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Jou nal o Plank on Resea ch, 2013 , doi:10.1093/plank / b 011
h p://plank .ox o djou nals.o g/con en /ea ly/2013/02/23/plank . b 011.abs ac
Spa ial pa e ns o plank on biomass and s able iso opes e lec he in luence o he
1
ni ogen- ixe T ichodesmium along he sub opical No h A lan ic
2
Ca men Mompeán*
1
, An onio Bode
1
, V.M. Bení ez-Ba ios
2
, J. F ancisco Domínguez-
3
Yanes
2
, José Escánez
2
and Eugenio F aile-Nuez
2
4
1
Ins i u o Español de Oceanog a ía. Cen o Oceanog á ico de A Co uña. E15080 A Co uña
5
(Spain)
6
2
Ins i u o Español de Oceanog a ía. Cen o Oceanog á ico de Cana ias. Vía Espaldón,
7
Dá sena Pesque a, Pa cela 8, E38180 San a C uz de Tene i e (Spain)
8
*Co esponding au ho . Phone: +34-981205362, Fax: +34-981229077, E-mail:
9
[email p o ec ed]
10
Abs ac
11
The spa ial a iabili y o biomass and s able iso opes in plank on size ac ions in he uppe
12
200 m was s udied in a high spa ial esolu ion ansec along 24ºN om Cana y Islands o
13
Flo ida o de e mine ni ogen and ca bon sou ces. Ve ical ad ec ion o wa e s p edomina ed
14
in la e al zones while he cen al A lan ic (30-70º W) was cha ac e ised by a s ong
15
s a i ica ion and oligo ophic su ace wa e s. Plank on biomass was low in he cen al zone
16
and high in bo h eas e n and wes e n sides, wi h mos o he a iabili y due o ei he la ge
17
(>2000 µm) and small plank on (<500 µm). Ca bon iso opes e lec ed mainly he ad ec ion
18
he deep wa e in la e al zones. S able ni ogen iso opes showed a nea ly symme ical spa ial
19
dis ibu ion in all ac ions, wi h he lowes alues (δ
15
N<1‰) in he cen al zone, and we e
20
in e sely co ela ed o ca bon s able iso opes (δ
13
C) and o he abundance o he ni ogen-
21
ixe T ichodesmium. Diazo ophy was es ima ed o accoun o >50% o o ganic ni ogen in
22
he cen al zone, and e en >30% in eas e n and wes e n zones. The impac o diazo ophy
23
inc eased wi h he size o he o ganisms, suppo ing he wide pa icipa ion o all ophic le els
24
in he p ocessing o ecen ly ixed ni ogen. These esul s indica e ha a mosphe ic sou ces o
25
2
ca bon and ni ogen p e ail o e deep wa e sou ces in he sub opical No h A lan ic and ha
26
he zone in luenced by diazo ophy is much la ge han epo ed in p e ious s udies.
27
Keywo ds: S able iso opes, plank on, Sub opical No h A lan ic, T ichodesmium
28
29
3
INTRODUCTION
30
La ge egions o he ocean a sub opical la i udes a e cha ac e ised by gy es o ocean cu en s
31
o a ing clockwise in he No he n Hemisphe e. Biological p oduc ion in he cen al egions
32
o hese gy es is gene ally low because o low nu ien inpu s while p oduc ion is enhanced a
33
hei bo de s (e.g. Beh en eld e al., 2006). Fo ins ance, he supply o ni ogen om deep
34
wa e s o he pho ic zone is lowes in he middle oceanic gy es, whe e a deep he mocline and
35
smoo h nu ien g adien s de e mine slow a es o nu ien supply by di usion (Mou iño-
36
Ca ballido e al., 2011). No wi hs anding hei low p oduc ion, hese gy es con ibu e a la ge
37
ac ion o global biogenic ca bon expo in o he deep ocean because o hei size (Eme son
38
e al., 1997; Ka l e al., 2008).
