Dissolved Organic Nitrogen Production and Export by Meridional Overturning in the Eastern Subpolar North Atlantic

Abstract

Dissolved organic matter (DOM) is produced in the surface and exported towards the deep ocean, adding ∼2 PgC yr−1 to the global carbon export. Due to its central role in the Meridional Overturning Circulation (MOC), the eastern subpolar North Atlantic (eSPNA) contributes largely to this export. Here we quantify the transport and budget of dissolved organic nitrogen (DON) in the eSPNA, in a box delimited by the OVIDE 2002 section and the Greenland‐Iceland‐Scotland sills. The MOC exports >15.9 TgN yr−1 of DON downwards and, contrary to the extended view that these are materials of subtropical origin, up to 33% of the vertical flux derives from a net local DON production of 7.1 ± 2.6 TgN yr−1. The low C:N molar ratio of DOM production (7.4 ± 4.1) and the relatively short‐transit times in the eSPNA (3 ± 1 yr) suggest that local biogeochemical transformations result in the injection of fresh bioavailable DOM to the deep ocean.

Full text

Confidential manuscript submitted to Geophysical Research Letters
Dissolved organic nitrogen production and export by1
meridional overturning in the eastern subpolar North2
Atlantic3
Bieito Fern´andez-Castro1,2, Marta ´
Alvarez3, Mar Nieto-Cid3, Patricia Zunino4,4
Herl´e Mercier4, Xos´e Ant´on ´
Alvarez-Salgado1
5
1Departamento de Oceanograf´ıa, Instituto de Investigaci´ons Mari˜nas (IIM-CSIC), 36208, Vigo, Spain.6
2Physics of Aquatic Systems Laboratory, Margaretha Kamprad Chair, Ecole Polytechnique F´ed´erale de7
Lausanne, Institute of Environmental Engineering, Lausanne, Switzerland8
3Centro Oceanogr´afico de A Coru˜na, Instituto Espa˜nol de Oceanograf´ıa, 15001, A Coru˜na, Spain9
4Laboratoire d’Oc´eanographie Physique et Spatiale, UMR 6523 CNRS/Ifremer/IRD/UBO, Ifremer Centre10
de Bretagne, Plouzan´e, 29280, France11
Key Points:12
•The eastern subpolar North Atlantic is a source of dissolved organic nitrogen (DON)13
•Up to one third of the DON exported by the overturning circulation is produced14
locally15
•Full-depth integrated net DON production roughly balances net nitrate uptake16
Corresponding author: Bieito Fern´andez-Castro, bieito.fernandezcastro@epfl.ch
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Confidential manuscript submitted to Geophysical Research Letters
Abstract17
Dissolved organic matter (DOM) is produced in the surface and exported towards the18
deep ocean, adding ∼2 PgC yr−1to the global carbon export. Due to its central role19
in the Meridional Overturning Circulation (MOC), the eastern subpolar North Atlantic20
(eSPNA) contributes largely to this export. Here we quantify the transport and bud-21
get of dissolved organic nitrogen (DON) in the eSPNA, in a box delimited by the OVIDE22
2002 section and the Greenland-Iceland-Scotland sills. The MOC exports >15.9 TgN23
yr−1of DON downwards and, contrary to the extended view that these are materials24
of subtropical origin, up to 33% of the vertical flux derives from a net local DON pro-25
duction of 7.1±2.6 TgN yr−1. The low C:N molar ratio of DOM production (7.4±4.1)26
and the relatively short-transit times in the eSPNA (3 ±1 yr) suggest that local bio-27
geochemical transformations result in the injection of fresh bioavailable DOM to the deep28
ocean.29
1 Introduction30
Dissolved organic matter (DOM) represents one of the largest reservoirs of reduced31
carbon on Earth (662 PgC) [Hansell et al., 2009]. The production of DOM occurs pri-32
marily in the euphotic zone as a result of phytoplankton photosynthesis and subsequent33
food web interactions [Carlson, 2002]. While labile DOM is remineralized within the source34
region, recalcitrant DOM (including the semi-labile, semi-refractory and refractory frac-35
tions; Hansell [2013]) escapes rapid degradation and can be transported horizontally by36
ocean currents [Letscher et al., 2013; Torres-Vald´es et al., 2009], or exported downwards37
by convergence in subtropical regions [Hansell et al., 2012], winter-convection [Carlson38
et al., 1994] and overturning circulation [Carlson et al., 2010; Fontela et al., 2016]. Glob-39
ally, 1.9 PgC yr−1of recalcitrant DOM are exported to the deep ocean [Hansell et al.,40
2009], contributing 15−38% to the global carbon export (5-12 Pg C yr−1) [Henson et al.,41
2011], and fuelling heterotrophic respiration in the dark ocean [Carlson et al., 2010; Hansell42
et al., 2012].43
The subpolar North Atlantic (SPNA) is a key region for the Meridional Overturn-44
ing Circulation (MOC) [Daniault et al., 2016]. The North Atlantic Current (NAC) car-45
ries warm and saline thermocline waters of subtropical origin into the SPNA gyre along46
its southeastern rim, constituting the northward flowing upper limb of the MOC in the47
region (Fig. 1a). Subtropical waters progressively gain density through air-sea exchange48
and are transformed into subpolar mode waters (SPMW) [Brambilla and Talley, 2008;49
Garc´ıa-Ib´a˜nez et al., 2015], whose densest variety is the Irminger SPMW (IrSPMW) [Krauss,50
1995; Garc´ıa-Ib´a˜nez et al., 2015]. Deep convection in the Irminger [Pickart et al., 2003;51
Piron et al., 2017] and Labrador Seas [Yashayaev et al., 2007] leads to the formation of52
the Labrador Sea Water (LSW) [McCartney and Talley, 1982]. These water masses, to-53
gether with the Denmark Strait Overflow Water (DSOW) and Iceland-Scotland-Overflow54
Water (ISOW), which enter the eastern SPNA from the Arctic and Nordic seas through55
the Denmark and Iceland-Scotland straits [Macrander et al., 2005; Nilsson et al., 2008;56
Jochumsen et al., 2017], respectively, compose the southward flowing lower limb of the57
MOC.58
The northward flowing DOM-rich waters of subtropical origin sink in the SPNA59
and return southward into the deep North Atlantic brought by the MOC [Hansell et al.,60
2009; Fontela et al., 2016]. Subsequent to this export, removal of the semi-refractory frac-61
tion of this DOM has been documented in the deep North Atlantic Ocean [Carlson et al.,62
2010; Hansell et al., 2012; Fontela et al., 2016]. However, these studies did not discuss63
the role of the SPNA as a source or sink of DOM [Carlson et al., 2010; Hansell et al.,64
2012], or they considered that the DOM imported from the thermocline waters of sub-65
tropical origin does not suffer significant biogeochemical transformations in the SPNA66
and it is only passively transported to the deep ocean by the MOC [Fontela et al., 2016].67
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Confidential manuscript submitted to Geophysical Research Letters
More recently, a diagnostic modelling study suggested instead that the production and68
export of DOM to the deep ocean are tightly coupled in the high-latitude North Atlantic69
Ocean [Roshan and DeVries, 2017]. Furthermore, all these studies have focused on dis-70
solved organic carbon (DOC), while it has been shown that the transport and reminer-71
alization of dissolved organic nitrogen (DON) is central to understand the biogeochem-72
ical nutrient budgets, both in low-latitude and polar regions [Torres-Vald´es et al., 2009;73
Letscher et al., 2013; Torres-Vald´es et al., 2016; Vidal et al., 2018].74
Here, we evaluate the transports and net budget of DON in the eastern SPNA (eS-75
PNA) to investigate the net production of DON along the transit through the subpo-76
lar gyre and its subsequent export by the lower limb of the MOC. In order to assess the77
biogeochemical relevance of the DON transports and budgets, equivalent calculations were78
performed for DOC and nitrate.79
2 Methods80
2.1 Nutrient budgets in the eastern SPNA81
The budgets of DON, DOC, and nitrate in the eSPNA were determined within a82
box delimited by the Greenland-Portugal OVIDE 2002 section to the southwest and the83
Greenland-Iceland-Scotland sills (GIS sills) to the northeast. The box has a surface area84
of 3.74 ×1012 m2and a volume of 7.96 ×1015 m3. The rate of change of an organic or85
inorganic nutrient N(∂N/∂t) in the eSPNA box is the sum of net transport across the86
OVIDE 2002 section (TOv.,0
N, where Ov. stands for the OVIDE 2002 section and the 087
superscript is to specify that volume-conserving transports are used, see below) and the88
GIS sills (TSills.,0
N), the input from rivers (Frivers
N) and atmospheric deposition (Fat.dep.
