Introduction
Map of the KEOPS2 study area. The locations of the stations are
marked by colored dots. Blue indicates the stations of the north–south
transect (TNS), green indicates the stations of the east–west transect
(TEW), orange indicates the stations E located in the meander of the polar
front (zoom panel). Red shows other stations located in the fertilized region
and black shows the station located in the high-nitrate low-chlorophyll
(HNLC) region. Detailed positions of the stations are given in Table S1.
The first scientific expeditions in the Southern Ocean discovered high
concentrations of major nutrients such as nitrate (NO3-) and
phosphate (PO43-) in surface waters south of 50∘ S (Hart,
1942). The general meridional overturning circulation that brings deep water
to the surface at the southern limits of the Antarctic Circumpolar Current
(Marshall and Speer, 2012) is the major mechanism supplying surface waters
with NO3- and PO43-. Most of the nutrient-rich upwelled
waters are transported northward and they leave the surface north of the
polar front through their transformation into intermediate and mode waters.
Despite the several-month-long northward transport during which the
NO3-- and PO43--rich waters are exposed to sunlight, not much
phytoplankton biomass develops. This system was characterized as high-nitrate
low-chlorophyll (HNLC). The major consequence of the HNLC status of the
Southern Ocean is that large amounts of unused nutrients are transported back
into the ocean interior where they feed the main thermocline and finally
supply low- and mid-latitude surface waters with essential nutrients
(Sarmiento et al., 2004). Another consequence is that similarly to
NO3- and PO43-, large amounts of upwelled dissolved inorganic
carbon (DIC) are not converted to particulate organic carbon (POC) and remain
in contact with the atmosphere for time periods long enough to degas carbon
dioxide (CO2) with important consequences for climate (Sigman and Boyle,
2000).
The iron hypothesis (Martin and Fitzwater, 1988) was a major advancement for
our understanding of the HNLC paradox. More than 2 decades of intense
research have confirmed that increasing iron supply stimulates primary
production, major nutrient utilization and the air-to-sea flux of CO2 in
surface waters. Nutrient utilization in surface waters is therefore a
diagnostic of the efficiency of the biological pump of CO2. Nitrate
utilization has also received much attention in paleoceanographic studies,
because it can be inferred from the isotopic composition of N in bulk
material or specific compounds of fossil organisms preserved in the sediment.
Recent results provide support to the enhanced NO3- utilization
related to higher dust deposition during the ice ages in the sub-Antarctic
region (Martinez-Garcia et al., 2014).
Early modeling studies on the iron hypothesis were conducted using models
that did not explicitly represent the iron cycle. The effect of iron
fertilization was mimicked using the extreme assumption that iron
fertilization results in the complete depletion of N or P in surface waters
(Gnanadesikan et al., 2003; Sarmiento and Orr, 1991). However, this was never
observed during artificial iron fertilization (Boyd et al., 2007), iron
addition during deck incubations (Moore et al., 2007) or in naturally
iron-fertilized regions (Blain et al., 2007). For most previous research in
this context, it was assumed that NO3- and PO43- behave in a
similar way. This is only true at first glance as interesting differences
have been noticed (Jenkins et al., 1984; Minas and Minas, 1992; Lourey and
Trull, 2001). Weber and Deutsch (2010) used zonal mean distributions of
NO3- and PO43- in the Southern Ocean to reveal that the
differential utilization of both nutrients is likely related to the
composition of the phytoplankton community. Detailed investigations of blooms
in varying regions of the Southern Ocean confirm different utilization of
NO3- and PO43- depending on the dominant species in the
phytoplankton community (Arrigo, 1999; De Baar et al., 1997; Moore et al.,
2007). In addition, the possible role of dissolved organic nitrogen (DON) and
dissolved organic phosphorus (DOP) for N and P decoupling has not been
investigated, although modeling studies suggest that these organic forms may
significantly contribute to the cycling of N and P in the Southern Ocean
(Wang et al., 2003).
Our work presents new data on dissolved inorganic and organic nitrogen and
phosphorus concentrations from the iron-fertilized regions near the Kerguelen
archipelago. We present their spatial and temporal distributions, and discuss
their stoichiometry.