39
A a ie y o physical mechanisms a e known o con ibu e o ni ogen inpu s in oligo ophic
40
gy es, including mesoscale and submesoscale u bulence (Oschlies and Ga çon, 1998), la e al
41
anspo om o he egions (Williams and Follows, 1998; To es-Valdes e al., 2009), and
42
a mosphe ic deposi ion (Duce e al., 2008). Howe e , biological ixa ion o a mosphe ic N
2
43
(diazo ophy) can be also a majo inpu o ni ogen in he oligo ophic ocean (G ube and
44
Sa mien o, 1997; Capone e al., 2005; Moo e e al., 2009). In he No h A lan ic, diazo ophy
45
con ibu ed o a la ge ac ion o new p oduc ion, e en exceeding he con ibu ions by ni a e
46
di usion ac oss he pycnocline (Capone e al., 2005; Fe nández e al. 2010; Mou iño-
47
Ca ballido e al., 2010). Ni ogen ixa ion is con olled by empe a u e (B ei ba h e al.,
48
2007), CO
2
(Ba celos e Ramos e al., 2007) and he a ailabili y o o he nu ien s, no ably
49
phospho us and i on, he la e p o ided by a mosphe ic dus inpu s (Moo e e al., 2009;
50
Sohm e al., 2011). Ni ogen o diazo ophic o igin is made a ailable o he pelagic ood web
51
h ough exc e ion and mo ali y o cyanobac e ia (Glibe and B onk, 1994) and u he
52
p ocessing by mic obes and plank onic me azoa (Mon oya e al., 2002).
53
The colonial cyanobac e ia o he genus T ichodesmium is he bes known diazo oph, wi h a
54
widesp ead dis ibu ion ac oss opical and sub opical egions o he ocean (Capone e al.,
55
1997; Luo e al., 2012) whe e su ace wa e empe a u e exceeds 20°C (B ei ba h e al.,
56
2007). In he No h A lan ic T ichodesmium is mo e abundan be ween 20°N and 20°S
57
(Ty ell e al., 2003; Da is and McGillicudy, 2006; Fe nández e al., 2010, 2012) bu mos
58
s udies on N
2
ixa ion ha e been ocused in he sub opical and opical egions whe e blooms
59
a e equen (Voss e al., 2004; Capone e al., 2005; Mulholland e al., 2006; Mon oya e al.,
60
2007). Only a ew s udies ha e measu ed concu en ly T ichodesmium abundance and
61
4
ni ogen ixa ion o e la ge spa ial scales in he A lan ic, as e iewed by Luo e al. (Luo e al.,
62
2012). Howe e , u he e idence o he impac o diazo ophy a egional scales was
63
p o ided by measu emen s o he na u al abundance o s able ni ogen iso opes in ses on and
64
plank on (Wase e al., 2000; Mino e al., 2002; Mon oya e al., 2002; Reynolds e al., 2007;
65
Land um e al., 2011).
66
S able iso opes can ace N
2
inpu s because a mosphe ic ni ogen is ela i ely deple ed in
67
hea y (
15
N) iso opes compa ed o ma ine ni a e (Owens, 1987). Assimila ion o his ligh N
2
68
by diazo ophs p oduces o ganic ma e wi h a cha ac e is ic iso opic signa u e ha can be
69
aced along he ood web. Because o he di e en u no e ime o plank onic o ganisms
70
(hou s o days in bac e ia and phy oplank on, and up o se e al mon hs in la ge zooplank on)
71
he iso opic signa u e o o ganic ma e in a ious compa men s p o ides an in eg a i e, in
72
si u ace o he mo emen and ans o ma ion o ni ogen in he wa e column beyond he
73
ins an aneous e ec s epo ed du ing N
2
-up ake measu emen s. Ni ogen iso opes in ses on
74
e lec he up ake o a mosphe ic N
2
by cyanobac e ia (e.g. Mon oya e al., 2002) while hose
75
in zooplank on show he assimila ion o o ganic ma e ini ially p oduced by diazo ophs
76
(McClelland e al., 2003). This ea u e allows an es ima ion o he con ibu ion o diazo ophy
77
o ne ni ogen assimila ion in di e en componen s o he ood web (Mino e al., 2002;
78
Mon oya e al., 2002; Reynolds e al., 2007; Land um e al., 2011). P e ious es ima es using
79
measu emen s o na u al abundance o ni ogen iso opes in ses on and zooplank on e ealed a
80
la ge con ibu ion o diazo ophic ni ogen (up o 100%) in he no h-eas e n opical and
81
sub opical A lan ic (Mon oya e al., 2002). Land um e al. (Land um e al., 2011) epo ed
82
lowe con ibu ions in he cen al and eas e n sub opical A lan ic compa ed o hose in he
83
eas e n egion, howe e his s udy was made in wa e s nea 30ºN, whe e T ichodesmium
84
abundances we e lowe han in sou he n wa e s (Ty ell e al., 2003; Da is and McGillicuddy,
85
2006; Fe nández e al., 2010). Di ec measu emen s e ealed signi ican N
2
ixa ion also in
86
he eas e n sub opical A lan ic (Fe nández e al., 2010; Wannicke e al., 2010; Bena ides e
87
al., 2011; Fe nández e al., 2012), al hough he e a e ew measu emen s o ei he abundance
88
o N
2
ixa ion in he cen al egion o he gy e (Luo e al., 2012).
