cy., 25,
GB4014, 10.1029/2010GB003970, 2011.
Garcia, N., Raimbault, P., and Sandroni, V.: Seasonal nitrogen fixation and
primary production in the Southwest Pacific: nanoplankton diazotrophy and
transfer of nitrogen to picoplankton organisms, Mar. Ecol-Prog. Ser., 343,
25–33, 2007.
Gimenez, A., Baklouti, M., and Moutin, T.: Investigation of the main
processes controlling the biological carbon pump in the Western Tropical
South Pacific using a 1DV biogeochemical-physical coupled model,
Biogeosciences, in preparation, 2017.Gruber, N.: The dynamics of the marine nitrogen cycle and its influence
on atmospheric CO2, in: The ocean carbon cycle and climate, edited by: Follows, M. and
Oguz, T., Kluwer Academic, Dordrecht, 2004.
Guidi, L.: Fate of a diazotrophs bloom followed by Underwater Video profiles
in the WTSP (OUTPACE cruise), Biogeosciences, in preparation, 2017.
Halm, H., Lam, P., Ferdelman, T. G., Lavik, G., Dittmar, T., LaRoche, J.,
D'Hondt, S., and Kuypers, M. M. M.: Heterotrophic organisms dominate nitrogen
fixation in the South Pacific Gyre, ISME J., 6, 1238–1249, 2012.
Hunt, B., Bonnet, S., Carlotti, F., Donoso, K., Pagano, M., and Moutin, T.:
Diazotroph derived nitrogen contribution to zooplankton biomass in the south
west Pacific, Biogeosciences, in preparation, 2017.INSU/CNRS LEFE CYBER: OUTPACE – Oligotrophy to UlTra-oligotrophy PACific
Experiment, available at:
http://www.obs-vlfr.fr/proof/php/outpace/outpace.php, last access: 5
July 2017.
Jackson, G. A.: Effect of coagulation on a model planktonic food web,
Deep-Sea Res. Pt. I, 48, 95–123, 2001.
Karl, D. M.: Microbially mediated transformations of phosphorus in the sea:
new views of an old cycle, Ann. Rev. Mar. Sci., 6, 279–337, 2014.
Karl, D. M., Letelier, R., Tupas, L., Dore, J., Christian, J., and Hebel, D.:
The role of nitrogen fixation in biogeochemical cycling in the subtropical
North Pacific Ocean, Nature, 388, 533–538, 1997.
Karl, D. M., Church, M. J., Dore, J. E., Letelier, R., and Mahaffey, C.:
Predictable and efficient carbon sequestration in the North Pacific Ocean
supported by symbiotic nitrogen fixation, P. Natl. Acad. Sci. USA, 109,
1842–1849, 2012.
Knapp, A. N., Sigman, D. M., and Lipschultz, F.: N isotopic composition of
dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time-series
Study site, Global Biogeochem. Cy., 19, 1–15, 2005.Knapp, A. N., Fawcett, S. E., Martínez-Garcia, A., Leblond, N., Moutin,
T., and Bonnet, S.: Nitrogen isotopic evidence for a shift from nitrate- to
diazotroph-fueled export production in the VAHINE mesocosm experiments,
Biogeosciences, 13, 4645–4657, 10.5194/bg-13-4645-2016, 2016.Knapp, A. N., Grosso, O., Leblond, N., Caffin, M., Moutin, T., and Bonnet,
S.: Zonal gradients in N2 fixation and its contribution to export
production in the Western Tropical South Pacific Ocean, Biogeosciences, in preparation, 2017.Kumar, P. K., Singh, A., Ramesh, R., and Nallathambi, T.: N2 Fixation in
the Eastern Arabian Sea: Probable. Role of Heterotrophic Diazotrophs, Front.
Mar. Sci., 4, 80, 10.3389/fmars.2017.00080, 2017.
Leblanc K., Cornet, V., Brunet, C., Rimmelin-Maury, P., Grosso, O.,
Helias-Nunige, S., and Quéguiner, B.: Siliceous plankton and silicon
biogeochemical cycle in the Tropical South Pacific. Biogeosciences, in preparation, 2017.
Lefevre, D., Grosso, O., Gimenez, A., Van Wambeke, F., Spungin, D., Belkin,
N., and Berman-Frank, I.: Net Community Production accross the Western
Tropical South Pacific. Implication for the ecosystem functioning,
Biogeosciences, in preparation, 2017.Lomas, M. W., Burke, A. L., Lomas, D. A., Bell, D. W., Shen, C., Dyhrman, S.
T., and Ammerman, J. W.: Sargasso Sea phosphorus biogeochemistry: an
important role for dissolved organic phosphorus (DOP), Biogeosciences, 7,
695–710, 10.5194/bg-7-695-2010, 2010.