Material and methods
Sampling
During KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2), the samples
were collected at the stations presented on the map in Fig. 1. The
coordinates and date of sampling are summarized in Table S1. Additional
samples were collected during the cruise KEOPSMOOR (Kerguelen Ocean and
Plateau compared Study, Moorings) in February 2013 at stations A3 and at
station TNS-6 (Table S1). The samples for dissolved nitrogen and phosphorus
analyses were collected with twenty-two 12 L Niskin bottles mounted on a
rosette equipped with a Seabird SBE911-plus CTD (conductivity, temperature
and depth) unit. In this work, we used potential temperature (θ) and
density anomaly (σ) to characterize the hydrology of the stations. A
more complete description of the hydrology and the circulation is presented
in Park et al. (2014).
For NO3-, PO43- and nitrite (NO2-), syringes (50 mL)
were directly connected to the spigot of the Niskin bottles. The samples were
drawn through a 0.45 µm Uptidisc (Whatman) adapted for the syringe.
Duplicate samples were collected. The second sample (25 mL) was poisoned
with mercuric chloride (HgCl2, 20 mg L-1, final concentration)
and stored in the dark at room temperature for later analysis.
For ammonium (NH4+), samples were collected from Niskin bottles in
two 50 mL Schott glass bottles. Following rinsing, the bottles were filled
with 40 mL of seawater and closed immediately to avoid contamination by air.
Back at the onboard laboratory, the oxidative reagent (Holmes et al., 1999) was
added. Samples for NH4+ determination were incubated for at least
3 h in the dark, at ambient temperature, before fluorescence measurements
(λexc=370, λemi=460 nm) with a
fluorimeter (Jabsco).
For dissolved organic nitrogen (DON) and phosphorus (DOP) analysis the
samples were collected from Niskin bottles in 100 mL Schott glass bottles.
The Schott glass bottles were rinsed with HCl (10 %) once and several times
with ultrapure water (prepared by deionization and UV sterilization) between
casts. The samples were then filtered through two combusted GF/F filters.
20 mL of the filtered samples were transferred to 20 mL PTFE vials and
poisoned with 100 µl of HgCl2 (4 g L-1, working
solution) before storage at 4 ∘C. All analyses were performed aboard
as described below.
Analytical methods
For NO3-, NO2-, PO43-, one sample was immediately
analyzed aboard with a segmented flow analyzer (Skalar) equipped with
colorimetric detection using methods described in (Aminot and Kérouel,
2007). The accuracy of the methods was assessed using reference material
(Certipur, Merck). The precisions were in the range of 1–4 %, and the limit
of detection was 0.02 µM for NO3-and NO2-, and
0.03 µM for PO43-.
Samples for DON and DOP determination were spiked with 2.5 mL of the
oxidative reagent (boric acid + sodium hydroxide + potassium
peroxodisulfate), and then heated at 120 ∘C for 30 min. After
cooling, the concentrations of NO3- and PO43- were determined
as mentioned above. This provides the concentrations of total dissolved
nitrogen (TDN) and total dissolved phosphorus (TDP). The concentrations of
DON and DOP were calculated as follows;
DON = TDN-[NO3-]-[NO2-] and
DOP = TDP-[PO43-].
Results
Most of the stations are located south of the polar front (PF), with the
exception of the coastal stations TEW-1–2 and the offshore stations
TNS-1–2, TEW-7–8 and F-L, which were located north of the PF (Fig. 1).
Station R-2, located west of the plateau, had low chlorophyll concentrations
in surface water throughout the season (∼ 0.3 mg m-3) (Lasbleiz
et al., 2014), an observation that is explained by the low iron supply
(Quéroué et al., 2015). In contrast, all other stations were
characterized by the development of large spring blooms consistent with
higher iron supply (Lasbleiz et al., 2014). However, the development of the
blooms within the iron-fertilized region was not homogenous in time and
space. A3-1 and stations TNS-1 to TNS-10 of the north–south transect,
sampled at the beginning of the spring bloom, were characterized by low
chlorophyll concentrations only slightly higher than that at station R-2.
Stations TEW-1 to TEW-8 of the east–west transect, stations E-2 to E-5, and
station A3-2 (the second visit to station A3), were sampled a few days later,
when the bloom rapidly developed with large spatial heterogeneity. The
largest stocks of chlorophyll a within the 0–200 m layer were observed at
stations F-L, north of the PF, and at station A3-2, above the plateau. Based
on the trajectories of two surface drifters (Zhou et al., 2014), stations
E-1, E-2, E-3, E4-E and E-5 are assumed to evolve in a quasi-Lagrangian
framework, and their succession in time can be considered approximately as a
time series.