89
The objec i e o his s udy is o cha ac e ize spa ial pa e ns o plank on in he oligo ophic
90
sub opical No h A lan ic by means o he analysis o size- ac iona ed plank on biomass and
91
na u al abundance o s able ca bon and ni ogen iso opes. The pa e ns a e ela ed o he
92
abundance o T ichodesmium and indica e a la ge in luence o diazo ophy ac oss plank on
93
size classes o e mos o he sub opical no he n A lan ic.
94
5
95
MATERIAL AND METHODS
96
Samples and wa e column measu emen s we e ob ained du ing Leg 8 o Malaspina-2010
97
expedi ion (h p://www.expedicionmalaspina.es) on R/V Sa mien o de Gamboa (Janua y-
98
Ma ch 2011) in a ansec mos ly along 24 °N be ween Cana y Islands and Flo ida (Fig. 1).
99
The ansec was a bi a ily di ided in eas e n, cen al, and wes e n zones o summa ize i s
100
oceanog aphic and plank on cha ac e is ics.
101
Plank on samples we e collec ed by e ical ows o a mic oplank on ne (40 µm mesh size)
102
and a mesoplank on ne (200 µm mesh size) h ough he uppe 200 m o he wa e column.
103
Sampling was made be ween 10:00 and 16:00 h GMT. Plank on was sepa a ed in o i e size
104
ac ions (40-200, 200-500, 500-1000, 1000-2000 and >2000 µm) by gen le il a ion o he
105
samples by a g aded se ies o nylon sie es (2000, 1000, 500, 200 and 40 µm). La ge
106
gela inous o ganisms we e emo ed be o e il a ion. Aliquo s o each size- ac ion we e
107
collec ed on p e-weigh ed glass- ib e il e s, d ied (60ºC, 48 h) and s o ed in a dessica o
108
be o e de e mina ion o biomass (d y weigh ), ca bon and ni ogen con en and na u al
109
abundance o s able ca bon and ni ogen iso opes asho e.
110
A e de e mina ion o d y weigh , inely g ound aliquo s o each size ac ion we e packed in
111
in capsules o elemen al and s able iso ope analysis by con e sion in o CO
2
and N
2
in an
112
elemen al analyse (Ca lo E ba CHNSO 1108) coupled o an iso ope- a io mass-spec ome e
113
(Finnigan Ma Del a Plus). Samples we e no acidi ied o emo e ca bona es because o he
114
s udies showed ha he acidi ica ion may no cause subs an ial modi ica ion in ca bon iso ope
115
esul s, bu i may a ec ni ogen de e mina ions (Bunn e al., 1995; Bode a al., 2003).
116
Simila ly no co ec ions we e made o lipid con en po en ially a ec ing ca bon iso ope
117
composi ion (Syman ec e al., 2007). In his case, he a e age (±se) C:N mola a io o all
118
samples was 4.8±0.0 (n=218) and showed li le a ia ions among size ac ions, sugges ing
119
low in luence o lipids. Ca bon and ni ogen s able iso ope abundance was exp essed as δ
13
C
120
and δ
15
N ela i e o VPDB (Vienna PeeDee Belemni e ca bona e) and a mosphe ic N
2
iso ope
121
s anda ds. P ecision (± s anda d e o ) o eplica e de e mina ions o bo h C and N s able
122
iso opes was <0.03‰.
123
Wa e p ope ies we e es ima ed om CTD cas s (SBE-911 Plus) in he uppe 300 m. In
124
absence o mo e de ailed obse a ions, sea su ace empe a u e (SST 0-10 m) was used as a
125

6
su oga e o nu ien supply o he su ace by ad ec ion om deepe laye s, and in i o
126
luo escence (SFluo ) as an es ima e o phy oplank on biomass. To al ni a e (NO
3-
+ NO
2-
)
127
and phospha e we e analysed colo ime ically (G asho e al., 1983) on ozen samples
128
collec ed by Niskin bo les a s anda d dep hs.