N)89
and a rate of net biological production (JBG
N):90
∂N
∂t =TOv.,0
N−TSills,0
N+Frivers
N+Fat.dep.
N+JBG
N(1)
As oceanic volume transports are defined positive to the north, a minus sign was added91
for the transports across the GIS sills (northward fluxes are out of the box). The net pro-92
duction rate (JBG
N) was diagnosed from the balance of oceanic, riverine and atmospheric93
fluxes by assuming steady state (∂N/∂t = 0). The nutrient transports across the south-94
ern boundary of the eSPNA were calculated by combining volume transports derived from95
an inverse model applied to hydrographic measurements [Lherminier et al., 2007] and96
nutrient concentration measurements [ ´
Alvarez-Salgado et al., 2013] along the OVIDE 200297
section. Transports across the northern boundary, Greenland-Iceland-Scotland (GIS) sills,98
and the atmospheric and riverine inputs were evaluated using data from the literature99
and public databases.100
2.2 Nutrient transports across the OVIDE 2002 section101
The cruise OVIDE 2002 was conducted from 19 June to 11 July 2002, on board102
R/V Thalassa. Ninety-one full-depth hydrographic stations were occupied, from the con-103
tinental shelf off Greenland to Lisbon. Nitrate profiles from Niskin bottle data were ob-104
tained at every station (maximum 30 pressure levels). DON and DOC data were deter-105
mined at 30 stations and selected depths (maximum 15 levels). The analytical determi-106
nation error was ±0.1
µ
mol kg−1,±0.32
µ
mol kg−1and ±0.7
µ
mol kg−1for nitrate, DON107
and DOC. Nitrate concentrations have been adjusted by applying a multiplicative fac-108
tor of 0.96 according to the GLODAPv2 adjustment table (https://glodapv2.geomar.109
de/). A detailed description of the instruments and calibrations associated with the phys-110
ical and chemical parameters are presented elsewhere [Lherminier et al., 2007; ´
Alvarez-111
Salgado et al., 2013].112
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Confidential manuscript submitted to Geophysical Research Letters
The absolute transport of an organic or inorganic nutrient Nacross the OVIDE113
2002 section can be computed as:114
TN=
Portugal
Z
Greenland
surface
Z
bottom
ρ(x, z)N(x, z)V(x, z)dx dz (2)
where Vis the velocity orthogonal to the section, and ρis the in situ density. Transports115
were defined positive to the north. The velocity field was calculated by combining geostrophic116
currents and acoustic Doppler current profiles in an inverse generalized least-squares method.117
The specifications of the method for the OVIDE 2002 cruise and the description of cur-118
rent field are detailed elsewhere [Lherminier et al., 2007]. The volume transports were119
estimated at the mid-distance between two stations with a vertical resolution of 1 dbar,120
and nutrient concentrations were obtained at each sampling point (i.e. bottle depth) for121
each hydrographic station. In order to match the grid of both fields, the nutrient fields122
were linearly interpolated at each 1 dbar and averaged in station pairs. The DOC and123
DON samples, collected in 1 out of 3 stations, were linearly interpolated in the horizon-124
tal coordinate to each station position prior to vertical interpolation. The cross-section125
velocity, nitrate and DON fields are shown in Fig. S1.126
In order to diagnose net biogeochemical rates from the nutrient budgets in the eS-127
PNA box, mass conservation must be ensured in the boxed region. To meet this require-128
ment, nutrient transports were decomposed into fluxes associated with the net volume129
transport across the section (TOv.,Net
N) and the volume-conserving fluxes (TOv.,0
N) [Zunino130
et al., 2014], TOv.
N=TOv.,Net
N+TOv.,0
N, by splitting the cross-section velocity (V) into131
two components,132
V(x, z) = V+v(x, z) (3)
where ¯
Vis the section-averaged velocity corresponding to the net transport across the133
section:134
TOv.,Net
Vol =¯
VZ Z dx dz (4)
With this, the volume-conserving nutrient fluxes were calculated as the product of the135
nutrient concentrations and the velocity anomalies along the section:136
TOv.,0
N=
Portugal
Z
Greenland
surface
Z
bottom
ρ(x, z)N(x, z)v(x, z)dx dz (5)
The net volume transport across the OVIDE 2002 was small (TOv.,Net
Vol =−0.03±137
2.85 Sv) and southward. The nutrient fluxes associated with this net volume transport,138
calculated as TOv.,Net
N=ρN T Ov.,Net
Vol , where ρN is the section-averaged volumetric nu-139
trient concentration, were −0.5±51.5 kmol s−1for nitrate, −0.1±9.8 kmol s−1for DON,140
and −1.4±141 kmol s−1for DOC.141
For display purposes, the transports were also divided in the contributions of the142
upper and lower limbs of the MOC, separated by the σ1= 32.15 kg m−3potential den-143
sity level (referenced to 1000 m). Errors in the transports were calculated as the stan-144
dard deviation of 1000 realizations generated by random perturbations of the velocity145
and nutrient fields based on the covariance matrix of the reference velocities [Lherminier146
et al., 2007] and the nutrient measurement errors [ ´
Alvarez-Salgado et al., 2013].147
2.3 Transports across the Greendland-Iceland-Scotland sills148
The nutrient transports across the GIS sills were calculated as the product of the149
volume transport of the water masses present at the GIS sills (n= 7) and an averaged150
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Confidential manuscript submitted to Geophysical Research Letters
nutrient concentration for the corresponding water mass (Ni):151
TSills
N=
7
X
i=1
Ti
VolρiNi(6)
Volume transports at the GIS sills were available from literature [Jeansson et al., 2011;152
Østerhus et al., 2005; Hansen et al., 2008] (Table S1). Briefly, 0.8±0.16 Sv of Atlantic153
Water (AW) and 1.5±0.5 Sv of Polar Water (PW) flow northward and southward [Nils-154
son et al., 2008; Østerhus et al., 2005], respectively, in the upper density levels of the Den-155
mark Strait (DS), while −3.4±0.4 Sv of dense Denmark Strait overflow water (DSOW)156
flow southward [Macrander et al., 2005]. In the Iceland-Faroe Ridge (IFR), 3.8±0.5157
Sv of AW flow northward [Østerhus et al., 2005] and −1.0±0.5 Sv of Iceland Strait over-158
flow water (ISOW) flow southward [Hansen et al., 2008]. Finally, 3.8±0.5 Sv of AW159
cross the Faroe-Shetland channel (FSC) to the north [Østerhus et al., 2005], and −2.1±160
0.3 Sv of ISOW enter the subpolar north Atlantic [Hansen et al., 2008].161
Similarly to the OVIDE 2002 section, the net nutrient transports were decomposed162
into the fluxes associated with the net transport of volume across the Sills (TSills,Net
Vol =163
0.4±1.2 Sv) and the volume-conserving flux. In order to calculate the latter, the vol-164
ume transports by each water mass with no net associated volume transport across the165
GIS sills were calculated as Ti,0
Vol =Ti
Vol−TSills,Net
Vol /7. Volume-conserving fluxes, which166
differ only slightly from the total fluxes in Table S1, are used throughout the manuscript.167
Thus, the flux of a nutrient corresponding to a net zero volume transport was:168
TSills,0
N=
7
X
i=1
Ti,0
VolρiNi(7)
Nitrate concentrations at the sills were obtained from the World Ocean Atlas 2013169
(WOA 2013, https://www.nodc.noaa.gov/OC5/woa13/), DOC concentrations were avail-170
able in Jeansson et al. [2011]. DON concentrations in the AW, DSOW and ISOW were171
obtained by extrapolating the mean DON in these water masses (i) along the OVIDE172
2002 section [ ´
Alvarez-Salgado et al., 2013], taking into account the removal of DON from173
the sills to the section:174
DONi,Sills = DONOv.+ (1/rCN)(DOCi,Sills −DOCOv.) (8)
where rCN is the C:N molar ratio of DON removal in the eSPNA, rCN = 6.9±0.6 for175
basin-scale remineralization [ ´
Alvarez-Salgado et al., 2013]. The correspondence between176
the water masses in the OVIDE 2002 section and the GIS sills and the calculated DON177
concentrations are given in Table S1. The Polar Water was not intercepted in the OVIDE178
2002 section. In order to obtain its DON concentration, the DOC value given by [Jeans-179