Vertical sections of dissolved nitrogen and phosphorus species along
the East-West transect (TEW). (a) nitrate, (b) ammonium,
(c) phosphate, (d) nitrite, (e) dissolved organic
nitrogen, (f) dissolved organic phosphorus. The isolines for density
anomaly (σ) are plotted on each panel.
Two-dimensional distributions of dissolved nitrogen and
phosphorus.
In the upper 200 m of the water column, concentrations of NO3- and
PO43- were ≥ 19 and ≥ 1 µM, respectively
(Figs. 2 and 3). Concentrations were higher west of the PF (transect E–W,
Fig. 2) and south of the PF (transect N–S, Fig. 3) and lower in surface
sub-Antarctic waters, north and east of the PF. Concentrations of NO2-
were the highest above 150 m, and below this depth NO2- decreased
rapidly to reach values close to the detection limit at 200 m. Above
150 m, NO2- concentrations were clearly higher at the stations in
the polar front zone (PFZ) (NO2- in the range 0.3–0.4 µM)
than at those in the Antarctic zone (AZ) (NO2- of 0.25 µM).
Along the transect E–W, the highest NO2- concentrations were measured
at TEW-1 (0.31–0.34 µM). Contrasting with the NO2-
distribution observed along the transect N–S, the stations of the AZ (i.e.,
west of the isocline σ = 27) had higher concentrations than those of
the PFZ. NH4+concentrations were highest at the coastal stations. At
stations TEW-1, concentrations of NH4+ increased from
0.19 µM (at 10 m) to 1.45 µM (close to the bottom). The
same trend was observed at TEW-2 (0.17 µM at 10 m and
0.39 µM close to the bottom). At all stations offshore and above
the plateau, a subsurface maximum of NH4+ peaking at
0.5–0.6 µM was observed between 50 and 150 m. The DON
distribution was characterized by a north–south gradient in the 0–150 m
layer. DON concentrations above the Kerguelen plateau at stations A3-1 and
TNS-10 (6.0 ± 1.0 µM) were similar to those in the meander of
the PF 6.4 ± 1.7 µM (stations TNS-3 to TNS-7). But higher
values were detected north of the PF (8.6 ± 1.2 µM for
stations TNS-1 and TNS-2).
Vertical sections of dissolved nitrogen and phosphorus species along
the North-South transect (TNS). (a) nitrate, (b) ammonium,
(c) phosphate, (d) nitrite, (e) dissolved organic
nitrogen, (f) dissolved organic phosphorus. The isolines for density
anomaly (σ) are plotted on each panel.
Dissolved nitrogen speciation at station A3-1 (a, b) and at
station A3-2 (c, d) during KEOPS2. Depth profiles of temperature and
σ–θ are plotted on each panel.
For DOP, the latitudinal gradient was less pronounced, but DOP
concentrations were lower above the Kerguelen plateau than at any other
sites.
Speciation of dissolved nitrogen at selected sites
The Kerguelen plateau station A3
The vertical distribution of different chemical nitrogen species during the
two visits at station A3 are detailed in Fig. 4. NO3- distributions
are discussed in more detail in Sect. 3.3. Concentrations of NO2-
were, during both visits, homogeneous in the mixed layer and revealed a small
maximum below the mixed layer depth (MLD). NO2- increased from
0.27 µM at A3-1 to 0.33 µM at A3-2 (Fig. 4b).
NH4+ concentrations roughly doubled between the two visits
(0.1 µM at A3-1 to 0.2 µM at A3-2) and clear maxima were
detectable at the base of the mixed layer. Concentrations of DON did not
change between visits; however, DON accounted for 20 % of TDN in the mixed
layer at A3-1, and this contribution increased to 25 % in the upper 40 m
water layer at A3-2 (data not shown). Both NO3- consumption and DON
release during the 4 weeks that separated the two visits explained the
increase in the percent DON of TDN. Below 200 m, TDN was higher at A3-1 than
at A3-2. This was mainly driven by the differences in DON concentrations that
were higher at A3-1 (4.7–6.7 µM) than at A3-2
(1.8–4 µM) in the 250–300 m layer (Fig. 4).