129
Abundance o he diazo oph T ichodesmium sp. was es ima ed by coun s o 50 ml aliquo s o
130
he sample om he mic oplank on ne p ese ed in glu a aldehyde (25% inal concen a ion)
131
using a FlowCAM® sys em (Fluid Imaging Technologies). P io o analysis he samples we e
132
sc eened by a 100 µm nylon mesh o p e en clogging o he FlowCAM cell. Resul s a e
133
epo ed as numbe o colonies ( ichomes) pe olume o seawa e . Abundance o o al
134
mic ozooplank on and phy oplank on (100 o 200 µm) was also de e mined in he same
135
samples. To al abundance o mesozooplank on was de e mined by coun s o aliquo s o he
136
200 µm ne p ese ed in 4% o malin and obse ed unde a binocula mic oscope. The
137
ela i e equency o he main axa was also eco ded.
138
The con ibu ion o ni ogen ixed by diazo ophs (diazo oph N) o plank on ac ions was
139
es ima ed using he iso ope mass balance app oach o Mon oya e al. (2002):
140
141
whe e δ
15
N
m
is he measu ed iso opic composi ion in he sample, δ
15
N
e
is he iso opic
142
e e ence alue o plank on no in luenced by diazo oph N and δ
15
N
d
is he iso opic
143
composi ion o diazo ophs (-2‰, Mon oya e al., 2002). Re e ence alues δ
15
N
e
we e 3.7,
144
4.3, 5.1 and 5.8 ‰ o 200-500, 500-1000, 1000-2000 and >2000 µm size-classes,
145
espec i ely, co esponding o plank on in opical equa o ial egions (Land um e al., 2011).
146
No es ima ions o diazo oph N con ibu ion we e made o he 40-200 µm class because o
147
po en ial bias caused by he p esence o T ichodesmium ilamen s.
148
RESULTS
149
Tempe a u e, salini y, luo escence and nu ien s
150
A la ge ange in empe a u e (10 o 25 °C) was ound in he uppe 300 m along he ansec
151
(Fig. 2). Iso he ms aised in he eas e n end acing he in luence o he Cana y upwelling, and
152
also a o he poin s along he ansec indica ing mesoscale ea u es a ou ing upwelling (e.g.
153
nea 50 and 70 °W). The highes su ace empe a u e alues we e ound in he wes e n and
154
%diazo oph= 100 δ
15
N
m
- δ
15
N
e
δ
15
N
d
- δ
15
N
e
7
cen al egions, and he lowes in he eas e n egion. Salini y showed a pa e n simila o he
155
desc ibed o empe a u e, bu in his case he e was a co e o high salini y (>37.4) be ween
156
25 and 48 °W in he uppe 150 m. S ong salini y g adien s cha ac e ised he eas e n egion
157
while he wes e n egion had in gene al low salini y alues.
158
Low alues o in i o luo escence p e ailed along he ansec and showed a cha ac e is ic
159
subsu ace maximum in nea ly all s a ions. This maximum was less de eloped in he eas e n
160
egion whe e luo escence was mo e uni o mly dis ibu ed in he uppe 100 m bu was sha pe
161
and deepe in he cen al and wes e n egions whe e i eached ca. 150 m deep.
162
Ni a e was almos deple ed (<0.05 µM) in he uppe 200 m o mos o he ansec bu in he
163
cen al zone a laye o ela i ely high concen a ion was ound be ween 75 and 100 m dep h
164
(Fig. 2). Phospha e also showed low concen a ions in mos o he ansec bu in his case he
165
whole wa e column had highe concen a ions (>0.05 µM) in he eas e n han in he o he
166
zones. Bo h nu ien s showed highe concen a ions in deep wa e s a he bo de s o he
167
ansec .
168
Spa ial pa e ns o plank on and s able iso opes
169
Plank on biomass dec eased owa ds he cen al zone o he ansec and had high alues a
170
bo h wes e n and eas e n zones (Fig. 3). This pa e n was simila in all size classes al hough
171
mean alues we e signi ican ly highe in he wes e n zone o plank on <500 µm and lowe in
172
he cen al zone o plank on >2000 µm (Table 1). The e we e signi ican di e ences in mean
173
alues o o al plank on biomass, wi h he lowes alue cen al zone, and he highes in he
174
wes e n zone.
