5208 
Q. Devresse et al.: Eddy-enhanced primary production sustains heterotrophic microbial activities 
(a) 
(b) 
Dee 
250 
40 
20°N 
x 20°N| 
30 
® 
© 18°N} 
20 © 
t 
F 
ZZ 16°N 
03 
20 ©) 
= 
ı 
150 ®% 
Br 
3 
100 © 
3 
N 
50 a 
18°N 
p 
P 
16°N 
‚u m 
24°W 22°W 20°W 18°W 16°W 
‚a 
x 
24°W 22°W 20°W 18°W 16°W 
=== Eddy periphery —— Eddyv Core 
Figure 3. Spatial distribution of maximum mixed layer depth (a) and integrated chlorophyll a (Chl a) over 100 m depth (b) during M156. 
tions (PPror/CR<1) occurred at the open ocean stations 
(hroughout the water column, while autotrophic conditions 
(PPror/CR>1) prevailed at the coastal St. E5 (PPTotr/CR 
ratio ranging from 0.7 to 1.9; Table 2). This pattern was 
preserved when data were integrated over the mixed layer 
(Fig. 6). PPpoc rates were sufficient to satisfy the BCD at the 
coastal St. E5 but not in the open ocean stations (Table 2). 
In the CE and at the frontal zone, integrated heterotrophic 
oacterial abundance ranged from 8.2-23.7 x 10° cells m”? 
(Table 1). In the CE, substantial variation in bacterial abun- 
dance occurred within the upper 20 m (Fig. 5a), with an abun- 
dance of <1 x 10° cells L7! in the western periphery of the 
CE and >3 x 10° cells L7! in the CE core stations. Depth- 
integrated BR and CR ranged between 59.1 and 320 and be- 
cween 135 and 592 mmol Cm”? d7!, respectively (Table 1). 
Elevated BR and CR rates (>1 and 2.5 umol CL7! d7!, re- 
spectively) were only present in the upper — 30-40 m of the 
CE (Figs. 5b, S4a). Integrated BP rates ranged from 5.6 to 
36.7 mmolC m? d7! in the CE and at the frontal zone sta- 
tions (Table 1). BP rates were elevated in the upper 40 m of 
‘he CE and at the frontal zone and significantly higher than 
in the majority of the coastal and open ocean stations (Tukey, 
p<0.05). Stations in the core of the CE had BGEs (Ta- 
le 2; Fig. 5d) significantly higher than at the stations located 
in the open ocean (Tukey, p<0.05). BGE had a range of 
2.7+2.9% to 18.3 + 1.0 % and 5.1 + 0.2 % to 5.5 + 2.4 % in 
che CE and the frontal zone stations, respectively. The high- 
est BGE was observed at 15m depth in the CE core (18.3 %, 
St. EDM-4E). The CE and frontal zone stations showed net 
heterotrophy and net autotrophy (Table 2), with a PPror/CR 
:atio ranging from 0.2 to 1.9. When integrated over the mixed 
layer (Fig. 6), stations within the core of the CE and at the 
frontal zone were net autotrophic, with a PPror/CR ratio 
ranging from 1.42 to 1.85, while net heterotrophy occurred 
at the eddy periphery. PPpoc was on average equivalent to 
71 % of the BCD within the CE and at the frontal zone, rang- 
ing from 27.9 % to 110 % (Table 2). 
Biogeosciences, 19. 51995219, 2022 
3.4 Semi-labile dissolved organic carbon 
Between coastal and open ocean stations, SL-DOC con- 
centration was not significantly different (Tukey, p>0.05; 
Fig. S4b) with ranges of 1.9-8.0 umolL7! at the coastal and 
1.6-18.9 umolL7! at the open ocean stations. At those sites, 
SL-DOC distribution was rather uniform in the upper 40m 
with SL-DOC >5 umolL7!, except from the station furthest 
offshore (St. E1) where SL-DOC > 5 umol L7! was limited 
to shallow depths (5 m). In the CE and at the frontal zone, SL- 
DOC concentration was clearly elevated and increased from 
east to west with an overall range of 1.4-54.4 umolL7!. At 
the frontal zone, SL-DOC concentration >5umolL7! was 
detectable down to 90 m depth. 
3.5 Correlation analysis 
We applied a Pearson correlation matrix (Fig. 7) to reveal 
significant correlations between the measured parameters in 
the stations outside (open ocean + coastal) and inside (cy- 
clonic eddy + frontal zone) the area influenced by the eddy. 
In both regimes, temperature correlated negatively with nu- 
trients (DIN, PO4, and Si(OH)4; r = —0.70, —0.67, and 
—0.67, respectively, for the stations outside and r = —0.97, 
0.96, and —0.95 for the stations inside the area influenced 
by the eddy, p<0.001) and positively with bacterial abun- 
dances (r = 0.51 and 0.68, respectively, p<0.001). 
In the stations outside the influence of the eddy, total 
(PPTorT) and dissolved primary production (PPpoc) rates 
were not correlated to Chl a or autotrophic pico- and 
nanoplankton biomass (p>0.05). In contrast, heterotrophic 
bacterial abundance (HB) and the bacterial biomass pro- 
duction (BP) were correlated to primary productivity rates 
(r = 0.85 and r = 0.82, respectively, for PPTort and r = 0.77 
and 0.77, respectively, for PPpoc, p<0.001), Chl a (r= 
0.64 and 0.72, respectively, p<0.001), autotrophic pico- 
and nanoplankton biomass (r = 0.42 and 0.46, respectively, 
p<0.001). and the concentration of semi-labile DOC (SL- 
https://doi.org/10.5194/bg-19-5199-2022