son et al., 2011] was divided by 14, a typical C:N ratio for bulk concentrations of DOM180
in upper and thermocline waters in the area [ ´
Alvarez-Salgado et al., 2013]. To the best181
of our knowledge, the only available measurements of DON in the area of the GIS sills182
were carried out in the East Greenland Current across the Denmark strait [Torres-Vald´es183
et al., 2016]. Our extrapolated DON values in the Denmark Strait were 4.83 and 4.88184
µ
mol kg−1for AW and PW, respectively, and 4.05
µ
mol kg−1for the DSOW (Table S1),185
in good agreement with Torres-Vald´es et al. [2016], who reported DON concentrations186
close to 5
µ
mol kg−1in the upper 100-200 m and fairly constant and slightly lower than187
4
µ
mol kg−1below.188
3 Results and Discussion189
3.1 Volume transports in the eastern SPNA190
The horizontal stream-function of the depth-integrated volume transport illustrates191
the main features of the circulation across the OVIDE 2002 section (Fig. 2a) described192
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Confidential manuscript submitted to Geophysical Research Letters
in detail elsewhere [Lherminier et al., 2007; Daniault et al., 2016]. The basin-scale trans-193
port in the eSPNA was constituted by the northward flow of the NAC located at ∼1600194
km from Greenland in the eastern rim of the gyre, and by the southward flow of the East195
Greenland Current (EGC), in the western rim of the gyre along the Greenland slope. The196
intensity of both currents was ∼30 Sv. The NAC flow carried 25.6±1.4 Sv of waters197
corresponding to the density levels of the upper MOC limb (defined by the σ1>32.15198
kg m−3isopycnal [Lherminier et al., 2007]), and a lower amount of lower limb waters (6.2199
Sv). A fraction of the upper limb waters (9.8 Sv) circulated anticyclonically in the east-200
ern end of the section (Fig. 2a), while most of the flux fed the cyclonic subpolar gyre201
circulation. Part of it (∼8.8 Sv) contoured anticyclonically the Reikjanes Ridge with202
the Iceland current (IC), and the rest flowed across the Reikjanes Ridge (Fig. 1a). The203
Irminger Sea presented a cyclonic circulation. The eastern rim of this circulation was con-204
stituted by a northward flow with similar contributions of the IC and recirculation from205
the Labrador Sea [Paillet et al., 1998; Garc´ıa-Ib´a˜nez et al., 2015]. Finally, the subpo-206
lar gyre was closed by the southward flow of the EGC, carrying 3.6 Sv in the upper limb207
and 26.5 Sv in the lower limb of the MOC. Most of this flux corresponds with the Irminger208
SPMW [Garc´ıa-Ib´a˜nez et al., 2015], the final product of the transformation of subtrop-209
ical into subpolar mode waters along the eastern North Atlantic subpolar gyre, before210
entering the Labrador Sea.211
Overall, 16.6±1.1 Sv and 5.3±4.0 Sv of upper and lower limb waters flowed into212
the eSPNA through the Western Iberian basin (WEB), while 0.7±1.4 Sv and 21.4±213
4.1 Sv, respectively, left the eSPNA through the Irminger and Iceland Basins (Fig. 1a).214
The transport across the OVIDE section was 16 Sv northward with the upper limb and215
southward with the lower limb of the MOC, respectively. At the GIS sills, 6.7±0.6 Sv216
of upper limb waters continued northward, while approximately the same volume of dense217
overflow waters (3.5±0.4 Sv of DSOW; and 3.3±0.5 Sv of ISOW) entered the eSPNA218
from the Nordic seas. Thus, the volume transport in the lower limb of the MOC increased219
from 6.7 Sv at the GIS sills to 16 Sv at the OVIDE section, implying a net formation220
of dense lower limb waters (i.e. overturning) [Garc´ıa-Ib´a˜nez et al., 2015] of 9.3±1.3 Sv221
inside the eSPNA box (Fig. 1a).222
3.2 DON transports across the OVIDE 2002 section223
The transport of DON across the OVIDE 2002 section was calculated as the prod-224
uct of the volume transports and the DON concentrations (see Methods). As the net vol-225
ume transport across the section has been previously subtracted in order to ensure vol-226
ume conservation inside the box, the net transport is solely dependent on the spatial cor-227
relation between volume transports and DON concentrations. The section-mean DON228
concentrations were higher in the upper (4.1±0.5
µ
mol kg−1), than in the lower limb229
of the MOC (3.2±0.4
µ
mol kg−1) (Fig. 2b). The large-scale variability patterns along230
the section revealed little deviations from the section-mean except in the western Irminger231
Sea where strong positive anomalies were found in the upper and lower limbs.232
The net northward DON transport in the WEB included 58±4 kmol s−1and 19±233
14 kmol s−1in the upper and lower limbs of the MOC, respectively (Fig. 1b). The net234
southward lower limb flux in the Irminger and Iceland basins was 82±14 kmol s−1, dom-235
inated by the EGC, carrying DON-enriched waters with a transport-weighted mean con-236
centration of 3.9±0.1
µ
mol kg−1, significantly higher than the section mean. Although237
smaller (7±6 kmol s−1), the southward flux of DON in the upper limb was also dispro-238
portionately large compared to the southward volume transport of 0.7 Sv. Due to this239
relative DON-enrichment in the western limit of the section, particularly in the Irminger240
Sea, the southward transport in the Irminger Sea and Iceland basins exceeded the north-241
ward transport in the WEB, and the net DON flux across the OVIDE section was south-242
ward at a rate of 12 ±3 kmol s−1.243
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Confidential manuscript submitted to Geophysical Research Letters
3.3 DON budget in the SPNA: net production and export by the MOC244
The DON budget in the eSPNA was calculated as the balance of inputs minus out-245
puts from the ocean, the rivers and the atmosphere. The oceanic inputs minus outputs246
include the DON transports across the OVIDE 2002 section and the GIS sills (see Meth-247
ods). At the GIS sills, the DSOW and ISOW transported 14 ±4 kmol s−1and 13 ±3248
kmol s−1of DON into the eSPNA, respectively, while the net upper limb flow was of 32±249
5 kmol s−1towards the Nordic Seas (Fig. 1b). The resulting net DON flux through the250
northern boundary of the eSPNA was of 5±5 kmol s−1northward. The net balance of251
DON transports across the OVIDE 2002 section and the GIS sills resulted in a net ex-252
port of 17±6 kmol s−1from the eSPNA. Furthermore, according to the global estimates253
of atmospheric nitrogen deposition [Jickells et al., 2017], 0.17−0.25 kmol s−1(central254
value: 0.21 kmol s−1) of DON are deposited annually into the eSPNA box. European,255
Iceland and Greenland rivers discharge 0.51−2.89 kmol s−1of inorganic nitrogen to the256
eSPNA box [Sharples et al., 2016], while globally, the DON discharge by rivers repre-257
sents 57% of the inorganic nitrogen flux [Kroeze et al., 2012]. With this information, the258
riverine DON supply to the eSPNA is in the range 0.29−1.65 kmol s−1(central value:259
∼1.0 kmol s−1). Considering the DON fluxes across the OVIDE 2002 section, the GIS260
sills, the atmosphere and the continents, we computed a net DON export of 16±6 kmol s−1
261
from the eSPNA. This net export requires an equivalent net DON production of 16±262
6 kmol s−1(7.1±2.6 TgN yr−1), in order to keep the steady state inside the box (Eq.263
1).264
Our transport calculations reveal that 70−75% of the DON produced in the eS-265
PNA (5.3±1.3 TgN yr−1) is exported southward across the OVIDE 2002 section, car-266
ried by the southward-flowing lower limb waters of the EGC. The remaining smaller frac-267
tion (2.3±1.3 TgN yr−1) is transported northward to the Nordic seas. DON is supplied268
to the eSPNA mainly by the waters of the upper limb of the MOC. In addition, local269
production of DON takes place in the surface layer of the eSPNA. Consequently, the DON270
transported to and produced in the upper limb of the MOC has to be transferred to the271
lower limb, that is, there should be an injection of DON from the surface to the deep ocean272
led by the overturning circulation. The DON vertical export associated with the MOC,273
calculated as the difference between the outputs (82 ±14 kmol s−1) and inputs (46 ±274