175
Mic ozooplank on abundance was simila in all zones while phy oplank on was signi ican ly
176
mo e abundan in he wes e n zone (Table 2). Mean abundance o mesozooplank on ollowed
177
a simila pa e n o o al plank on biomass, wi h equi alen alues in he eas e n and wes e n
178
zones and minimum alues in he cen al zone (Table 2). Copepods we e gene ally dominan
179
in all zones, bu some gene a we e mo e equen in he la e al zones (Oi hona) han in he
180
cen al egion (Calanus, Mac ose ella). Os acoda we e also mo e equen in la e al zones
181
while salps and appendicula ia showed highe equencies in cen al and eas e n zones.
182
The spa ial a iabili y in ni ogen iso opes was simila o he pa e n desc ibed o biomass
183
(Fig. 4), as signi ican , posi i e co ela ions (P<0.05) we e ound be ween δ
15
N and biomass
184
in all plank on size- ac ions. In his case all ac ions showed mean δ
15
N alues in he cen al
185
8
zone (<2‰) signi ican ly lowe han alues in ei he eas e n o wes e n zones (Table 1).
186
Iso opic en ichmen was only no iceable be ween he smalles and la ges size classes, while
187
plank on be ween 200 and 2000 µm showed simila mean δ
15
N alues wi hin zones.
188
In con as , δ
13
C displayed
an opposi e pa e n o he one desc ibed
o δ
15
N and biomass, bu
189
wi h mo e di e ences be ween size- ac ions (Fig. 5). Mean alues o 40-200, 200-500 and
190
1000-2000 µm classes we e signi ican ly lowe in he eas e n zone, while no signi ican
191
di e ences be ween zones we e ound o o he classes (Table 1). Biomass was only
192
signi ican ly co ela ed wi h δ
13
C o 200-500 and 500-1000 µm classes.
193
Rela ionships wi h su ace empe a u e, salini y and in i o luo escence
194
Biomass was nega i ely co ela ed wi h su ace empe a u e only o he la ges size-class,
195
and also nega i ely wi h su ace salini y o <500 µm classes, while non signi ican
196
co ela ions esul ed be ween biomass and su ace luo escence (Fig. 6). Howe e ,
197
luo escence was posi i ely co ela ed wi h δ
15
N o all classes and wi h δ
13
C o <1000 µm
198
classes. Su ace empe a u e was also co ela ed wi h δ
15
N (nega i ely) o δ
13
C (posi i ely)
199
o <1000 µm (and in case o δ
13
C also o >1000 µm) classes.
200
Linea i y be ween δ
δδ
δ
15
N and δ
13
C
201
Ca bon and ni ogen iso ope abundances showed a signi ican nega i e linea ela ionship
202
wi hin size-classes, excep o he >2000 µm class (Fig. 7). The slopes and in e cep s o he
203
lines we e equi alen o classes <1000 µm and also o classes >1000 µm, while wi hin hese
204
g oups he e we e no signi ican di e ences (ANCOVA, P<0.05).
205
T ichodesmium abundance and δ
δδ
δ
15
N
206
Wi h he excep ion o he wo eas e nmos s a ions, T ichodesmium was eco ded a all
207
s a ions o he ansec (Fig. 8). I s abundance showed an ab up inc ease a ca. 25ºW ollowed
208
by a gene al dec ease o he wes . Mean alues we e signi ican ly highe in he eas e n and
209
cen al zones (mean±se = 4.77±0.73 ichomes L
-1
n=29) han in he wes e n zone (2.04±0.57
210
ichomes L
-1
n=14, ANOVA, P<0.05).
211
A nega i e linea ela ionship was ound be ween δ
15
N and log- ans o med T ichodesmium
212
abundance o all size-classes (Fig. 9). The slope o he line was simila o all classes (mean
213
slope = -1.42±0.11, n=84) while he e we e di e ences in he in e cep be ween he 40-200
214
9
µm and he o he classes and be ween >2000 µm and classes 200-1000 µm (ANCOVA,
215
P<0.05).
