Introduction
The primary source of nitrogen (N) to the ocean is the biologically mediated
reduction of dinitrogen (N2) gas to ammonia, which is then assimilated
into the biomass of the organisms carrying out this process, known as
diazotrophs (Gruber, 2004). While the distribution and rates of this process
in the ocean play a central role in regulating the fertility and community
structure of marine ecosystems, these first-order properties of marine
N2 fixation remain poorly constrained. Historically, the highest rates
of N2 fixation in the global ocean have been associated with the
tropical North Atlantic (Mahaffey et al., 2005; Sohm et al., 2011). The high
15N2 incubation-based N2 fixation rates observed in the
tropical Atlantic (Luo et al., 2012) are consistent with both the preference
of diazotrophs for warm waters (Breitbarth et al., 2007; Stal, 2009) as well
as the high atmospheric dust flux to the region (Mahowald et al., 2009;
Prospero, 1996) that helps fulfil the high iron requirement of the enzyme,
nitrogenase, carrying out N2 fixation (Berman-Frank et al., 2001; Kustka
et al., 2003). Additionally, the elevated ratio of nitrate (NO3-) to
phosphate (PO43-) concentrations (Gruber and Sarmiento, 1997) and low
δ15N-NO3- (Knapp et al., 2008) in the upper thermocline of
the North Atlantic are attributed to high regional N2 fixation rates
and have supported the hypothesis that iron availability plays a key role in
regulating the spatial distribution of N2 fixation in the ocean (Moore
et al., 2009; Moore and Doney, 2007) (“δ15N”, where
δ15N ={15N/14Nsample/15N/14Nreference-1}×1000, with atmospheric N2 as the reference).
While the highest inputs of N to the ocean have traditionally been associated
with the North Atlantic, it has also been argued that this association
results from the significant sampling bias in favour of the tropical Atlantic
(Sohm et al., 2011), with large regions of the South Pacific and Indian Ocean
under-sampled with respect to direct N2 fixation rate measurements (Luo
et al., 2012). More recently, the eastern tropical South Pacific (ETSP) has
seen increased sampling due to nutrient distribution-based modelling
predictions that the highest global N2 fixation rates would be found in
surface waters above and adjacent to oxygen-deficient zones (ODZs), where
significant phosphorus (P) would be available to support N2 fixation
(Deutsch et al., 2007). However, field campaigns have found exceedingly low
rates of N2 fixation in the ETSP gyre (Knapp et al., 2016a; Raimbault
and Garcia, 2008; Moutin et al., 2008), which have been attributed to limited
iron availability (Dekaezemacker et al., 2013). Consequently, existing
measurements indicate that the dominant sinks for N in the ocean, benthic and
water column denitrification and anaerobic ammonium oxidation, focused in the
ODZs of the eastern tropical Pacific and Arabian Sea (Gruber and Galloway,
2008), are spatially segregated from the dominant N2 fixation inputs in
the tropical Atlantic. This spatial decoupling of N inputs and outputs
necessarily corresponds to a temporal decoupling, requiring the timescale of
ocean circulation for N2 fixation to respond to changes in rates of
denitrification, and vice versa. In spite of the apparent spatial decoupling
in the modern ocean, paleoceanographic evidence indicates that N fluxes to
and from the ocean have been closely balanced over ≥ 20 kyr, requiring
feedbacks in the N cycle to operate on timescales shorter than ocean
circulation, and thus implying a tighter spatial coupling of N sources and
sinks (Brandes and Devol, 2002; Deutsch et al., 2004). While N loss in the
ocean is constrained to suboxic sediments and water column ODZs, similar
constraints on the location of the largest N2 fixation fluxes to the
ocean are lacking, and thus the degree to which marine N sources and sinks
have been coupled through time remains uncertain.