Stations F-S and F-L north of the polar front
Distinct vertical profiles of NO2- and NH4+ were observed at
station F-S. Concentrations of NO2- decreased from 0.39 µM
at 10 m to 0.22 µM at 93 m. However, we note a remarkably low
value of 0.15 µM at 79 m (Fig. 5a). The NH4+ profile
presented the same anomaly, resulting in two subsurface maxima. This feature
contrasts with most other stations where a single subsurface maximum was
observed, as for example at station F-L (Fig. 5b) located a few nautical
miles away from F-S. We suggest that this anomaly is due to the position of
F-S within the polar front where a complex mixing event at a small scale could
have occurred. The contribution of DON to TDN at F-S decreased continuously
from 34 % at 20 m to 9 % at 120 m. However, close to the surface,
the contribution of DON was only 17 % (Fig. 5d).
Dissolved nitrogen speciation at stations F-L and F-S during
KEOPS2.
Dissolved nitrogen speciation at station R-2.
The HNLC station R-2
The vertical distribution of NO3- and DON revealed small variations
between the surface and 200 m (Fig. 6a). DON accounted for 19 to
24 % of TDN, representing intermediate values as compared to the range
observed in the fertilized region. Concentrations of NO2- and
NH4+ presented similar vertical distributions, decreasing rapidly
below the mixed layer (Fig. 6b). Concentrations of NH4+ in the mixed
layer (0.07 µM) were at least two-fold lower than at any other
stations, and NO2- concentrations in the mixed layer
(0.3 µM) were similar to those of the mixed layers in the
fertilized regions.
The Lagrangian sites E
All stations were characterized by similar vertical distributions of
NO2- and NH4+. Concentrations in the mixed layer were in the
range 0.25–0.3 µM decreasing to 0.02-0.03 µM below
200 m. The vertical distributions of NH4+ are characterized by a
subsurface maximum with concentrations (0.4–0.65 µM) two-fold
higher than at the surface (0.2–0.3 µM). NO3-
distributions are described in more detail in the next section. The
contribution of DON to TDN in the mixed layer was in the range of 15–25 %.
No clear temporal evolution was detectable.
Temporal evolution of the vertical distributions of nitrate and
phosphate
The Lagrangian sites E
The vertical profiles of NO3- and PO43- concentrations in the
upper 200 m of five stations located in the center of a meander of the PF are
presented in Fig. 7. In addition, we show data from two other cruises.
Samples collected in early October 1995 during the cruise ANTARES3 (Blain et
al. 2001) provided data typical of winter conditions. Samples of the
KEOPSMOOR profile were collected in February 2013, representing post-bloom
conditions.
Concentrations of NO3- were almost identical among visits at 150 m
(mean 27.5 ± 0.8 µmol L-1, Fig. 7a). Above 150 m,
NO3- concentrations change along the season. In winter,
concentrations were homogenous from the surface to 150 m, resulting in a mean
integrated stock of 4.22 ± 0.08 mol m-2. In spring, the KEOPS2
profiles qualitatively clustered into two groups. The first cluster is composed
of stations TNS-5, TNS-6, E-1, E-2 and E-3 with higher NO3-
concentrations (mean integrated stock 0–150 m of
4.10 ± 0.05 mol m-2) than in the group formed by stations E4-E
and E-5 (mean integrated stock 0–150 m of
3.90 ± 0.04 mol m-2). Finally, the lowest concentrations were
measured in summer (mean integrated stock 0–150 m of 3.48 mol m-2).
Vertical profiles of PO43- presented similar characteristics as
NO3-, with the exception of the winter profile (Fig. 7b). The winter
profile indicates that PO43- concentrations are homogenously mixed in
the upper 150 m. The concentrations seem overestimated at 150 m and above.
We do not think that the differences result from interannual variability
because this would have also impacted NO3- concentrations. The high
concentrations of PO43- measured in winter 1995 lead to a
NO3- : PO43- ratio of 12.5 which is low. The overestimation
of PO43- could result from methodological issues. The ANTARES3
samples were not analyzed aboard, but a few months later in a laboratory by a
different analytical protocol. The lack of certified international standards
necessary for strong quality control of the accuracy precludes rigorous
comparison of samples collected in 1995 with more recent samples.
Similarly to NO3-, we consider the mean concentration of
PO43- at 150 m (excluding the ANTARES3 PO43- value) to
estimate a mean winter PO43-concentration in the surface layer of
1.93 ± 0.09 µmol L-1 that yields an integrated winter
stock of 0.30 ± 0.02 mol m-2. The integrated stock for the group
of stations E-1–E-3 (0.280 ± 0.004 mol m-2) was higher than for
the group E-4–E-5 (0.274 ± 0.005 mol m-2). At the end of the
season the integrated PO43- stock was 0.250 mol m-2.