216
Con ibu ion o N om diazo ophs
217
Diazo oph N con ibu ed o all size ac ions in almos all s a ions (Fig. 10). Only ew
218
s a ions in he eas e n zone wi hou T ichodesmium showed ze o con ibu ion while maximum
219
alues (>70%) occu ed in gene al in he cen al zone. Mean con ibu ions we e ca. 50% in
220
he cen al zone bu be ween 22 and 38% in he eas e n and wes e n zones (Table 3). The
221
con ibu ions o diazo oph N inc eased o la ge classes. On a e age he e was an inc ease in
222
he con ibu ion o diazo oph N be ween he 200-500 and he >2000 µm classes o 16, 10 and
223
4% o he eas e n, cen al and wes e n zones.
224
225
DISCUSSION
226
Plank on biomass ac oss he sub opical A lan ic
227
The measu ed plank on biomass e lec ed well he oligo ophy o mos o he sub opical
228
No h A lan ic. To ou knowledge hese esul s a e he i s ob ained in his egion o he deep
229
ocean a such small spa ial esolu ion showing a g adual dec ease om zones nea he
230
con inen al shel es o he cen al basin. Howe e , he s udied ansec included also
231
p oduc i e a eas. Biomass o nea ly all size classes was highe in he wes e n han in he
232
eas e n and cen al zones, wi h mean alues equi alen o hose p e iously epo ed o bo h
233
wes e n (Madin e al., 2001) and eas e n zones (He nández-León e al., 2007). The high
234
p oduc i i y o la e al zones can be a ibu ed o he in luence o he Cana y upwelling in he
235
eas (indica ed by he upwa d end in iso he ms and he shallow chlo ophyll maxima in Fig.
236
2) and o mesoscale eddy pumping (McGillicudy e al., 2001) in he wes (as indica ed by
237
sha p changes in iso he ms nea 70°W). Zooplank on biomass has been shown o ack
238
upwelling dynamics and ex end he in luence o he high p oduc i e wa e s nea no h-
239
wes e n A ica well in o he deep ocean (He nández-León e al., 2007). Also, zooplank on
240
biomass peaks ollow win e -sp ing blooms in he wes e n zone (Madin e al., 2001). The
241
high plank on p oduc i i y is di ec ly linked o la ge ad ec i e luxes o ni a e om deep
242
wa e s in bo h eas e n (Peleg í e al., 2005) and wes e n zones (Lipschulz, 2001) while low
243
p oduc ion in he cen al zone o he basin is a ibu ed o low nu ien ad ec ion (Ma añón e
244
al., 2000).
245
16
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501
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20
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550
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551
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564
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567
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568
569
21
Figu e legends
570
Figu e 1. CTD and plank on sampling s a ions du ing c uise Leg 8 o Malaspina-2010
571
expedi ion along 24 ⁰N (dashed line). The e ical lines indica e he limi s o he eas e n (E),
572
cen al (C) and wes e n (W) egions desc ibed in he ex .
573
Figu e 2. Tempe a u e, salini y and in i o luo escence in he uppe 300 m along 24 ⁰N.
574
CTD s a ions a e indica ed in he uppe panel. The dashed lines indica e he limi s be ween
575
eas e n (E), cen al (C) and wes e n (W) zones desc ibed in he ex .
576
Figu e 3. Accumula ed biomass (mg d y weigh m
-3
) o size ac iona ed plank on along 24
577
⁰N. The dashed lines indica e he limi s be ween eas e n (E), cen al (C) and wes e n (W)
578
zones desc ibed in he ex .
579
Figu e 4. Na u al abundance o s able ni ogen iso opes (δ
15
N, ‰) o size ac iona ed
580
plank on along 24 ⁰N. The dashed lines indica e he limi s be ween eas e n (E), cen al (C)
581
and wes e n (W) zones desc ibed in he ex .
582
Figu e 5. Na u al abundance o s able ca bon iso opes (δ
13
C, ‰) o size ac iona ed plank on
583
along 24 ⁰N. The dashed lines indica e he limi s be ween eas e n (E), cen al (C) and wes e n
584
(W) zones desc ibed in he ex .
585
Figu e 6. Co ela ion coe icien s (Pea son ) be ween size ac iona ed plank on: a) biomass
586
(mg DW m-3), b) δ15N o c) δ13C o and sea su ace empe a u e (SST, ⁰C), salini y (SSS) o
587
in i o luo escence (S luo ) along 24 ⁰N. The dashed lines indica e he signi icance alue
588
(P<0.05).