Longhurst, A. R.: Ecological Geography of the Sea, Academic press, 1998.Luo, Y.-W., Doney, S. C., Anderson, L. A., Benavides, M., Berman-Frank, I.,
Bode, A., Bonnet, S., Boström, K. H., Böttjer, D., Capone, D. G.,
Carpenter, E. J., Chen, Y. L., Church, M. J., Dore, J. E., Falcón, L. I.,
Fernández, A., Foster, R. A., Furuya, K., Gómez, F., Gundersen, K.,
Hynes, A. M., Karl, D. M., Kitajima, S., Langlois, R. J., LaRoche, J.,
Letelier, R. M., Marañón, E., McGillicuddy Jr., D. J., Moisander, P.
H., Moore, C. M., Mouriño-Carballido, B., Mulholland, M. R., Needoba, J.
A., Orcutt, K. M., Poulton, A. J., Rahav, E., Raimbault, P., Rees, A. P.,
Riemann, L., Shiozaki, T., Subramaniam, A., Tyrrell, T., Turk-Kubo, K. A.,
Varela, M., Villareal, T. A., Webb, E. A., White, A. E., Wu, J., and Zehr, J.
P.: Database of diazotrophs in global ocean: abundance, biomass and nitrogen
fixation rates, Earth Syst. Sci. Data, 4, 47–73, 10.5194/essd-4-47-2012,
2012.
McMahon, K. W., McCarthy, M. D., Sherwood, O. A., Larsen, T., and Guilderson,
T. P.: Millennial-scale plankton regime shifts in the subtropical North
Pacific Ocean, Science, 350, 1530–1533, 2015.Moisander, P. H., Beinart, R. A., Hewson, I., White, A. E., Johnson, K. S.,
Carlson, C. A., Montoya, J. P., and Zehr, J. P.: Unicellular Cyanobacterial
Distributions Broaden the Oceanic N2 Fixation Domain, Science, 327,
1512–1514, 2010.
Moore, C. M., Mills, M. M. M., Arrigo, K. R., Berman-Frank, I., Bopp, L.,
Boyd, P. W., Galbraith, E. D., Geider, R. J., Guieu, C., Jaccard, S. L.,
Jickells, T. D., La Roche, J., Lenton, T. M., Mahowald, N. M., Maranon, E.,
Marinov, I., Moore, J. K., Nakatsuka, T., Oschlies, A., Saito, M. A.,
Thingstad, T. F., Tsuda, A., and Ulloa, O.: Processes and patterns of oceanic
nutrient limitation, Nat. Geosci., 6, 701–710, 2013.
Moutin, T., Van Den Broeck, N., Beker, B., Dupouy, C., Rimmelin, P., and
LeBouteiller, A.: Phosphate availability controls Trichodesmium spp. biomass
in the SW Pacific ocean, Mar. Ecol.-Prog. Ser., 297, 15–21, 2005.Moutin, T., Karl, D. M., Duhamel, S., Rimmelin, P., Raimbault, P., Van Mooy,
B. A. S., and Claustre, H.: Phosphate availability and the ultimate control
of new nitrogen input by nitrogen fixation in the tropical Pacific Ocean,
Biogeosciences, 5, 95–109, 10.5194/bg-5-95-2008, 2008.Moutin, T., Van Wambeke, F., and Prieur, L.: Introduction to the
Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean
(BOUM) experiment, Biogeosciences, 9, 3817–3825, 10.5194/bg-9-3817-2012,
2012.
Moutin, T., Wagener, T., Fumenia, A., Gimenez, A., Caffin, M., Lefevre, D.,
Leblanc, K., Bouruet-Aubertot, P., Helias-Nunige, S., Rougier, G., Grosso,
O., and de Verneil, A.: Phosphate availability and the ultimate control of
the biological carbon pump in the Western Tropical South Pacific Ocean
(OUTPACE cruise), Biogeosciences, in preparation, 2017.Murnane, R. J., Sarmiento, J. L., and Le Quere, C.: Spatial distribution of
air-sea CO2 fluxes and the interhemispheric transport of carbon by the
oceans, Global Biogeochem. Cy., 13, 287–305, 1999.
Nelson, D. M., Treguer, P., Brezezinski, M. A., Leynaert, A., and Queguiner,
B.: Production and dissolution of biogenic silica in the ocean: Revised
global estimates, comparison with regional data and relationship to biogenic
sedimentation, Global Biogeochem. Cy., 9, 359–372, 1995.
Panagiotopoulos, C., Pujo-Pay, M., Benavides, M., and R. Sempéré.
Composition and distribution of dissolved carbohydrates across the Western
Tropical South Pacific (OUTPACE cruise), Biogeosciences, in preparation, 2017.Polovina, J. J., Howell, E. A., and Abecassis, M.: Ocean's least productive
waters are expanding, Geophys. Res. Lett., 35, L03618, 10.1029/2007GL031745, 2008.Raimbault, P. and Garcia, N.: Evidence for efficient regenerated production
and dinitrogen fixation in nitrogen-deficient waters of the South Pacific
Ocean: impact on new and export production estimates, Biogeosciences, 5,
323–338, 10.5194/bg-5-323-2008, 2008.