While prior modelling analyses emphasized the importance of iron or
phosphorus in supporting N2 fixation, the most recent modelling studies
reflect the importance of elevated surface temperatures, adequate iron,
and the potential for low surface ocean
NO3- : PO43- concentration ratios to support a unique
ecological niche for diazotrophs (Dutkiewicz et al., 2012; Monteiro et al.,
2011; Weber and Deutsch, 2014). Attention has consequently shifted to the
relatively undersampled western tropical South Pacific (WTSP) Ocean, where
atmospheric dust fluxes to warm surface waters are higher than in the central
and eastern tropical South Pacific (Mahowald et al., 2009), and where surface
ocean NO3- and PO43- concentrations and ratios are relatively
advantageous for diazotrophs (Moutin et al., 2005; Van Den Broeck et al.,
2004). While seasonally some regions nearer to islands experience
PO43- drawdown to lower levels (e.g. Van Den Broeck et al., 2004;
Moutin et al., 2018), in parts of the WTSP gyre surface ocean NO3-
concentrations are ≤ 0.1 µM and PO43- concentrations
are ∼ 0.05 to 0.2 µM (Garcia et al., 2014), with
corresponding positive P* values (where P*=[PO43-]-[NO3-]/16) (Deutsch et al., 2007). Additionally, early remote
sensing work detected significant and persistent blooms of
Trichodesmium spp. in the WTSP (Dupouy et al., 2000), consistent
with more recent direct observations of elevated Trichodesmium spp.
abundance and N2 fixation rates observed near Melanesian islands (i.e.
New Caledonia, Vanuatu, and Fiji) (Moisander et al., 2010; Shiozaki et al.,
2014; Stenegren et al., 2018; Yoshikawa et al., 2015) and in the Solomon Sea
(Bonnet et al., 2009, 2015; Berthelot et al., 2017). These high
Trichodesmium spp. abundances and N2 fixation rates have been
attributed to sea surface temperatures > 25 ∘C and continuous
nutrient inputs of terrigenous and volcanic origin (Labatut et al., 2014;
Radic et al., 2011). Prior molecular work has also shown higher rates of
N2 fixation in the WTSP at locations where surface ocean dissolved iron
(DFe) concentrations were higher and where Trichodesmium spp. were
less stressed for iron (Chappell et al., 2012). Together, these observations
and modelling-based predictions highlight the potential for significant
N2 fixation rates in regions of the WTSP where diazotrophs can meet
their iron and phosphorus requirements.
Location, subsurface NO3-+ NO2-δ15N,
PNsinkδ15N, and N2 fixation rate and contribution to
export at the OUTPACE long-duration stations.
Average
150 m trap
Subsurface
Latitude
Longitude
PNsink flux
PNsinkδ15N*
NO3-+ NO2-δ15N
% export
N2 fixation rate
Station
(∘ N)
(∘ E)
(µmol N m-2 d-1)
(‰)
(‰)
N2 fixation
(µmol N m-2 d-1)
LD A
-19.22
163.59
303
0.6 ± 1
7.0 to 8.4
80 to 83 ± 13 %
219 to 290
LD B
-18.18
-170.74
30
3.1 ± 1
7.2 to 8.3
50 to 56 ± 12 %
11 to 20
LD C
-18.5
-165.79
47
7.7 ± 1
7.0 to 8.4
0 to 8 ± 11 %
0 to 9
* Flux-weighted mean PNsinkδ15N.
Map of the OUTPACE cruise with “long-duration” (LD) stations A, B,
and C noted (a); water column NO3-+ NO2-
δ15N measurements from the OUTPACE cruise (b); and CTD
fluorescence (green line), NO3-+ NO2- concentration (filled
circles), NO3-+ NO2-δ15N (open circles), and
PNsinkδ15N (filled inverted triangles) from OUTPACE
stations LD A (c), LD B (d), and LD C (e). Error
bars represent 1 standard deviation and are smaller than the symbol size for
NO3-+ NO2- concentration and most NO3-+ NO2-δ15N analyses. The range of NO3-+ NO2-δ15N
endmember values used for δ15N budget calculations are represented
by the shaded regions. The N2 fixation endmember δ15N value,
-1 ‰, is represented by the arrows on the upper x axis.