The Kerguelen plateau station A3
At station A3, vertical profiles of changes of NO3- and PO43-
concentrations were observed between spring and summer (Fig. 8). Although the
stations were sampled in November 2011 and February 2013, we consider these
variations as seasonal changes. The profiles of both nutrients merge at
200 m in early spring and summer (A3-1 and A3-2). However, during the second
visit at A3 (A3-2), we observed that the surface layer was mixed down to 170
m. We propose that the concentrations at 200 m are a good estimate of the
winter concentrations of NO3- and PO43- at this station.
Thus, winter stocks (0–200 m) were 6.27 and 0.43 mol m-2 for
NO3- and PO43-, respectively. At the first visit at station
A3 the stocks had decreased to 5.96 and 0.41 mol m-2. Four weeks later
(A3-2) they reached 5.29 and 0.36 mol m-2. Finally, in February the
stocks were 4.77 and 0.35 mol m-2.
Temporal variability of the vertical profiles of concentrations of
NO3- (a) and PO43- (b) for stations located
in the meander of the polar front. Details for profiles of KEOPSMOOR
(February 2013) and ANTARES3 (October 1995) are provided in the text.
Temporal variability of the vertical profiles of concentrations of
NO3- (a) and PO43- (b) at the station A3.
(a) Comparison of concentrations of NO3- versus
PO43-. Dots denote the samples, and lines show different values of
N* = NO3--rN:P PO43-. (b)
Comparison of concentrations of TDN versus TDP, dots denote the samples and
lines show different values of TNxs = TDN -rN:P TDP.
Depth profiles of N* at stations A3 (a) and TNS-6
(b) for the month of November (in red) and February (in blue).
Vertical profiles of density anomaly are shown with the same color.
Discussion
The distributions of NO3- and PO43- in the world's
oceans have been extensively studied over the past decades. A major rationale for
this research is the critical role of these major nutrients for
phytoplankton growth and therefore marine primary production. Further,
concentrations of NO3- and PO43- are used as tracers for
biogeochemical processes in the ocean (Deutsch and Weber, 2012). In the
Southern Ocean, south of the sub-Antarctic front, NO3- and
PO43- concentrations are high. They are therefore considered as
non-limiting and much less attention has been paid to their distributions as
compared to other nutrients such as silicic acid or dissolved iron. However,
the relaxation of iron limitation by natural or artificial fertilizations offers
a different perspective because NO3- and PO43- should be
consumed as the bloom develops. This has motivated the present detailed
study of dissolved N and P in the naturally fertilized region of Kerguelen.
To explore the dynamics of NO3- and PO43- we examined their
stoichiometry in the study region. This is commonly done by establishing the
ratio rN:P=[NO3-] : [PO43-] for comparison
with the Redfield ratio of 16 (Redfield et al., 1963). However, the
significance of rN:P is limited because this ratio is not
conserved by mixing or biological processes such as uptake or
remineralization (Deutsch and Weber, 2012). We therefore calculated the
linear combination N* = [NO3-]-16 [PO43-], similar to
the parameter first introduced by Michaels et al., 1996, but omitting the
constant term required to obtain a global average of N* equal to 0. N*
traces the impact of processes that add or remove N and P with a
stoichiometry different from the Redfield ratio of 16. At almost all stations
and depths, N* was close to -3 µM (Fig. 9a). This value agrees
well with the mean N* computed for regions of the Southern Ocean close to
the PF (Weber and Deutsch, 2010). A noticeable deviation from this value was
observed for a set of data where N* increased from
N* = -3 µM to N* = 6 µM. All data with
N*>0 are for samples collected in the mixed layer north of the PF, and
located in a bloom where diatoms contributed 70 % of the carbon biomass
in the euphotic layer (Lasbleiz et al., 2014).