589
Figu e 7. Rela ionships be ween δ
15
N and δ
13
C o size ac iona ed plank on (µm). All
590
eg ession lines a e signi ican wi h P<0.01 excep o he >2000 µm ac ion (dashed line,
591
P<0.05).
592
Figu e 8. Abundance o T ichodesmium ( ichomes L
-1
) along 24 ⁰N. The dashed lines
593
indica e he limi s be ween eas e n (E), cen al (C) and wes e n (W) zones desc ibed in he
594
ex .
595

22
Figu e 9. Rela ionships be ween δ
15
N and T ichodesmium abundance (log
10
( ichomes L
-1
)).
596
All eg ession lines a e signi ican wi h P<0.001.The dashed line indica es he eg ession line
597
o he >2000 µm ac ion.
598
Figu e 10. Diazo ophic N con ibu ion (%) o plank on size-classes >200 µm es ima ed om
599
δ
15
N along 24⁰ N.
600
601
23
Table 1. Mean (±se) biomass (mg DW m
-3
), δ
15
N and δ
13
C by size- ac ions in he wes e n (W), cen al (C) and eas e n (E) zones along 24°N. 602
To al biomass (To al) is he sum o biomass o all size- ac ions. n: numbe o da a. Shaded alues and le e s indica e signi ican di e ences 603
be ween means (ANOVA and C-Dunne a pos e io i es , P<0.05). 604
DW
δ
15
N δ
13
C
Size ac ion
(µm) W C E W C E W C E
40-200 4.65±0.24
b
3.38±0.11
a
3.46±0.18
a
1.7±0.2
b
0.6±0.1
a
2.1±0.2
b
-19.6±0.2
b
-19.7±0.1
b
-20.3±0.1
a
200-500 3.00±0.24
b
1.89±0.08
a
2.04±0.12
a
2.0±0.2
b
1.2±0.1
a
2.5±0.3
b
-19.7±0.1
b
-19.6±0.3
b
-20.4±0.2
a
500-1000 2.69±0.24
a
1.95±0.09
a
2.48±0.22
a
2.3±0.2
b
1.2±0.1
a
2.5±0.3
b
-20.0±0.2
a
-20.1±0.3
a
-20.6±0.2
a
1000-2000 2.31±0.16
a
1.74±0.20
a
2.08±0.11
a
2.4±0.2
b
1.5±0.2
a
2.6±0.3
b
-19.4±0.4
b
-18.7±0.4
b
-20.9±0.7
a
>2000 2.39±0.17
b
1.65±0.11
a
2.71±0.23
b
3.2±0.3
b
1.6±0.3
a
2.9±0.3
b
-19.5±0.5
a
-19.3±0.5
a
-20.9±0.6
a
To al 15.04±0.93
c
10.60±0.38
a
12.78±0.66
b
n 14 12 17 14 12 17 14 12 17
24
Table 2. Mean (±se) abundance o mic oplank on (n L
-1
), and dominan axa (% equency)
and mean (±sd) o al abundance (n m
-3
) o mesozooplank on in he wes e n (W), cen al (C)
and eas e n (E) zones along 24°N. Shaded alues and le e s indica e signi ican di e ences
be ween means (ANOVA and C-Dunne a pos e io i es , P<0.05).
zone
G oup Taxa W C E
To al abundance (n L
-
1
)
Mic oplank on
(40-200 µm)
Phy oplank on
Mean ± se 8.1±1.0
b
4.5±0.5
a
3.7±0.4
a
Zooplank on Mean ± se 11.0±1.0
a
7.3±0.7
a
7.5±1.4
a
F equency (%)
Mesozooplank on Calanus 4.7
9.9
10.0
(>200 µm) Co ycaeus 8.1
8.3
5.6
Mac ose ella 4.0
7.4
4.4
Oi hona 8.7
7.4
10.0
Appendicula ia 2.0
2.5
6.1
Chae ogna ha 6.7
7.4
7.8
Os acoda 8.7
5.0
7.2
Polychae a 7.4
3.3
5.6
Salps 2.7
5.8
5.0
To al abundance (n m
-
3
)
Mean ± se 186.9±29.1
a
124.3±16.6
a
178.0±32.4
a
n 14
12
17
25
Table 3. Mean (±se) con ibu ion o N om diazo ophs (%) o plank on size- ac ions in he
wes e n (W), cen al (C) and eas e n (E) zones along 24°N. N: numbe o da a. Shaded alues
and le e s indica e signi ican di e ences be ween means (ANOVA and C-Dunne a
pos e io i es , P<0.05).