Here we use
geochemical tools to quantify rates of N2 fixation along a zonal
transect in the WTSP where surface waters are ≥ 25 ∘C and have
favourable macronutrient concentrations and ratios, and where DFe
concentrations are an order of magnitude higher than in the South Pacific
Gyre and are mainly attributable to shallow hydrothermal input (Guieu et al.,
2018). We then compare these geochemical estimates of N2 fixation rates
with other metrics of N2 fixation evaluated on this cruise, as well as
with the global distribution of marine N2 fixation rates.
Methods
Sample collection
Sampling for the Oligotrophic to UlTra-oligotrophic PACific Experiment
(“OUTPACE”) cruise was conducted on the R/V L'Atalante, which left
Nouméa, New Caledonia, on 18 February 2015 and arrived in Papeete, Tahiti, on
2 April 2015. This cruise followed a roughly zonal transect along 18 to
19∘ S between 159∘ E and 160∘ W. Details of the
cruise and experimental design are described comprehensively in Moutin et
al. (2017), but briefly, sediment traps were deployed at three “long-duration” (LD) stations A, B, and C (Table 1) (Fig. 1a). Water column
samples were collected from Niskin bottles deployed on a CTD rosette at both
LD and “short-duration” (SD) stations (Fig. 1a), and water was
stored at -20 ∘C in HDPE bottles for analysis on land.
NO3-+ NO2- concentration and
δ15N measurements
The concentrations of NO3-+ NO2- in water column samples
collected on the OUTPACE cruise were measured by colorimetric methods (Aminot
and Kerouel, 2007). The δ15N of NO3-+ NO2- in
samples collected on the OUTPACE cruise was measured using the denitrifier
method (Casciotti et al., 2002; Sigman et al., 2001), with modifications
(McIlvin and Casciotti, 2011) (Fig. 1b). Typical standard deviation of the
NO3-+ NO2-δ15N analyses was ≤ 0.2‰,
with error bars for individual analyses shown in Fig. 1c.
Sinking particulate N flux and δ15N
measurements
Surface-tethered floating particle-interceptor traps (PPS5) were deployed on
the OUTPACE cruise at 150, 330, and 520 m for ∼ 5 days at stations LD A
and LD B, and at 150 and 330 m at LD C (Moutin et al., 2017). The mass flux
(“PNsink flux”) and δ15N of the PNsink flux
was determined by combustion–gas chromatography interfaced to an isotope ratio mass
spectrometer at the Mediterranean Institute of Oceanography with a lower
detection limit of 2.2 µg N and precision of ±0.3 ‰
for 80 µg samples, with a precision of ±1.0 ‰ for 10
to 20 µg samples typical of what was collected in the sediment
traps at the LD stations.
δ15N budget calculations
Here we compare the δ15N of the two dominant sources of “new” N
to surface waters, subsurface NO3- and N2 fixation, with the
δ15N of the sinking particulate N (PNsink) flux to
estimate the relative importance of both NO3- and N2 fixation as
a source of new N to surface waters. This approach relies on subsurface
NO3- and N2 fixation having distinct isotopic compositions.
N2 fixation introduces new N to the ocean with a δ15N of
∼ -1 ‰ (Carpenter et al., 1997; Hoering and Ford, 1960;
Minagawa and Wada, 1986). In contrast, in the Pacific, NO3- mixed up
from the subsurface is impacted by water column denitrification and can have
a NO3-δ15N > 20 ‰ (e.g. Brandes et al., 1998;
Casciotti et al., 2013; Rafter and Sigman, 2016), although as upper
thermocline waters move westward in the Pacific, the very high NO3-δ15N signal is diluted and typical values are between 5 and
10 ‰ (Lehmann et al., 2018; Rafter et al., 2013). The relative
importance of each source for supporting export production can be determined
using the two-endmember mixing model described in Eq. (1) (“δ15N
budget”), where the fractional importance of N2 fixation for supporting
export production (x) is defined as
PNsinkδ15N=x(-1‰)+(1-x)(NO3-+NO2-δ15N).
Rearranging and solving for x yields
x=NO3-+NO2-δ15N-PNsinkδ15N1+NO3-+NO2-δ15N.