Richardson, T. L. and Jackson, G. A.: Small phytoplankton and carbon export
from the surface ocean, Science, 315, 838–840, 2007.
Rousselet, L., de Verneil, A., Doglioli, A. M., Petrenko, A. A., Duhamel, S., Maes, C., and Blanke, B.: From large to submesoscale circulation influence during
the OUTPACE cruise (Western Tropical South Pacific), Biogeosciences, in preparation, 2017.
Rousset G., Dupouy C., Lefèvre, J., De Boissieu, F., Ridoux, V., Rodier,
M., and Menkes, C.: Remote sensing of Trichodesmium in open Ocean of the
South Pacific, Biogeosciences, in preparation, 2017.Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J.
L., Wanninkhof, R., Wong, C. S., Wallace, D. W. R., Tilbrook, B., Millero, F.
J., Peng, T. H., Kozyr, A., Ono, T., and Rios, A. F.: The oceanic sink for
anthropogenic CO2, Science, 305, 367–371, 2004.
Sarmiento, J. and Gruber, N.: Ocean Biogeochemical Dynamics, Princeton University Press, Princeton,
Woodstock, 503 pp., 2006.
Scharek, R., Latasa, M., Karl, D. M., and Bidigare, R. R.: Temporal
variations in diatom abundance and downward vertical fux in the oligotrophic
North Pacific gyre, Deep-Sea Res. Pt. I, 46, 1051–1075, 1999a.
Sharek, R. M., Tupas, L. M., and Karl, D. M.: Diatom fluxes to the deep sea
in the oligotrophic North Pacific gyre at Station ALOHA, Mar. Ecol.-Prog.
Ser., 82, 55–67, 1999b.Sohm, J. A., Webb, E. A., and Capone, D. G.: Emerging patterns of marine
nitrogen fixation, Nat. Rev. Microbiol., 9, 499–508, 2011.
Stenegren, M., Caputo, A., Berg, C., Bonnet, S., and Foster, R. A.:
Distribution and drivers of symbiotic and free-living diazotrophic
cyanobacteria in the Western Tropical South Pacific, Biogeosciences Discuss.,
10.5194/bg-2017-63, in review, 2017.
Subramaniam, A., Yager, P. L., Carpenter, E. J., Mahaffey, C., Bjorkman, K.
M., Cooley, S., Kustka, A. B., Montoya, J., Sanudo-Wilhelmy, S., Shipe, R.,
and Capone, D. G.: Amazon River enhances diazotrophy and carbon sequestration
in the tropical North Atlantic Ocean, P. Natl. Acad. Sci. USA, 105,
10460–10465, 2008.Takahashi, K., Montecinos, A., Goubanova, K., and Dewitte, B.: ENSO regimes:
Reinterpreting the canonical and Modoki El Nino, Geophys. Res. Lett., 38, 5 pp., 10.1029/2011GL047364, 2011.
Valdés, V., Donoso, K., Carlotti, F., Pagano, M., Molina, V., Escribano,
R., and Fernandez, C.: Nitrogen and phosphorus recycling mediated by
copepods in western tropical south Pacific, Biogeosciences, in preparation, 2017.
Van Den Broeck, N., Moutin, T., Rodier, M., and Le Bouteiller, A.: Seasonal
variations of phosphate availability in the SW Pacific Ocean near New
Caledonia, Mar. Ecol.-Prog. Ser., 268, 1–12, 2004.
Van Wambeke, F., Duhamel, S., Gimenez, A., Lefèvre, D., Pujo-Pay, M., and
Moutin, T.: Dynamics of phytoplankton and heterotrophic bacterioplankton in
the Western Tropical South Pacific Ocean along a gradient of diversity and
activity of nitrogen fixers, Biogeosciences, in preparation, 2017.
Wagener, T., Metzl, N., Lo Monaco, C., Fin, J., Caffin, M., Lefevre, D., and
Moutin, T.: Anthopogenic carbon accumulation in the South West Pacific,
Biogeosciences, in preparation, 2017.
White, A. E., Foster, R. A., Benitez-Nelson, C. R., Masqué, P., Verdeny,
E., Popp, B. N., Arthur, K. E., and Prahl, F. G.: Nitrogen fixation in the
Gulf of California and the Eastern Tropical North Pacific, Prog. Oceanogr.,
109, 1–17, 2012.
Zehr, J. P. and Kudela, R. M.: Nitrogen Cycle of the Open Ocean: From Genes
to Ecosystems, Ann. Rev. Mar. Sci., 3, 197–225, 2011.
Zehr, J. P., Mellon, M. T., and Zani, S.: New nitrogen-fixing microorganisms
detected in oligotrophic oceans by amplification of nitrogenase (nifH) genes,
Appl. Environ. Microb., 64, 3444–3450, 1998.Zehr, J. P., Bench, S. R., Carter, B. J., Hewson, I., Niazi, F., Shi, T.,
Tripp, H. J., and Affourtit, J. P.: Globally Distributed Uncultivated Oceanic
N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II, Science, 322,
1110–1112, 2008.