Nutrient drawdown lower than the Redfield ratio has been observed previously
in the Southern Ocean. During the artificial iron fertilization experiment
EIFEX, an apparent differential consumption of Δ(NO3-) : Δ(PO43-) of 6.4 was reported (Smetacek et
al., 2012). Arrigo (1999) and De Baar et al. (1997) determined a nutrient
drawdown ratio in diatom blooms of 9.7 and 4.4–6.1, respectively. Near the
Crozet Islands, the removal of NO3- versus PO43- measured in
situ and during iron-addition experiments revealed that the ratio was
inversely related to the proportion of diatoms in the phytoplankton community
(Moore et al., 2007). All these studies confirm the impact of diatom blooms
on nutrient stoichiometry in the surface layer. However, the interpretations
of these observations are diverse. De Baar et al. (1997) suggested that the
preferential drawdown of PO43- during the bloom of
Fragilariopsis kerguelensis in the PF could be due to the dominance
of Fragilariopsis kerguelensis because of the physiological trait of
low N : P ratio. These hypotheses could not explain our observations
because the stations with a nutrient drawdown anomaly were located in an
iron-fertilized region and the diatom community was not dominated by
Fragilariopsis kerguelensis but rather by Chaetoceros (Hylochaete) spp., Pseudo-nitzschia spp. and Centric sp.
(Lasbleiz et al., 2014).
Thus, we interpret the positive values of N* as a result of the
preferential uptake of PO43- versus NO3- by fast-growing
diatoms. Diatoms have a mean elemental N : P stoichiometry of 10 ± 4
(Sarthou et al., 2005), which differs from the Redfield value. Indeed, the
elemental particulate matter composition determined at the stations with
positive N* during KEOPS2 (Lasbleiz et al., 2014) exhibits a mean ratio of
particulate organic nitrogen to particulate organic phosphorus (PON : POP)
of 10.5 ± 3.3 which is consistent with the observed nutrient drawdown
Δ(NO3-) : Δ(PO43-) of 8. We suggest that
the preferential allocation of resources to the P-rich assembly of the cell
machinery by exponentially growing cells is the most likely explanation for
our observations (Klausmeier et al., 2004). The anomaly observed for the
present data set is not linked to a particular species but to general traits
of the diatom community responding to iron fertilization.
As a variant of N* , the tracer DINxs, takes into account
NO2- and NH4+ (Hansell et al., 2007), but none of those
tracers consider the organic pools of N and P. Landolfi et al. (2008) have
defined the tracer TNxs= [TDN]-16[TDP] and have shown that the
dissolved organic fraction significantly contributes to changes in
TNxs. For example, relying on N* only, can lead to an
underestimation of N2 fixation at the global scale (Landolfi et al.,
2008). In the case of KEOPS2, the contribution of DON and DOP to TDN and TDP
reached 30 %. We have therefore considered TDN and TDP at all KEOPS2
stations where these measurements were available (Fig. 9b). Plotting TDN as a
function of TDP (TDN = f(TDP)) reveals more dispersion of the data than
NO3-= f(PO43-), mainly due to the lower analytical
precision for DOP and DON determinations. Still, clear trends are detectable.
TNxs values were negative for most stations and depths, and
relatively constant in the 0–500 m layer. As for N* the stations north
of the PF had higher TNxs in the 0–100 m layer.
When a water parcel is considered, N* is affected by the redistribution of
N and P between the inorganic and the organic pools, whereas TNxs
is only affected by net non-Redfield sources or sinks of N and P.
Consequently, the positive anomaly observed for TNxs in surface
waters north of the PF can be explained by three possible mechanisms:
deposition of N-rich material from the atmosphere, N2 fixation and
export of P-rich material. The region of Kerguelen receives low quantities of
atmospheric material (Heimburger et al., 2012; Wagener et al., 2008) which
is mainly of natural origin, such as desert dust, which contains little
nitrogen compared to phosphorus (Zamora et al., 2013). This is confirmed by
the low N deposition rate estimated around the Crozet Islands
(2 nmol m-2 d-1; Planquette et al., 2007). We can therefore
refute the deposition of N-rich material as the cause of the TNxs
anomaly. The second hypothesis involves N2 fixation. To date, N2
fixation has not been reported to occur in the cold waters of the Southern Ocean.