Multiplying the fraction of export production supported by N2 fixation
(x) by the PNsink mass flux provides a time-integrated N2
fixation rate that can be compared with 15N2 incubation-based
N2 fixation rate measurements (Knapp et al., 2016a). Here it is
hypothesized that both rates of N2 fixation and its importance for
fuelling export production will be higher at stations in the western vs.
central and eastern regions of the WTSP because of their closer proximity to
iron sources (Guieu et al., 2018).
Results
NO3-+ NO2- concentration and δ15N, and
PNsinkδ15N
Samples collected in the upper 70 m at the LD stations had
≤ 0.1 µM NO3-+ NO2- (Caffin et al., 2017)
and increased with depth, consistent with prior regional observations (Garcia
et al., 2014) (Fig. 1c). All nutrient concentration data are available at
http://www.obs-vlfr.fr/proof/php/outpace/outpace.php (last access: 10 April 2018).
Water column
NO3-+ NO2-δ15N data (Knapp et al., 2018) are available at
https://www.bco-dmo.org/dataset/733237/data (last access: 11 April 2018) and show similar trends at the LD stations, with 650 m
NO3-+ NO2- δ15N ∼ 7‰, increasing to
∼ 8.5 ‰ at 400 m (Fig. 1b, c) (Knapp et al., 2018), which
fall within the range of previous regional measurements (Yoshikawa et al.,
2015). The elevation of thermocline NO3-+ NO2-δ15N
relative to the mean ocean NO3-+ NO2-δ15N of
5 ‰ is attributed to denitrification and/or anammox occurring in the
ODZs of the ETSP, where thermocline NO3-δ15N can exceed
20 ‰ (e.g. Altabet et al., 2012; Casciotti et al., 2013). The
average, mass-weighted δ15N of the PNsink flux collected
in the 150 m trap increased from the western to eastern stations, from
0.6 ± 1.0 ‰ at LD A, to 3.1 ± 1.0 ‰ at LD B,
and to 7.7 ± 1.0 ‰ at LD C (Table 1) (Fig. 1c).
Results of the δ15N budget: N2 fixation rates and
their contribution to export production
Estimates of N2 fixation rates and their contribution to export
production determined using δ15N budgets include the quantitatively
dominant fluxes of N into and out of the surface ocean. Here, the dominant
fluxes of N into the surface ocean include subsurface NO3- and newly
fixed N introduced from diazotrophs, and the dominant loss term is
represented by the PNsink flux (Eq. 1). In the event that total
dissolved N (TDN) concentrations vary in space/time, they may be included as
well; however, surface ocean TDN concentrations from the OUTPACE cruise show
little to no zonal gradient, and were typically between 5 and 7 µM
in the upper 100 m (Moutin et al., 2018), and so are not included in
δ15N budget calculations. Additionally, the importance of N in
atmospheric deposition has recently received significant attention,
especially in the northwest Pacific (e.g. Kim et al., 2014), raising the
possibility that atmospheric N deposition might also be an important source
of N in the WTSP. However, the atmospheric N deposition flux measured on the
OUTPACE cruise, 0.2 µmol N m-2 d-1 (Caffin et al.,
2017), is several orders of magnitude lower than the mass flux captured in
the 150 m sediment traps, 30–300 µmol N m-2 d-1
(Table 1), indicating that atmospheric N deposition is an insignificant
source of new N to regional surface waters, and so is neglected in our
δ15N budget calculations.
While gradients with depth in subsurface NO3-+ NO2-
δ15N at the OUTPACE LD stations are modest compared to those in the
ETSP, due to the relatively low sampling resolution in the upper thermocline
where NO3- is likely sourced, we calculate δ15N budgets
using a range of NO3-+ NO2-δ15N endmember values,
which are represented by the shaded regions in Fig. 1c. At each LD station,
the NO3-+ NO2-δ15N lower bound is represented by
the 650 m sample and the upper bound is represented by the 400 m sample.