Size ac ion
(µm) W C E
200-500 29.4±3.9
a
43.1±1.4
b
22.5±3.9
a
500-1000 31.9±2.7
a
49.2±1.9
b
29.0±4.4
a
1000-2000 38.3±3.2
a
50.4±3.2
b
35.5±4.1
a
>2000 33.2±4.3
a
53.5±4.1
b
38.7±4.7
a
n 14 12 17
-26
-22
-18
-14
δ
13
C
> 2000
ECW
-14
-
22
-18
-14
δ
13
C
500
-
1000
-26
-22
-18
-14
δ
13
C
1000-2000
Figu e 5. Na u al abundance o s able ca bon iso opes (δ13C, ‰) o size ac iona ed plank on
along 24 ⁰N. The dashed lines indica e he limi s be ween eas e n (E), cen al (C) and wes e n
(W) zones desc ibed in he ex .
-26
-22
-18
-14
020406080100
δ
13
C
40-200
-26
-22
-18
δ13C
200-500
-26
-
22
500
-
1000
Longi ude

-1.0
-0.5
0.0
0.5
1.0
40-200 200-500 500-1000 1000-2000 > 2000
co ela ion coe icien
size class (µm)
SST
SSS
SFluo
Biomass
(DW)
-0.5
0.0
0.5
1.0
co ela ion coe icien
δ
15
N
Figu e 6. Co ela ion coe icien s (Pea son ) be ween size ac iona ed plank on biomass (mg
DW m-3)δ15N o δ13C o and sea su ace empe a u e (SST, ⁰C), salini y (SSS) o in i o
luo escence (S luo ) along 24 ⁰N. The dashed lines indica e he signi icance alue (P<0.05).
-1.0
40-200 200-500 500-1000 1000-2000 > 2000
size class (µm)
-1.0
-0.5
0.0
0.5
1.0
40-200 200-500 500-1000 1000-2000 > 2000
co ela ion coe icien
size class (µm)
δ
13
C
-1
0
1
2
3
4
5
6
-26 -22 -18 -14
δ
15
N
δ
13
C
40-200
-1
0
1
2
3
4
5
6
-26 -22 -18 -14
δ
15
N
δ
13
C
200-500
3
4
5
6
15
N
3
4
5
6
15
N
-1
0
1
2
-26 -22 -18 -14
δ
15
δ
13
C
500-1000
-1
0
1
2
-26 -22 -18 -14
δ
15
δ
13
C
1000-2000
>2000
Figu e 7. Rela ionships be ween δ15N and δ13C o size ac iona ed plank on (µm). All eg ession
lines a e signi ican wi h P<0.01 excep o he >2000 µm ac ion (dashed line, P<0.05).
10
15
20
T ichodesmium
( ichomes L
-1
)
WCE
0
5
10
020406080100
T ichodesmium
Longi ude
Figu e 8. Abundance o T ichodesmium ( ichomes L-1) along 24 ⁰N. The dashed lines indica e
he limi s be ween eas e n (E), cen al (C) and wes e n (W) zones desc ibed in he ex .
-1
0
1
2
3
4
5
6
-2 -1 0 1 2
δ
15
N
log
10
(T ichodesmium)
40-200
-1
0
1
2
3
4
5
6
-2 -1 0 1 2
δ
15
N
log
10
(T ichodesmium)
200-500
5
6
5
6
-1
0
1
2
3
4
5
-2 -1 0 1 2
δ
15
N
log
10
(T ichodesmium)
500-1000
-1
0
1
2
3
4
5
-2 -1 0 1 2
δ
15
N
log
10
(T ichodesmium)
1000-2000
> 2000
Figu e 9. Rela ionships be ween δ15N and T ichodesmium abundance (log10( ichomes L-1)). All
eg ession lines a e signi ican wi h P<0.001.The dashed line indica es he eg ession line o he
>2000 µm ac ion.
80
20
40
60
80
% N diazo ophs
1000-2000 0
20
40
60
80
% N diazo ophs
>2000
ECW
Figu e 10. Diazo ophic N con ibu ion (%) o plank on size-classes >200 µm es ima ed om
δ15N along 24⁰ N.
0
20
40
60
80
020406080100
% N diazo ophs
Longi ude W
200-500
0
20
40
60
% N diazo ophs
500-1000
0