13 kmol s−1) to the eSPNA by lower limbs waters, was at least 36±16 kmol s−1(15.9±275
7.1 TgN yr−1) (Fig. 1b). This figure represents a lower estimate because the possible rem-276
ineralization within lower limb waters of the eSPNA was unresolved. Thus, up to one277
third (5.3±1.3 TgN yr−1) of the DON exported to the deep ocean (>15.9±7.1 TgN yr−1)278
by the MOC is locally produced in the eSPNA.279
3.4 Stoichiometry and lability of the produced DOM280
The DON leaving the eSPNA with the EGC subsequently flows with the Western281
Greenland Current into the Labrador Sea where it joins the Deep Western Boundary Cur-282
rent [Cuny et al., 2002], being exported to the deep North Atlantic where it can contribute283
to remineralization processes in the ocean interior at lower latitudes [Carlson et al., 2010;284
Fontela et al., 2016]. The bioavailability of the exported DOM determines its decay time-285
scale and its potential to fuel heterotrophic activity downstream of the source region.286
Here we compute the stoichiometry of the produced dissolved organic materials and an287
upper-end estimate of their age as proxies of their lability.288
The C:N ratio provides an indirect measure of bioavailability as fresh DOM has rel-289
atively low C:N ratios (∼10) and aged DOM is progressively enriched in carbon, with290
respect to nitrogen [Hopkinson and Vallino, 2005]. In order to obtain the C:N ratio of291
the DOM transported and cycled in the eSPNA, the DOC transports and budget were292
calculated following the same methodology used for DON (Fig. S2). The C:N ratios of293
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Confidential manuscript submitted to Geophysical Research Letters
the transported and produced DOM were calculated as the ratio of the DOC and DON294
fluxes and net production rates.295
The DOM imported into the eSPNA with the NAC had a C:N molar ratio of 15.4±296
0.5, while the DOM exported from the Irminger Sea with the EGC and through the Iceland-297
Scotland ridge was characterized by lower C:N ratios, ∼14.6 and 12 ±2, respectively,298
indicating that the bulk C:N ratio of DOM entering the eSPNA with the NAC is reduced299
within this region (Fig. 1b). This result is partly achieved by the supply of DOM with300
relatively low C:N (13.6−14.3) ratios from the overflows. Furthermore, according to301
our DOC budget, 111 ±45 kmolC s−1were produced in the eSPNA, leading to a C:N302
molar ratio of 7.4±4.1 for the locally-produced DOM (Fig. 1b). This ratio was only303
slightly higher than the Redfield proportion of recently produced particulate organic mat-304
ter (6.7), and even slightly lower than the characteristic C:N ratios of bioavailable DOM305
(∼10) [Hopkinson and Vallino , 2005]. This result suggests that, together with the im-306
port of DOM with relatively low C:N from the overflows, local production of fresh DON307
(with low C:N) reduces the C:N ratios of DOM in the eSPNA, before being exported away308
from the region, preferentially towards the deep ocean with the MOC.309
The DON budget and the C:N ratios of organic matter production could be sen-310
sitive to the choice of the C:N ratio used to calculate DON concentrations at the GIS311
sills (rCN ). In order to rule out this possibility, an alternative calculation of the DON312
fluxes at the sills was performed with an upper estimate rCN of 13±2 [Hopkinson and313
Vallino , 2005]. In this test the net DON flux across the sills was 3 ±3 kmol s−1, and314
a net DON budget in the eSPNA, −14±4 kmol s−1. Both figures were very close to the315
values obtained with the initial proportion of rCN = 6.9±0.6 (5±5 kmol s−1and −16±316
6 kmol s−1, respectively; see Fig. 1b). The alternative C:N ratio of DOM production (7.9±317
4.1) was also practically unchanged. Therefore, the results are not significantly altered318
by the choice of the C:N ratio of the DOM in the GIS sills.319
To further support the coherence of a low C:N ratio as indicator of DON lability,320
we estimated a transit time of 3 ±1 years for a water parcel contouring the subpolar321
gyre in the eSPNA using a collection of Argo float trajectories (http://www.argo-france.322
fr/fr/home/) (Fig. S3). This transit time sets the upper limit for the age of the DON323
produced in the eSPNA at <3 years. This age is only slightly larger than the life-time324
of semi-labile DOM (∼1.5 years), but significantly shorter than the life-time of semi-325
refractory DOM (∼20 years) [Hansell , 2013]. The transit time is thus too short for the326
locally-produced semi-refractory DON to fully decay before being exported. Hence, the327
exported DON is at least semi-refractory and can contain a potentially significant frac-328
tion of (semi-)labile materials, as suggested by the low C:N ratio. Thus the exported DOM329
has the potential to fuel heterotrophic processes in mid-latitudes of the deep North At-330
lantic the time-scales of months to decades. This is consistent with several previous stud-331
ies reporting important remineralization of DOM in the deep North Atlantic ocean be-332
tween polar and tropical latitudes [Carlson et al., 2010; Hansell et al., 2012; Fontela et al.,333
2016]. In particular, Hansell et al. [2012] found the largest global rates of DOC removal334
from the deep ocean in the North Atlantic south of Greenland at ∼55 ◦N, in the vicin-335
ity of the Labrador Sea and downstream of the EGC. It is a common view that the ma-336
terials remineralized in the deep ocean at these latitudes are predominantly originated337
in the subtropical oceans and subduct with the MOC in the Labrador and Irminger Seas338
[Hansell et al., 2009]. Our results suggest instead that relatively bioavailable DOM pro-339
duced in the eSPNA could significantly contribute to explain the high rates of DOM rem-340
ineralization in the North Atlantic.341
3.5 The contribution of DON to the nitrogen budget in the eSPNA342
In this section we compare the DON and the nitrate budgets in the region. To do343
so, the nitrate transports and budget were calculated following the same methodology344
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Confidential manuscript submitted to Geophysical Research Letters
as for DON. The nitrate transport in the upper limb of the MOC through the eastern345
part of the OVIDE 2002 section (the Western Iberian Basin) was 229±15 kmol s−1north-346
ward (Fig. 1c), dominated by the NAC (Fig. 2c), while the lower limb flux was 109±347
75 kmol s−1northward. The nitrate flux in the western part of the gyre (Iceland plus348
Irminger basins) was 345±71 kmol s−1southward in the lower limb and non-significant349
in the upper limb (4±20 kmol s−1northward). The net nitrate transport across the OVIDE350
2002 section was 3 ±8 kmol s−1southward.351
The DSOW and ISOW transported 45 ±5 kmol s−1and 46 ±5 kmol s−1of ni-352
trate southward, respectively, into the eSPNA through the GIS sills, while the net north-353
ward flow of waters corresponding to the upper MOC limb carried 73±13 kmol s−1into354
the Nordic Seas (Fig. 1c). This resulted in a net southward transport of 18±15 kmol s−1.355
Combining the fluxes across the OVIDE 2002 section and the sills, the net oceanic trans-356
port was 15 ±14 kmol s−1into the eSPNA box. According to the global estimates of357
atmospheric nitrogen deposition [Jickells et al., 2017], 0.02−0.03 gN m−2yr−1and 0.06−358
0.09 gNm−2yr−1are deposited annually in our study region in the form of ammonium359
and nitrate, respectively, adding up to a total flux of inorganic nitrogen of 0.68 −1.0360
kmol s−1(central value: 0.8 kmol s−1). The riverine supply of inorganic nitrogen was 0.51−361
2.89 kmol s−1(central value: 1.7 kmol s−1) [Sharples et al., 2016]. The riverine and at-362
mospheric supplies (amounting 1.2−3.9 kmol s−1in total) were added to the net ni-363
trate input by the ocean circulation, summing up 17±15 kmol s−1(7.5±6.6 TgN yr−1).364
The net nitrate input into the eSPNA must be balanced by an equivalent net bi-365
ological consumption (Fig. 1c), reinforcing the observation of a net full-depth autotrophic366