Samples collected shallower than this (i.e. ≤ 200 m) either have
isotopic compositions which fall within this range or show elevation in
NO3-+ NO2-δ15N as the NO3-+ NO2-
concentration decreases, which reflects the effect of NO3-
assimilation. This elevation of shallow NO3- + NO2-δ15N is commonly observed below the euphotic zone in other
oligotrophic regions (Knapp et al., 2016a, 2008), and is not thought to represent the δ15N of
the source NO3-. Using the PNsinkδ15N
(±1 ‰, 1 SD) and the range in subsurface
NO3-+ NO2-δ15N endmember values in Eq. (2)
corresponds to 80 to 83 ± 13 %, 50 to 56 ± 12 %, and 0 to
8 ± 11 % of export production supported by N2 fixation at
stations LD A, LD B, and LD C, respectively (Table 1). Multiplying the
fractional importance of N2 fixation by the PNsink mass flux
yields a range of estimated N2 fixation rates of 219 to 290, 11 to 20,
and 0 to 9 µmol N m-2 d-1 at stations LD A, LD B, and
LD C, respectively (Table 1), where the range includes uncertainty in both
the PNsinkδ15N measurement as well as the
NO3-+ NO2-δ15N endmember.
Discussion
Comparison of δ15N budget results with other N2
fixation metrics from the OUTPACE cruise
The N2 fixation rates derived from the δ15N budgets described
above are lower than those measured by in situ 15N2 incubations at
the same OUTPACE stations, with depth-integrated average N2 fixation
rates of 593 ± 51, 706 ± 302, and
59 ± 16 µmol N m-2 d-1 at LD A, LD B, and LD C,
respectively (Caffin et al., 2017). Previous work has also found lower
δ15N budget-derived N2 fixation rates relative to
15N2 incubation-based N2 fixation rates (Knapp et al., 2016a).
To the extent that sediment traps under collect the export flux, the two
different metrics of N2 fixation may be reconciled by multiplying x
from Eq. (2), the fractional importance of N2 fixation for export
production, by other metrics of new or export production such as
O2 / Ar ratios, 234Th deficits, or 14C uptake rates (Knapp
et al., 2016a). This explanation may reconcile the δ15N budget and
15N2 incubation-based N2 fixation rate estimates at LD A,
which differ by a factor of ∼ 2.5, and potentially the rates at LD C as
well, which, while they differ by a factor ≥ 6, both correspond to
relatively low N2 fixation rates. However, the δ15N budget and
15N2 incubation-based N2 fixation rates observed at LD B, 11
to 20 and 706 µmol N m-2 d-1, respectively, are more
difficult to reconcile based on sediment trap under-collection alone, and may
be partially attributable to variability encountered while sampling at the
end of a phytoplankton bloom as well as the fate of newly fixed N at that
station (Caffin et al., 2018; de Verneil et al., 2017). We note that the
zonal trend in increasing PNsinkδ15N to the east is
similar to a zonal gradient in suspended particulate N (PNsusp)δ15N (Bonnet et al., 2018), suggesting that the δ15N of
the PNsink observed at LD B is consistent with other regional
geochemical data. Additionally, the 15N2 incubation-based N2
fixation rate at LD B has relatively large error bars, resulting from
observations of decreasing in situ N2 fixation rates over the course of
several daily observations at LD B (Caffin et al., 2017), which may also
contribute to the offset between the 15N2 incubation and δ15N budget-based N2 fixation rate estimates. Further, the
PNsink flux collected in the 150 m trap at LD B,
0.030 mmol N m-2 d-1, was somewhat lower than the
PNsink flux collected in the 330 and 520 m traps at the same
station, 0.034 and 0.036 mmol N m-2 d-1, respectively, which is
unexpected given the more typical mass flux attenuation with depth observed
at LD A and LD C, as well as elsewhere in the ocean (Martin et al., 1987).