However, during KEOPS2 detectable N2 fixation rates were measured at
different stations with a few exceptionally high values
(∼ 250 µmol m-2 d-1) in the mixed layer of station
F-L (Gonzàlez et al., 2015). Such high fixation rates could
contribute to an enrichment of about 1 % of TDN that is not enough to
create the observed anomaly. If N2 fixation was a dominant process
driving the N : P stoichiometry at this station, particulate organic matter (POM) elemental composition
should also be affected. Generally, N2 fixing microorganisms have a high
N : P ratio (Laroche and Breitbarth, 2005). Such high ratios are at odds
with the low N : P measured in the POM at station F-L (Lasbleiz et al.,
2014). The third hypothesis for explaining the anomaly relies on the export
of P-rich material from the mixed layer. We do not have direct measurements
of N : P in the exported material, but we have already mentioned above that the
elemental composition of particulate matter at station F-L yielded the lowest
N : P ratio in POM (Lasbleiz et al., 2014). This provides support for the
export of P-rich material resulting in high TNxs values north
of the PF. We propose that the anomaly of TNxs results from the
imprint on stoichiometry of the diatom bloom which consumed and exported
phosphorus with a N : P ratio below the Redfield value.
During KEOPS2, rapidly growing diatom blooms were also sampled at other
stations located south of the PF, but anomalies similar to those at F-L were
not observed. We discuss here the case of stations A3 and E-4W, which had
similar chlorophyll concentrations as F-L. Station A3 had a contribution of
diatoms to carbon biomass and dominant diatom species similar to F-L (these
observations are not available for E4-W) (Lasbleiz et al., 2014). There is no
reason that the physiological features of exponentially growing diatoms as
revealed for station F-L do not apply to the diatoms growing at stations A3
and E4-W. It is, however, possible that the resulting effect is not large
enough to translate into N* or TNxs anomalies. A possible
explanation could be the differences in the age of the blooms. The
stoichiometry would be less affected in a younger bloom as compared to a
bloom of longer duration. This hypothesis cannot be fully verified due to the
poor temporal resolution of the satellite ocean color images available (see
supplementary animations provided in Trull et al., 2014). Another or
complementary explanation is the difference in the mixed layer depths that
were 50 and 150 m at stations F-L and A3-2, respectively. Such a deep
mixed layer as observed at station A3-2 likely resulted from a deep episodic
mixing event generated by strong winds prevailing during the day preceding our
visit. The deepening could have dampened the anomaly by diluting and mixing
the affected water parcel with underlying water having a typical
stoichiometry (e.g., N* or TNxs around -3).
In February 2013, 2 years after the KEOPS2 cruise, we were able to return to
two sites visited during KEOPS2 (stations A3 and TNS-6) and obtain
measurements for the concentrations of NO3- and PO43-. These
data, in combination with KEOPS2 data, allowed us to compare N* during two
different seasons (Fig. 10). In the mixed layer, little change in N* was
observed between spring and summer. However, in summer, N* exhibited a
clear subsurface minimum between 100–200 m at both stations.
Denitrification is a process that could produce this subsurface feature. But
denitrification would require low oxygen concentrations that are not observed
at these stations. In a general manner, preferential remineralization of P
versus N in the water column is supported by an increase of N : P in high
molecular dissolved organic matter (Clark, et al., 1998) in particulate
matter (Copin-Montegut and Copin-Montegut, 1978) and in supernatant of
sediment trap material (Lourey et al., 2003). The observation of the N*
subsurface minimum at the end of the season, but not at the beginning implies
a temporal cumulative effect. The minimum is located just below the mixed
layer in the region of the pycnocline that presents the highest density
gradient. This could represent a zone with a higher residence time for
sinking particles resulting in an accumulation of biomass. Additional
evidence for intensive remineralization at shallow depths at this location is
provided by the strong attenuation of the particles fluxes as observed with
moored sediment trap (Rembauville et al., 2015). Consequently, the
remineralization would also be increased in this layer compared to the rest
of the water column resulting in a higher accumulation of PO43-
relative to NO3-. This effect might be amplified by the occurrence of
particulate organic matter with a low N : P ratio resulting from diatom
accumulation at the pycnocline. As the subsurface minimum is located above
200 m depth, it is erased when winter mixing occurs.
To our knowledge, such a subsurface minimum has not be reported in the
Southern Ocean. This could be due to the limited studies that investigate
concurrently dissolved N and P biogeochemistry, and to the lack of
samples collected at the appropriate vertical and temporal time scale. Our
finding raises several further questions. Is the subsurface minimum of N*
a particular feature of iron-fertilized regions? What is the link between its
occurrence and the strength of stratification of the water column? And what
is the role of this layer in the remineralization of carbon? These questions
argue for future detailed investigation of the cycling of both elements in
the upper layer of the Southern Ocean.