balance in the eSPNA, in good agreement with previous studies in the area [Maz´e et al.,367
2012; Zunino et al., 2015]. Our budgets also show that the water-column integrated rates368
of net uptake of nitrate (7.5±6.6 TgN yr−1) and net production of DON (7.1±2.6 TgN yr−1)369
are roughly equivalent within error bars, such that the net nitrate consumption could370
be counterbalanced by net DON production. Torres-Vald´es et al. [2013] computed a net371
deficit of nitrate in the Arctic Ocean and hypothesized that the nitrogen budget could372
be closed by a net import of DON. We found an opposite situation in the eSPNA, where373
our budget calculations indicate that the net nitrate import could end up as net DON374
production and export out of the region. Although subsequent studies failed to demon-375
strate the hypothesis of Torres-Vald´es et al. [2013] for the Arctic Ocean [Torres-Vald´es376
et al., 2016], our results reveal that DON production and export could represent a first377
order contribution to the annual nitrogen budget in the eSPNA. Despite previous spec-378
ulation and attempts [eg. ´
Alvarez et al., 2002; Torres-Vald´es et al., 2016], to the best of379
our knowledge, this is the first time that in situ data is reported showing a distinct cen-380
tral contribution of DON to the nitrogen budget of an ocean basin.381
Previous studies addressing the role of dissolved organic matter in the biological382
carbon pump quantified either the contribution of DOC to net organic carbon export383
from the photic layer [Romera-Castillo et al., 2016; Roshan and DeVries, 2017] or to oxy-384
gen consumption in the dark ocean [Ar´ıstegui et al., 2002; Carlson et al., 2010; Fontela385
et al., 2016]. All these studies found a consistent contribution of 17-40% of DOC to the386
carbon export from the surface or the respiration in the deep ocean. The budgets pre-387
sented here differ from previous studies because they are integrated through the whole388
water column of the eSPNA and, therefore, do not capture the vertical export of ma-389
terials from the euphotic zone but the net balance between production in the photic layer390
and remineralization in the deep ocean. If this balance is positive for net organic mat-391
ter production, as it is the case, the fate of these materials is either burial in the sed-392
iments or/and horizontal export to other ocean regions. Despite sinking particles rep-393
resenting the major contribution to the export of organic matter from the euphotic zone394
in the eSPNA [Martin et al., 2011; Giering et al., 2014], sinking particulate material suf-395
fers remineralization in the meso- and bathypelagic waters before reaching the sediments396
[Alkire et al., 2012; Lemaitre et al., 2018]. As a consequence, the burial of organic nitro-397
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Confidential manuscript submitted to Geophysical Research Letters
gen in the sediments of the open ocean (0.20 kmol s−1) [Brunneg˚ard et al., 2004] and shelves398
(1.2 kmol s−1) [Wollast , 1993] is one order of magnitude lower than the net nitrate up-399
take computed here, and the organic nitrogen budget is then dominated by lateral fluxes400
of DON. Recent studies show that the transport of suspended material can also be im-401
portant for the export of organic matter in different ocean basins [Baker et al., 2017].402
Due to the large error bars of our nitrate and DON budgets we cannot discount the pos-403
sibility that the lateral flux of suspended particulate nitrogen could also play a role in404
our study area.405
The budgets presented here rely on mass transports and nutrient concentrations406
that were derived from climatological data in GIS sills, but from a single-cruise realiza-407
tion in the OVIDE section. One must consider to which extent the biogeochemical rates408
are representative of the mean state of the eSPNA, as both currents and nutrient dis-409
tributions can suffer significant interannual fluctuations [Maz´e et al., 2012; Mercier et al.,410
2013]. For the period 1997-2010, the mean MOC across the OVIDE section was 16 Sv411
with an interannual variation of ±1 Sv (SD of 6 realizations), and the mean amplitude412
of EGC and NAC was ∼30 Sv. During the OVIDE 2002 cruise, the amplitude of the413
MOC and these currents were very close to the climatological values of Mercier et al.414
[2013] and, thus, representative of the mean state of the system. The combined impact415
of the interannual variability of circulation and nutrient concentrations was assessed in416
Maz´e et al. [2012]. Those authors computed the nitrate, phosphate and oxygen budgets417
using data from 3 different OVIDE occupations (2002, 2004, 2006). They reported a mean418
nitrate consumption of −7.8±6.5 kmol s−1and −8.4±6.6 kmol s−1in the Irminger419
Sea and the Iceland + Western European basin, respectively, so that the total nitrate420
uptake was -16.2 kmols−1, in very good agreement with our estimate for 2002 (−17±421
15 kmol s−1). The authors also estimated the amplitude of the interannual variations of422
the nitrate flux across the OVIDE section at 7 kmol s−1, not distinguishable from the423
mean state given the error bars. These evidences suggest that the budgets computed with424
the OVIDE 2002 cruise are representative of the mean state of the eSPNA in the early425
21st century, at least within the reported error bars.426
4 Conclusions427
The eSPNA has long been recognized as a key region for vertical export of dissolved428
organic matter into the deep ocean by the Meriodional Overturning Circulation. The429
DON and nitrate budgets presented here show that this region does not only act as a430
passive belt transporting DOM originated in the subtropics, but it also behaves as a net431
autotrophic system, with a net production of DON at the expense of net nitrate uptake.432
The lower-limb waters enriched with relatively bioavailable DOM originated in the eS-433
PNA are subsequently transported southward towards the Labrador Sea and the tem-434
perate North Atlantic, with potential impacts for the basin-scale microbial remineral-435
ization and biogeochemistry. The eSPNA is sensitive to natural climate variability and436
human-induced global change affecting both gyre-scale and overturning circulation [Yashayaev437
et al., 2015; Smeed et al., 2018] and primary production [Boyce et al., 2010; Zhang et al.,438
2018], which are intimately linked in this region [Zhang et al., 2018; Johnson et al., 2013].439
Understanding these interactions, and particularly the role of organic nutrients, is key440
to predict the evolution of the nitrogen cycle in the North Atlantic in the forthcoming441
decades.442
Acknowledgments443
B. F.-C. is supported by a Juan de La Cierva Formaci´on fellowship (FJCI-641 2015-25712,444
Ministerio de Econom´ıa y Competitividad, Spanish Goverment). The OVIDE project445
was funded by the Institut Fran¸cais de Recherche pour l’Exploitation de la Mer (IFRE-446
MER), the Centre National de la Recherche Scientifique (CNRS), the Institut National447
des Sciences de l’Universe (INSU), and the French National Program Les Enveloppes Flu-448
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Confidential manuscript submitted to Geophysical Research Letters
ides et l’Environnement (LEFE). Additional funding comes from the Spanish Govern-449
ment project REN2001-4965-E to X. A. ´
A.-S. Hydrographic CTD-O2 data collected dur-450
ing the OVIDE 2002 cruises are available online at http://www.seanoe.org/data/00353/451
46448/. Biogeochemistry data are available on the CCHDO (CLIVAR & Carbon Hy-452
drographic Data Office) webpage (http://cchdo.ucsd.edu). Dissolved organic carbon453
and nitrogen concentrations are available in the supplementary material of this article.454
References455
Alkire, M. B., E. D’Asaro, C. Lee, M. Jane Perry, A. Gray, I. Cetini´c, N. Briggs,456
E. Rehm, E. Kallin, J. Kaiser, and A. Gonz´alez-Posada (2012), Estimates457
of net community production and export using high-resolution, Lagrangian458
