Q. Devresse et al.: Eddy-enhanced primary production sustains heterotrophic microbial activities 
5215 
frontal zone are unknown. Despite the low HB abundance, 
BP was particularly stimulated in these areas. On average, 
BP was 3-fold higher in the eddy-influenced stations com- 
pared to the open ocean ones when integrated over 100m. 
This is in accordance with earlier studies from the Sargasso 
Sea (Ewart et al., 2008), the CanUS (Baltar et al., 2010), and 
che Mediterranean Sea (Belkin et al., 2022), where enhanced 
BP has been observed in CEs. As stated previously, the up- 
welling induced by the CE and the frontal zone led to higher 
phytoplankton biomass, which was likely responsible for this 
overall increase in BP. However, it is noteworthy that BP and 
PPror rates were less correlated than in the zonal transect. 
BR rates were also enhanced at the surface of the CE and fol- 
lowed a similar trend as BP. SL-DOC concentrations showed 
a strong positive correlation with BR, indicating that high- 
molecular-weight DOC compounds (>1kDa) are an avail- 
able carbon source for heterotrophic microbes (Amon and 
Benner, 1994, 1996; Benner and Amon, 2015). PPpoc rates 
ın the CE covered 27.9 % to 110 % of the BCD, suggesting a 
moderate to strong trophic dependence of bacteria on phyto- 
plankton in CE. Although PPror may satisfy the BCD in the 
CE (43.1 % to 341 %), a question remains about why BGE 
was so variable and low in some parts of the CE with values 
down to 2.7 %. One explanation might be that variability in 
nutrient availability in the surface waters limited the building 
of bacterial biomass (Thingstad et al., 1997; Jansson et al., 
2006; Berggren et al., 2010), but this requires further study. 
Overall, we showed that autotrophy prevails in the upper 
100 m depth of Mauritanian coastal waters, while heterotro- 
phy prevailed offshore. This is consistent with a modelling 
study from Lovecchio et al. (2017). The CE and the associ- 
ated frontal zone fuelled phytoplankton nutrient needs and 
maintained autotrophy further offshore inside of the eddy 
and especially in the frontal zone, where the highest PPror 
were measured. Mourifio-Carballido (2009) reported from 
indirect estimations of net community production that the 
frontal zones between CEs and ACEs are among the most 
productive areas in the northwest subtropical Atlantic Ocean. 
Previous studies have shown that the trophic balance could 
switch from autotrophy to heterotrophy in an eddy within 
a month (Maixandeau et al., 2005; Mourifio-Carballido and 
McGillicuddy, 2006). Here we showed that both autotrophy 
and heterotrophy can occur within a single eddy. This urges 
the need for more high-resolution eddy studies in order to 
better estimate their impact on plankton metabolic activities 
and carbon cyvcling. 
pensate for bacterial metabolic needs. Even if BP was en- 
hanced in the CE, the BGE was rather low and varied sub- 
stantially. Instead, heterotrophic bacteria preferentially used 
DOM for respiration. Microbial metabolic activity dynamics 
within eddies are complex and require further investigations 
to better understand and unravel carbon cycling in these fea- 
tures. 
Data availability. All data will be made available at the 
PANGAEA database https://doi.pangaea.de/10.1594/PANGAEA. 
950510 (Devresse et al.. 2022). 
Supplement. The supplement related to this article is available on- 
line at: https://doi.org/10.5194/bg-19-5199-2022-supplement. 
Author contributions. QD, KWB, and AE designed the scientific 
study, analysed the data, and wrote the paper. AB did the eddy re- 
construction, and both AB and JH commented on the paper. 
Competing interests. The contact author has declared that none of 
the authors has any competing interests. 
Disclaimer. Publisher’s note: Copernicus Publications remains 
neutral with regard to jurisdictional claims in published maps and 
institutional affiliations. 
Special issue statement. This article is part of the special issue 
“Concerted multidisciplinary and multi-platform eddy studies off 
West Africa: mesoscale and sub-mesoscale dynamics and physical- 
chemical-biological coupling (BG/ESSD/OS inter-journal SI)”. It is 
not associated with a conference. 
Acknowledgements. We thank the captain and the crew of the R/V 
Meteor for their support during the M156 cruise. We thank Jon Roa, 
Tania Klüver and Lindsay Scheidemann for sampling on board. We 
thank the two anonymous reviewers for their constructive comments 
on earlier versions of the manuscript. We thank Jon Roa and San- 
dra Golde additionally for the analysis of dissolved organic matter 
and Tania Klüver for cell counting and bacterial and phytoplankton 
activity analyses. We thank Bettina Domeyer and Regina Surberg 
for the nutrient analyses. This study has been conducted using EU 
Copernicus Marine Service information. The results contain modi- 
fied Copernicus Climate Change Service information for 2020. Nei- 
ther the European Commission nor ECMWF is responsible for any 
use that may be made of the Copernicus information or data it con- 
tains. This study is a contribution of the REEBUS project (Role of 
Eddies in the Carbon Pump of Eastern Boundary Upwelling Sys- 
tems) sub-proijects WP1 and WP4. 
5 Conclusions 
Our results highlight the ability of a CE to be an autotrophic 
vector toward the open ocean with organic matter freshly 
produced by the phytoplankton community inside. Yet, de- 
spite the strong autotrophy associated with the CE, phyto- 
plankton exudation of DOM was not always enough to com- 
https://doi.org/10.5194/bg-19-5199-202} 
Biogeosciences, 19, 51995219, 2022