324 
Q. Devresse et al.: Eddy-enhanced primary production sustains heterotrophic microbial activities 
Community respiration (CR) rates were estimated from qua- 
druplicate incubations by measuring changes in dissolved 
oxygen over 24-36 h at the same temperature as used for BP 
(14°C) using optode spot mini sensors (PreSens PSt3; Preci- 
sion Sensing GmbH, Regensburg, Germany). The detection 
limit (DL) for CR was 0.55 umol O2 L7! a7}. 
CR at 22°C was estimated using the extrapolation from 
Regaudie-De-Gioux and Duarte (2012): 
CR22°C = CRı4°c x 2.011 — 0.013. (5) 
CRo22°C was converted into bacterial respiration (BRo22°Cc) 
based on Aranguren-Gassis et al. (2012): 
BRo2°C = 0.30 x CR152- — 0.013. (6) 
A respiratory quotient of 1 was used to convert oxygen 
consumption into carbon respiration (del Giorgio and Cole, 
1998). 
We estimated the bacterial carbon demand (BCD) as fol- 
lows: 
BCD = BP +BR. 
Bacterial growth efficiency (BGE) was calculated from BP 
and BCD: 
BGE = BP 
“ BCD' 
a salinity between 35.55-36.79 in the upper 200m depth 
(Fig. 2a and b). The average mixed layer depth was 35 + 7m 
(Fig. 3a; Table S1). Oxygen concentrations (Fig. 2c) de- 
creased with depth, while nutrient concentrations increased 
(Fig. 2d-e). Nutrients were depleted (<0.5, <0.2, and 
<0.5 umol L7! for DIN, PO4, and Si(OH)a4, respectively) in 
the mixed layer. 
At the coastal stations (16.51—16.92° W), the temperature 
had a range of 14.6-26.1 °C and a salinity between 35.53 
and 36.08 in the upper 200 m depth (Fig. 2a and b). Here, 
the mixed layer was shallower than in the open ocean but 
not significantly (Tukey, p>0.05), with an average depth 
of 24.54 9m (Fig. 3a; Table S1). Oxygen was decreasing 
with depth, and a shallow oxygen minimum zone (OMZ; 
<50 umol kg!) was detected between 80 and 200m depth 
(Fig. 2c). Nutrients (Fig. 2d-e) were depleted at the surface 
(5m depth), while the deeper coastal waters (80 to 200m 
depth) were colder and richer in nutrients than the open ocean 
waters, with on average 3.4-fold higher nutrient concentra- 
tions (DIN, PO4, Si(OH)4) when integrated over 100 m depth 
‘data not shown). 
In the CE (“periphery” and “core”), waters had a tempera- 
ture range of 13.2-24.2 °C and a salinity between 35.48 and 
36.36 in the upper 200 m depth (Fig. 2a and b). A compres- 
sion of isopycnals with a strong doming of the isotherms, 1so- 
halines, and nutrient isolines was observed (Fig. 2a-b, d—f). 
A shallow OMZ was detected from — 30 to — 100m depth 
with the lowest oxygen concentration (<10umolkg 7!) be- 
tween 3040 m. The mixed layer was significantly shallower 
(Tukey, p<0.05) in the CE periphery and in the CE core than 
ın the open ocean with an average of 15 +6 m and 204+2m 
depth, respectively (Fig. 3a). At the surface (5m depth), 
nutrients were depleted (<0.5, <0.2, and <0.5umolL7! 
for DIN, PO4, and Si(OH)y4, respectively) only in the most 
eastern (17.11° W, 18°N) and western (18.83—-19.11° W. 
18.58° N) parts of the CE periphery (Fig. 2d—f). In the core, 
nutrient concentrations were also lowest in the surface water 
hut richer in nutrients than in the ambient waters. 
The frontal zone station E3 (19.55° W) was distinct from 
the adjacent stations with respect to surface temperature 
(1°C colder; Fig. 2a). A doming of the nutrient isolines was 
observed (Fig. 2d—f), and nutrient concentrations integrated 
over 100m depth at St. E3 were — 3-fold higher than at the 
open ocean $4 (20.3° W) and — 1.2-fold higher than at the 
CE periphery at St. EDZ-1 (19.11° W). 
3.2 Chlorophyll ag and primary production 
Detailed information on procedures and calculations of mi- 
crobial activities are provided in the Supplement. 
2.4 Data analysis 
Statistical analyses and calculations were conducted using 
the software R (v4.0.3) in R studio (v1.1.414; Ihaka and Gen- 
leman, 1996). Analysis of variances (ANOVA) and Tukey’s 
test were performed on the different parameters by grouping 
‘he stations by their position (Table S1). Seawater density 
was calculated using R package oce v1.3.0 (Kelley, 2018), 
and the mixed layer maximum depth was determined as 
the depth at which a change from the surface density of 
0.125kg m”? has occurred (Levitus, 1982). Erroneous esti- 
mates of mixed layer maximum depth have been corrected 
manually on five profiles. Other R packages used in this 
study include corrplot v0.84 (Dray, 2008) and ggplot2 v3.3.3 
(Wickham, 2016). Section plots were made using Ocean 
Data View v5.6.2 (Schlitzer, 2020). Depth integrated values 
were calculated using the midpoint rule. 
In order to compare stations along the zonal transect and 
within the eddy, data were integrated over the water column 
(0-100 m depth). Along the zonal transect, depth-integrated 
Chl a concentration ranged between 11.7 and 58.7 mg m”? 
and decreased from the coastal to the open ocean stations 
(Table 1; Fig. 3b). Depth distribution showed a Chl a max- 
imum in the open ocean around —75m from 23.61 to 
3 Results 
3.1 Hydrographic conditions 
Along the zonal transect, open ocean waters (from 20 to 
24.5° W) had a temperature range of 13.45—-24.2 °C and 
Biogeosciences, 19. 51995219. 2027 
https://doi.org/10.5194/bg-19-5199-2022