measurements of O2, NO3-, and POC through the evolution of a spring di-459
atom bloom in the North Atlantic, Deep-Sea Research I,64, 157–174, doi:460
10.1016/j.dsr.2012.01.012.461
´
Alvarez, M., H. L. Bryden, F. P´erez, A. F. R´ıos, and G. Ros´on (2002), Physical462
and biogeochemical fluxes and net budgets in the subpolar and temperate North463
Atlantic, Journal of Marine Research,60 (1), 191–226.464
´
Alvarez-Salgado, X. A., M. Nieto-Cid, M. Alvarez, F. F. P´erez, P. Morin, and465
H. Mercier (2013), New insights on the mineralization of dissolved organic matter466
in central, intermediate, and deep water masses of the northeast North Atlantic,467
Limnology and Oceanography,58 (2), 681–696, doi:10.4319/lo.2013.58.2.0681.468
Ar´ıstegui, J., C. M. Duarte, S. Agust´ı, M. Doval, X. A. Alvarez-Salgado, and D. A.469
Hansell (2002), Dissolved organic carbon support of respiration in the dark ocean.,470
Science,298, 1967, doi:10.1126/science.1076746.471
Baker, C. A., S. A. Henson, E. L. Cavan, S. L. Giering, A. Yool, M. Gehlen,472
A. Belcher, J. S. Riley, H. E. Smith, and R. Sanders (2017), Slow-sinking par-473
ticulate organic carbon in the Atlantic Ocean: Magnitude, flux, and potential con-474
trols, Global Biogeochemical Cycles,31 (7), 1051–1065, doi:10.1002/2017GB005638.475
Boyce, D. G., M. R. Lewis, and B. Worm (2010), Global phytoplankton decline over476
the past century, Nature,466 (7306), 591–596, doi:10.1038/nature09268.477
Brambilla, E., and L. D. Talley (2008), Subpolar mode water in the northeastern478
Atlantic: 1. Averaged properties and mean circulation, Journal of Geophysical479
Research: Oceans, doi:10.1029/2006JC004062.480
Brunneg˚ard, J., S. Grandel, H. St˚ahl, A. Tengberg, and P. O. Hall (2004), Nitro-481
gen cycling in deep-sea sediments of the Porcupine Abyssal Plain, NE Atlantic,482
Progress in Oceanography,63 (4), 159–181, doi:10.1016/j.pocean.2004.09.004.483
Carlson, C., H. Ducklow, and A. Michaels (1994), Annual flux of dissolved organic484
carbon from the euphotic zone in the northwestern Sargasso Sea, Nature,371,485
405–408.486
Carlson, C. A. (2002), Production and removal processes, in Biogeochemistry of Ma-487
rine Dissolved Organic Matter., edited by D. A. Hansell and C. A. Carlson, pp.488
91–151, Academic Press, San Diego, CA.489
Carlson, C. A., D. A. Hansell, N. B. Nelson, D. A. Siegel, W. M. Smethie, S. Khati-490
wala, M. M. Meyers, and E. Halewood (2010), Dissolved organic carbon export491
and subsequent remineralization in the mesopelagic and bathypelagic realms492
of the North Atlantic basin, Deep-Sea Research II,57 (16), 1433–1445, doi:493
10.1016/j.dsr2.2010.02.013.494
Cuny, J., P. B. Rhines, P. P. Niiler, and S. Bacon (2002), Labrador Sea Boundary495
Currents and the Fate of the Irminger Sea Water, Journal of Physical Oceanogra-496
phy,32 (2), 627–647, doi:10.1175/1520-0485(2002)032%3C0627:lsbcat%3E2.0.co;2.497
Daniault, N., H. Mercier, P. Lherminier, A. Sarafanov, A. Falina, P. Zunino, F. F.498
P´erez, A. F. R´ıos, B. Ferron, T. Huck, V. Thierry, and S. Gladyshev (2016),499
The northern North Atlantic Ocean mean circulation in the early 21st century,500
–11–

Confidential manuscript submitted to Geophysical Research Letters
Progress in Oceanography,146 (July), 142–158, doi:10.1016/j.pocean.2016.06.007.501
Fontela, M., M. I. Garc´ıa-Ib´a˜nez, D. A. Hansell, H. Mercier, and F. F. P´erez (2016),502
Dissolved Organic Carbon in the North Atlantic Meridional Overturning Circula-503
tion, Scientific Reports,6(November), 26,931, doi:10.1038/srep26931.504
Garc´ıa-Ib´a˜nez, M. I., P. C. Pardo, L. I. Carracedo, H. Mercier, P. Lherminier, A. F.505
R´ıos, and F. F. P´erez (2015), Structure, transports and transformations of the wa-506
ter masses in the Atlantic Subpolar Gyre, Progress in Oceanography,135, 18–36,507
doi:10.1016/j.pocean.2015.03.009.508
Giering, S. L. C., R. Sanders, R. S. Lampitt, T. R. Anderson, C. Tamburini,509
M. Boutrif, M. V. Zubkov, C. M. Marsay, S. a. Henson, K. Saw, K. Cook, and510
D. J. Mayor (2014), Reconciliation of the carbon budget in the ocean’s twilight511
zone., Nature,507, 480–483, doi:10.1038/nature13123.512
Hansell, D., C. A. Carlson, D. J. Repeta, and R. Schlitzer (2009), Dissolved Organic513
Matter in the Ocean: A Controversy Stimulates New Insights, Oceanography,514
22 (4), 202–11, doi:10.5670/oceanog.2009.109.515
Hansell, D. A. (2013), Recalcitrant Dissolved Organic Carbon Frac-516
tions, Annual Review of Marine Science,5(1), 421–445, doi:10.1146/517
annurev-marine-120710-100757.518
Hansell, D. A., C. A. Carlson, and R. Schlitzer (2012), Net removal of ma jor marine519
dissolved organic carbon fractions in the subsurface ocean, Global Biogeochemical520
Cycles,26 (1), 1–9, doi:10.1029/2011GB004069.521
Hansen, B., S. Østerhus, W. R. Turrell, S. J´onsson, H. Valdimarsson, H. H´at´un, and522
S. M. Olsen (2008), The Inflow of Atlantic Water, Heat, and Salt to the Nordic523
Seas Across the Greenland-Scotland Ridge, in Arctic-Subarctic Ocean Fluxes, pp.524
15–43, Springer Netherlands, Dordrecht, doi:10.1007/978-1-4020-6774-7 2.525
Henson, S. a., R. Sanders, E. Madsen, P. J. Morris, F. Le Moigne, and G. D. Quar-526
tly (2011), A reduced estimate of the strength of the ocean’s biological carbon527
pump, Geophysical Research Letters,38 (4), 10–14, doi:10.1029/2011GL046735.528
Hopkinson, C., and J. Vallino (2005), Efficient expert of carbon to the deep ocean529
through dissolved organic matter, Nature,433 (January), 142–145.530
Jeansson, E., A. Olsen, T. Eldevik, I. Skjelvan, A. M. Omar, S. K. Lauvset, J. E. Ø.531
Nilsen, R. G. J. Bellerby, T. Johannessen, and E. Falck (2011), The Nordic Seas532
carbon budget: Sources, sinks, and uncertainties, Global Biogeochemical Cycles,533
25 (4), 1–16, doi:10.1029/2010GB003961.534
Jickells, T. D., E. Buitenhuis, K. Altieri, A. R. Baker, K. Fennel, M. . Kanakidou,535
J. Laroche, K. Lee, P. Liss, and J. J. Middelburg (2017), A re-evaluation of the536
magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the537
ocean, Global Biogeochemical Cycles,31, 289–305, doi:10.1002/2016GB005586.538
Jochumsen, K., M. Moritz, N. Nunes, D. Quadfasel, K. M. Larsen, B. Hansen,539
H. Valdimarsson, and S. Jonsson (2017), Revised transport estimates of the Den-540
mark Strait overflow, Journal of Geophysical Research: Oceans,122 (4), 3434–541
3450, doi:10.1002/2017JC012803.542
Johnson, C., M. Inall, and S. H¨akkinen (2013), Declining nutrient concentrations543
in the northeast Atlantic as a result of a weakening Subpolar Gyre, Deep-Sea544
Research I,82, 95–107, doi:10.1016/j.dsr.2013.08.007.545
Krauss, W. (1995), Currents and mixing in the Irminger Sea and in the Iceland546
Basin, Journal of Geophysical Research,100 (C6), 10,851, doi:10.1029/95JC00423.547
Kroeze, C., L. Bouwman, and S. Seitzinger (2012), Modeling global nutrient export548
from watersheds, Current Opinion in Environmental Sustainability,4(2), 195–202,549
doi:10.1016/j.cosust.2012.01.009.550
Lemaitre, N., H. Planquette, F. Planchon, G. Sarthou, S. Jacquet, M. I. Garc´ıa-551
Ib´a˜nez, A. Gourain, M. Cheize, L. Monin, L. Andr´e, P. Laha, H. Terryn, and552
F. Dehairs (2018), Particulate barium tracing significant mesopelagic car-553
bon remineralisation in the North Atlantic, Biogeosciences, p. In press, doi:554
–12–

Confidential manuscript submitted to Geophysical Research Letters
10.5194/bg-2017-400.555
Letscher, R. T., D. A. Hansell, C. A. Carlson, R. Lumpkin, and A. N. Knapp (2013),556
Dissolved organic nitrogen in the global surface ocean: Distribution and fate,557
Global Biogeochemical Cycles,27 (1), 141–153, doi:10.1029/2012GB004449.558
Lherminier, P., H. Mercier, C. Gourcuff, M. Alvarez, S. Bacon, and C. Kermabon559
(2007), Transports across the 2002 Greenland-Portugal Ovide section and com-560
parison with 1997, Journal of Geophysical Research: Oceans,112 (7), 1–20, doi:561
10.1029/2006JC003716.562
Macrander, A., U. Send, H. Valdimarsson, S. J´onsson, and R. H. K¨ase (2005),563
Interannual changes in the overflow from the Nordic Seas into the Atlantic564
Ocean through Denmark Strait, Geophysical Research Letters,32 (6), 1–4, doi:565