This unusual trend in mass flux with depth suggests either non-steady-state
sinking flux conditions (Caffin et al., 2018) or a problem with the sediment
trap sample collection at LD B. Regardless, using the 14C-uptake-based
estimate of net N community production
at LD B, 1.91 mmol N m-2 d-1, instead of the PNsink
mass flux to multiply x from Eq. (2) by yields an N2 fixation
rate of 2300 µmol N m-2 d-1. These significant
disparities in productivity metrics and resulting N2 fixation rates at
LD B suggests the potential for temporal decoupling of production and export
and/or the underestimation of the export flux by the sediment trap, and
indicate that N2 fixation rates are probably higher than those resulting
from δ15N budget calculations based on the mass flux to the 150 m
trap at LD B. Regardless, we take the zonal trend in PNsinkδ15N to indicate a decreasing contribution from N2 fixation to
export from the west to the east to be robust as it is consistent with both
the PNsuspδ15N measurements and the broad trends in
15N2 incubation-based N2 fixation rate estimates that decrease
from the west to east.
Comparing the absolute magnitude of the δ15N-budget-based N2
fixation rates with previous measurements, we find that the 219 to
290 µmol N m-2 d-1 rate estimated for LD A represents
a significant N2 fixation rate relative to prior global measurements
(Luo et al., 2012), in particular if it should be revised upwards to account
for the under-collection of the export flux by the sediment trap. In
contrast, the estimated rate range at LD B, 11 to
21 µmol N m-2 d-1, is quite low, as is the range of 0
to 9 µmol N m-2 d-1 at LD C, and both of these rates
are broadly similar to the rates previously measured in the ETSP (Knapp et
al., 2016a; Moutin et al., 2008; Raimbault and Garcia, 2008). Similarly, the
δ15N-budget-based estimate of the contribution of N2 fixation
to export production at LD C is low and similar to previous
δ15N-budget measurements in the North Pacific (Casciotti et al.,
2008) and North Atlantic (Altabet, 1988; Knapp et al., 2005). However, the
fractional contribution of N2 fixation to export production at LD A, 80
to 83 %, is higher than all previous δ15N budget results. The
contribution of N2 fixation to export production at LD B, 50 to
57 %, is also notably high. While the previous δ15N budgets of
Karl et al. (1997) and Dore et al. (2002) found evidence for ∼ 50 %
of export production being supported by N2 fixation near Hawaii, newer methods
capable of measuring the NO3-+ NO2-δ15N at the
lower NO3-+ NO2- concentrations found in the upper
thermocline that represent a more realistic estimate of the endmember
NO3- source suggest that N2 fixation may support closer to
25 % of export during the summer in the North Pacific Gyre (Bottjer et
al., 2017; Casciotti et al., 2008). Consequently, the findings of 50 to
57 % and 80 to 83 % of export production being supported by N2
fixation at stations LD B and LD A, respectively, indicate that N2
fixation plays a significant role in supporting carbon fixation and export
production in this region of the WTSP, consistent with the high e ratios (up
to 9.7) reported by Caffin et al. (2017). Direct export of diazotrophs has
been reported by Caffin et al. (2017), but most export is likely indirect,
i.e. after the transfer of diazotroph-derived N to non-diazotrophic
plankton, which is subsequently exported (Caffin et al., 2018), as has been
observed elsewhere in the WTSP (Bonnet et al., 2016; Knapp et al., 2016b).
Environmental sensitivities of N2 fixation and the
basin-scale coupling of N sources and sinks
The zonal gradient in both N2 fixation rates as well as their
contribution to export production in the OUTPACE study supports emerging
hypotheses regarding the controls on the distribution of marine N2
fixation fluxes in the global ocean. Specifically, the low rates of N2
fixation documented in this study at LD C and in the ETSP (Knapp et al.,
2016a) indicate that low NO3- : PO43- concentration ratios
in the absence of adequate iron (Blain et al., 2008; Fitzsimmons et al.,
2014) are insufficient to support significant fluxes of new N to the ocean.
Instead, the results presented here are consistent with recent modelling work
that has included both the high iron requirements of diazotrophs and the potential for low NO3- : PO43- concentration
ratios to support elevated diazotroph abundance and N2 fixation inputs
to the ocean (Dutkiewicz et al., 2012; Monteiro et al., 2011; Weber and
Deutsch, 2014). Indeed, these new modelling efforts have identified the WTSP
as a unique region where PO43- concentrations are relatively high,
NO3- concentrations are low, and atmospheric dust fluxes provide a
moderate source of iron to warm surface waters, conditions seemingly
favourable for significant N2 fixation fluxes. While regions within the
WTSP nearer to islands experience significant PO43- drawdown, with
seasonal PO43- turnover times comparable to those observed in the
Sargasso Sea (Van Den Broeck et al., 2004; Van Mooy et al., 2009), these
modelling predictions are supported by recent reports of high regional
15N2 incubation-based N2 fixation rates (Bonnet et al., 2017).