10.1029/2004GL021463.566
Martin, P., R. S. Lampitt, M. Jane Perry, R. Sanders, C. Lee, and E. D’Asaro567
(2011), Export and mesopelagic particle flux during a North Atlantic spring di-568
atom bloom, Deep Sea Research I,58 (4), 338–349, doi:10.1016/J.DSR.2011.01.569
006.570
Maz´e, G., H. Mercier, V. Thierry, L. Memery, P. Morin, and F. F. P´erez (2012),571
Mass, nutrient and oxygen budgets for the northeastern Atlantic Ocean, Biogeo-572
sciences,9(10), 4099–4113, doi:10.5194/bg-9-4099-2012.573
McCartney, M. S., and L. D. Talley (1982), The Subpolar Mode Water of the574
North Atlantic Ocean, Journal of Physical Oceanography,12 (11), 1169–1188,575
doi:10.1175/1520-0485(1982)012h1169:TSMWOTi2.0.CO;2.576
Mercier, H., P. Lherminier, A. Sarafanov, F. Gaillard, N. Daniault, D. Des-577
bruy`eres, A. Falina, B. Ferron, C. Gourcuff, T. Huck, and V. Thierry (2013),578
Variability of the meridional overturning circulation at the Greenland-Portugal579
OVIDE section from 1993 to 2010, Progress in Oceanography,132, 250–261, doi:580
10.1016/j.pocean.2013.11.001.581
Nilsson, J., G. Bj¨ork, B. Rudels, P. Winsor, and D. Torres (2008), Liquid freshwater582
transport and Polar Surface Water characteristics in the East Greenland Cur-583
rent during the AO-02 Oden expedition, Progress in Oceanography,78 (1), 45–57,584
doi:10.1016/j.pocean.2007.06.002.585
Østerhus, S., W. R. Turrell, S. J´onsson, and B. Hansen (2005), Measured volume,586
heat, and salt fluxes from the Atlantic to the Arctic Mediterranean, Geophysical587
Research Letters,32 (7), 1–4, doi:10.1029/2004GL022188.588
Paillet, J., M. Arhan, and M. S. McCartney (1998), Spreading of Labrador Sea589
Water in the eastern North Atlantic, Journal of Geophysical Research: Oceans,590
103 (C5), 10,223–10,239, doi:10.1029/98JC00262.591
Pickart, R. S., F. Straneo, and G. W. Moore (2003), Is Labrador Sea Wa-592
ter formed in the Irminger basin?, Deep-Sea Research I,50 (1), 23–52, doi:593
10.1016/S0967-0637(02)00134-6.594
Piron, A., V. Thierry, H. Mercier, and G. Caniaux (2017), Gyre-scale deep convec-595
tion in the subpolar North Atlantic Ocean during winter 2014-2015, Geophysical596
Research Letters,44 (3), 1439–1447, doi:10.1002/2016GL071895.597
Romera-Castillo, C., R. T. Letscher, and D. A. Hansell (2016), New nutrients exert598
fundamental control on dissolved organic carbon accumulation in the surface At-599
lantic Ocean, Proceedings of the National Academy of Sciences,113 (38), 10,497–600
10,502, doi:10.1073/pnas.1605344113.601
Roshan, S., and T. DeVries (2017), Efficient dissolved organic carbon production602
and export in the oligotrophic ocean, Nature Communications,8(1), 2036, doi:603
10.1038/s41467-017-02227-3.604
Sharples, J., J. J. Middelburg, K. Fennel, and T. D. Jickells (2016), What propor-605
tion of riverine nutrients reaches the open ocean?, Global Biogeochemical Cycles,606
pp. 39–58, doi:10.1002/2016GB005483.607
–13–

Confidential manuscript submitted to Geophysical Research Letters
Smeed, D. A., S. A. Josey, C. Beaulieu, W. E. Johns, B. I. Moat, E. Frajka-608
Williams, D. Rayner, C. S. Meinen, M. O. Baringer, H. L. Bryden, and G. D.609
Mccarthy (2018), The North Atlantic Ocean Is in a State of Reduced Overturning,610
Geophysical Research Letters, pp. 1527–1533, doi:10.1002/2017GL076350.611
Torres-Vald´es, S., V. M. Roussenov, R. Sanders, S. Reynolds, X. Pan, R. Mather,612
A. Landolfi, G. A. Wolff, E. P. Achterberg, and R. G. Williams (2009), Dis-613
tribution of dissolved organic nutrients and their effect on export produc-614
tion over the Atlantic Ocean, Global Biogeochemical Cycles,23 (4), 1–16, doi:615
10.1029/2008GB003389.616
Torres-Vald´es, S., T. Tsubouchi, S. Bacon, A. C. Naveira-Garabato, R. Sanders,617
F. A. McLaughlin, B. Petrie, G. Kattner, K. Azetsu-Scott, and T. E. Whitledge618
(2013), Export of nutrients from the Arctic Ocean, Journal of Geophysical Re-619
search: Oceans,118 (4), 1625–1644, doi:10.1002/jgrc.20063.620
Torres-Vald´es, S., T. Tsubouchi, E. Davey, I. Yashayaev, and S. Bacon (2016),621
Relevance of dissolved organic nutrients for the Arctic Ocean nutrient budget,622
Geophysical Research Letters,43 (12), 6418–6426, doi:10.1002/2016GL069245.623
Vidal, M., E. Aspillaga, I. Teixidor-Toneu, and A. Delgado-Huertas (2018), Lateral624
Transport of N-Rich Dissolved Organic Matter Strengthens Phosphorus Defi-625
ciency in Western Subtropical North Atlantic, Global Biogeochem. Cycles,32 (9),626
1350–1366, doi:10.1029/2017GB005868.627
Wollast, R. (1993), Interactions of Carbon and Nitrogen Cycles in the Coastal628
Zone, in Interactions of C, N, P and S Biogeochemical Cycles and Global629
Change, pp. 195–210, Springer Berlin Heidelberg, Berlin, Heidelberg, doi:630
10.1007/978-3-642-76064-8 7.631
Yashayaev, I., H. M. van Aken, N. P. Holliday, and M. Bersch (2007), Transfor-632
mation of the Labrador Sea Water in the subpolar North Atlantic, Geophysical633
Research Letters,34 (22), 1–8, doi:10.1029/2007GL031812.634
Yashayaev, I., D. Seidov, and E. Demirov (2015), A new collective view of oceanog-635
raphy of the Arctic and North Atlantic basins, Progress in Oceanography,132,636
1–21, doi:10.1016/j.pocean.2014.12.012.637
Zhang, M., Y. Zhang, Q. Shu, C. Zhao, G. Wang, Z. Wu, and F. Qiao (2018), Spa-638
tiotemporal evolution of the chlorophyll a trend in the North Atlantic Ocean,639
Science of the Total Environment,612, 1141–1148, doi:10.1016/j.scitotenv.2017.08.640
303.641
Zunino, P., M. I. Garcia-Iba˜nez, P. Lherminier, H. Mercier, A. F. Rios, and F. F.642
P´erez (2014), Variability of the transport of anthropogenic CO2 at the Greenland-643
Portugal OVIDE section: Controlling mechanisms, Biogeosciences,11 (8), 2375–644
2389, doi:10.5194/bg-11-2375-2014.645
Zunino, P., P. Lherminier, H. Mercier, X. A. Pad´ın, A. F. R´ıos, and F. F. P´erez646
(2015), Dissolved inorganic carbon budgets in the eastern subpolar North At-647
lantic in the 2000s from in situ data, Geophysical Research Letters,42, doi:648
10.1002/2015GL066243.Received.649
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c) Nitrate budget
345±71
kmol s-1
4±20
kmol s-1
229±15 kmol s-1
109±75 kmol s-1
Atm.dep.:
0.8±0.2
Rivers:
1.7±1.2
Net budget:
17±15 kmol s-1
145 kmol s-1
45±5 kmol s-1
46±5 kmol s-1
73±13 kmol s-1
Greenland
Iceland
B.I.
Iberian
Peninsula
R.R.
Irminger
Sea
Iceland Basin
Western
European Basin
NAC
EGC
IC
6.7±0.6 Sv
Labrador
Sea
0.7±1.4 Sv
3.5 ±0.4 Sv
3.3±0.5 Sv
I.A.P.
ISOW
DSOW
9.3 Sv
a) Mass transport
GIS sills
21.4±4.1 Sv
5.3±4.0 Sv
16.6±1.1 Sv
13±3 kmol s-1
32±5 kmol s-1
C:N = 12
b) DON budget
82±14
kmol s-1
7±6
kmol s-1
58±4 kmol s-1
19±14 kmol s-1
C:N = 14.6
Atm.dep.:
0.21±0.04
Rivers:
1.0±07
Net budget:
-16±6 kmol s-1
C:N = 7.4±4.1
36 kmol s-1
14±4 kmol s-1
C:N = 14.3
C:N = 15.4
C:N = 13.6
OVIDE 2002
Figure 1. Map of the study area in the eastern Subpolar North Atlantic and (a) volume
transports (1 Sv = 106m3s−1) across the OVIDE 2002 section and the Greendland-Iceland-
Scotland sills (GIS sills), (b) dissolved organic nitrogen (DON), and (c) nitrate transports and
net budget (kmol s−1). Red (upper) and blue (lower) arrows and numbers represent the circula-
tion in the density levels corresponding to the upper and lower limbs of the Atlantic Meridional
Overturning Circulation (MOC), limited by the σ1= 32.15 kg m−3isopycnal. NAC, IC and
EGC represent the North Atlantic, Iceland and East Greenland Currents. DSOW and ISOW
are the Denmark Strait and Iceland Strait overflows. R.R. is the Reikjanes Ridge and I. A. P.,
the Iberian Abyssal plain. Fluxes across the OVIDE 2002 section are given for the Irminger Sea
plus Iceland basin (West) and Western European Basin (East). Fluxes across the GIS sills are
reported for the DSOW, the ISOW (blue) and the Atlantic Water (plus Polar Water) (red). The
⊗symbol represents the downward transport by the overturning circulation. The supply by at-
mospheric deposition and rivers is represented by the purple and green arrows, respectively. In
panel (b), the C:N molar ratios, calculated as the ratio of the DOC and DON transports and
budget, are shown in black font.