However, prior to the OUTPACE cruise, our knowledge of DFe concentrations and
their sources in the WTSP was limited, especially in the western and central
sectors. During OUTPACE, Guieu et al. (2018) reported high DFe concentrations
in the western sector of the WTSP (from 160∘ E to 165∘ W,
average 1.7 nM within the photic layer), i.e. significantly (p<0.05)
higher than those reported in the eastern sector (165–160∘ W,
average 0.3 nM within the photic layer). The high DFe concentrations
measured in the west were previously undocumented and reveal several maxima
(> 50 nM), suggesting significant iron inputs to this region. Guieu et
al. (2018) found that atmospheric deposition in this region was too low to
explain the observed DFe concentrations in the water column, and that the
iron in the euphotic layer may instead derive from shallow (∼ 500 m)
hydrothermal sources associated with the Tonga–Kermadec subduction zone.
Recent studies performed in the western end of the WTSP in the Solomon,
Bismarck (Berthelot et al., 2017; Bonnet et al., 2009, 2015), and Arafura
(Messer et al., 2015; Montoya et al., 2004) seas also reveal extremely high
N2 fixation rates (> 600 µmol N m-2 d-1),
indicating that high N2 fixation rates have been found over a
significant region of the WTSP, extending west to east from Australia to
Tonga and north to south from the Equator to 25 to 30∘ S, or
∼ 13×106 km2 (i.e. ∼ 20 % of the South
Pacific Ocean area). These significant N inputs may offset the N loss
occurring in the ODZs of the eastern tropical Pacific. The ability for marine
N inputs and outputs to compensate for each other within the same ocean basin
corresponds to a spatial and thus temporal coupling on the scale of years to
decades, consistent with the paleoceanographic record (Brandes and Devol,
2002; Deutsch et al., 2004; Weber and Deutsch, 2014), and represents an
intermediate view of the distribution of global marine N2 fixation
fluxes consistent with that proposed by Weber and Deutsch (2014), where iron
availability controls local N2 fixation rates but phosphorus
availability regulates basin-scale N2 fixation rates (Moutin et al.,
2008, 2018).
Conclusions
The goal of this study was to address the question: do regions other than the
tropical Atlantic contribute significantly to global N2 fixation fluxes?
While our results should be taken as a “snapshot” view that cannot
necessarily be scaled up to annual fluxes, at stations proximal to iron
sources, geochemically derived N2 fixation rates of 219 to
290 µmol N m-2 d-1 were observed, and could
potentially represent a lower bound of N2 fixation rates due to the
potential under-collection of the PNsink flux by sediment traps.
Moreover, at stations LD A and LD B, separated by ∼ 27∘
longitude, N2 fixation was found to support > 50 % of export
production, a finding that has not been replicated elsewhere with sensitive
NO3-+ NO2-δ15N methods to our knowledge. Together
with similar findings from 15N2 uptake experiments, these results
suggests that N2 fixation can support a significant fraction of export
production over a large region of the WTSP. At the eastern station most
distant from iron sources, both rates and the contribution of N2
fixation to export production were low, ∼ 0 to
9 µmol N m-2 d-1 and 0 to 8 %, respectively,
similar to previous measurements in the ETSP where diazotrophs may also be
challenged by iron availability (Dekaezemacker et al., 2013; Knapp et al.,
2016a; Moutin et al., 2008). Significant N2 fixation fluxes in the WTSP
may provide a means of balancing N loss occurring in the ODZs of the eastern
tropical Pacific, and thus may help reconcile the paleoceanographic record
requiring N inputs and losses to balance each other on timescales shorter
than ocean circulation (Dutkiewicz et al., 2012; Monteiro et al., 2011; Weber
and Deutsch, 2014).