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EGC Irminger
Sea (East) Iceland
Basin Western European
Basin Iberian Abyssal
Plain
-30
-20
-10
0
10
20
30
Acc. Trp. Mass (Sv)
-30.1
-3.6
-26.5
16.8
6.4
10.4
-8.8
-3.5
-5.3
31.9
25.6
6.2
-9.8
-9.0
-0.9
EGC IC NAC
a)
2
3
4
5
6
<DON> ( mol/kg)
4.1 0.5
3.2 0.4
b)
0 725 1300 2600 3500
Distance from Greenland (km)
10
15
20
<NO3> ( mol/kg)
13.2 1.3
18.0 1.4
c)
Figure 2. (a) Depth-integrated volume transports (1 Sv = 106m3s−1) across the OVIDE
2002 section horizontally accumulated from Greenland and distribution of the (b) Dissolved or-
ganic nitrogen (DON) and (c) nitrate (NO3) mean concentrations (
µ
mol kg−1) along the section.
Transports and concentrations corresponding to the upper and lower limbs of the Meriodional
Overturning Circulation (MOC), limited by the σ1= 32.15 kg m−3isopycnal, are shown in red
and blue colors, respectively. Net upper plus lower limb transports are represented by a black
line. The horizontal red and blue lines and numbers in panels (b,c) represent the DON and ni-
trate section-mean concentrations for the upper and lower limbs. NAC, IC and EGC represent
the North Atlantic, Iceland and East Greenland Currents. Integrated transports over the differ-
ent regions are given in this order: total (larger font), upper limb (red) and lower limb (blue).
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Figure S1. Distribution of the (a) cross-section velocities (V, cm s−1), (b) nitrate (NO3,
µ
mol kg−1) and (c) dissolved inorganic nitrogen (DON,
µ
mol kg−1) concentrations along the
OVIDE 2002 section. Measured and interpolated concentrations are shown as coloured dots and
contours, respectively. Numbers in the top of the panels indicate the station pair numbers. The
main circulation features intersected by the OVIDE 2002 section are labeled in panel (a): East
Greenland Current (EGC), Irminger Current (IC), North Atlantic Current (NAC), and three
anticyclonic eddies (A-C Eddy). Two branches of the NAC have been intersected, a western more
intense branch (NAC, station pairs 42.47) and a eastern weaker branch (ENAC, station pairs 60-
64). The realms of the principal water masses present in the section are indicated: Eastern North
Atlantic Central Water (ENACW), Subpolar Mode Water (SPMW), its variety at the Irminger
Basin (IrSPMW), Mediterranean Water (MW), Labrador Sea Water (LSW), Iceland Strait Over-
flow Water (ISOW), Denkmark Strait Overflow Water (DSOW) and Lower Deep Water (LDW).
The different regions in which the section has been divided for description purposes are also indi-
cated: East Greenland Current (EGC), eastern Irminger Basin, Iceland Basin, Western European
Basin and Iberian Abyssal Plain. The σ1= 32.15 kg m−3isopycnal, separating the upper and
lower limbs of the Meriodinal Overturning Circulation is depicted in all panels.
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177±36 kmol s-1
380±59 kmol s-1
DOC budget
1228
±212 kmol s-1
76±74
kmol s-1
911±59 kmol s-1
271±194 kmol s-1
Atm.dep.:
5.0
Rivers:
0.6
Net budget:
-111±45 kmol s-1
573 kmol s-1
207±32 kmol s-1
Figure S2. Dissolved organic carbon (DOC) transports and net budget (kmol s−1) in the
eastern Subpolar North Atlantic. A schematic representation of the main circulation patterns in
the area is outlined. Red (upper) and blue (lower) arrows and numbers represent the circulation
in the density levels corresponding to the upper and lower limbs of the Atlantic Meridional Over-
turning Circulation (MOC), limited by the σ1= 32.15 kg m−3isopycnal. DON fluxes across the
OVIDE 2002 section are given for the Irminger Sea plus Iceland basin (West) and Western Euro-
pean Basin (East). Fluxes across the GIS sills are reported for the DSOW, the ISOW (blue) and
the Atlantic Water (plus Polar Water) (red). The ⊗symbol represents the downward transport
by the overturning circulation. DOC supply by atmospheric deposition and rivers are represented
by the purple and green arrows, respectively. Those fluxes were obtained from Fontela et al.
[2016] and references herein.
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30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
2.40 years
Float: 5904175
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
2.17 years
Float: 6900638
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
4.23 years
Float: 6900640
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
4.69 years
Float: 6900966
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
3.95 years
Float: 6901022
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
3.73 years
Float: 6901023
30°N
50°N
70°N
60°W 40°W 20°W 0°
τ
=
2.31 years
Float: 6901568
0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 4.8 5.4
Time (yr)
Figure S3. Trajectories of seven selected Argo floats (http://www.argo-france.fr/fr/
home/) contouring the eastern North Atlantic subpolar gyre. The bubble color-scale represents
the time since deployment. The float trajectories were used to estimate the transit time inside
the eastern subpolar North Atlantic (τ) corresponding to a water parcel following the main path
of the subpolar gyre. The floats are parked at 1000 m between profiles and, hence, the trajec-
tories represent the mean flow at this depth. As a consequence, the transit time is possibly an
upper bound estimate of the renewal time of surface waters, where the flow is more energetic.
The positions of the Argo floats were downloaded from ftp://ftp.ifremer.fr/ifremer/argo/.
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Table S1. Volume fluxes, density and nutrient concentrations for water masses at the
Greenland-Iceland-Scotland (GIS) sills. DOC and DON concentrations for the corresponding
water masses in the OVIDE 2002 section are also reported. Those were used to estimate DON
concentrations at the GIS sills according to Eq. 8 DS: Denmark strait, IFR: Iceland-Faroe Ridge,
FSC: Faroe-Shetland channel, AW: Atlantic Water, PW: Polar Water, DSOW/ISOW: Den-
mark/Iceland strait Overflow water, SPMW: Subpolar Mode Water. Superscripts indicate the
bibliography from where the data were obtained. Note that the flux values differ from those re-
ported in Fig. 1a, which correspond to volume-conserving transports (a net volume transport of
0.4±1.2 Sv across the GIS sills has been substracted, see Methods).
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714
715
716
717
718
719
GIS sills OVIDE 2002
Reg. WM Volume ρ1NO3(1) DOC2DON WM DOC3DON3
Trp. (Sv) (kg m−3) (
µ
mol kg−1) (
µ
mol kg−1)
DS AW 0.8±0.16 (4) 1027.8 10.1±1.5 59 ±4 4.83 ±0.62 SPMW 50.5±0.6 3.58 ±0.08
PW −1.5±0.5 (5) 1027.7 9.0±1.6 70 ±10 4.88 ±0.81 - - -
DSOW −3.4±0.4 (6) 1030 12.6±0.5 58 ±6 4.05 ±0.94 DSOW 50.8±2.0 2.97 ±0.14
IFR AW 3.8±0.5 (4) 1028 11.0±1.7 58 ±4 4.69 ±0.61 SPMW 50.5±0.6 3.58 ±0.08
ISOW −1.0±0.5 (7) 1029.9 13.8±0.3 53 ±5 3.90 ±0.74 ISOW 46.9±0.6 3.04 ±0.10
FSC AW 3.8±0.5 (4) 1027.8 9.6±2.4 58 ±4 4.69 ±0.62 SPMW 50.5±0.6 3.58 ±0.08
ISOW −2.1±0.3 (7) 1031.6 13.8±0.2 53.5±5 3.96 ±0.75 ISOW 46.9±0.6 3.04 ±0.10
References 1: World Ocean Atlas 2013 (https://www.nodc.noaa.gov/OC5/woa13/), 2:Jeansson et al. [2011],
3:´
Alvarez-Salgado et al. [2013], 4:Østerhus et al. [2005], 5:Nilsson et al. [2008],
6:Macrander et al. [2005], 7:Hansen et al. [2008]
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