Biogeosciences, 19, 5199-5219, 2022
https://doi.org/10.5194/bg-19-5199-2022
© Author(s) 2022. This work is distributed under
che Creative Commons Attribution 4.0 License.
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Eddy-enhanced primary production sustains heterotrophic
microbial activities in the Eastern Tropical North Atlantic
Quentin Devresse!, Kevin W. Becker!, Arne Bendinger!”, Johannes Hahn'->, and Anja Engel‘
!GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
’Laboratoire d’Etudes en Ge&ophysique et Oc&anographie Spatiales (LEGOS),
Universite Toulouse, IRD, CNRS, CNES, UPS, Toulouse, France
Bundesamt für Seeschifffahrt und Hydrographie, Hamburg, Germany
Correspondence: Quentin Devresse (qdevresse@ geomar.de)
Received: 18 February 2022 — Discussion started: 21 February 2022
Revised: 17 October 2022 — Accepted: 18 October 2022 — Published: 17 November 2022
Abstract. Mesoscale eddies modulate the ocean’s physi-
cal, chemical, and biological properties. In cyclonic eddies
(CEs), nutrient upwelling can stimulate primary produc-
tion by phytoplankton. Yet, how this locally enhanced au-
cotrophic production affects heterotrophy and consequently
the metabolic balance between the synthesis and the con-
sumption of dissolved organic matter (DOM) remains largely
ınknown. To fill this gap, we investigated the horizontal and
vertical variability in auto- and heterotrophic microbial ac-
tivity (biomass production and respiration) within a CE that
formed off Mauritania and along the — 900 km zonal corri-
dor between Mauritania and the Cape Verde islands in the
Eastern Tropical North Atlantic (ETNA). Our results show
how the physical disturbances caused by the CE affected
the biomass distribution of phyto- and bacterioplankton and
‘heir metabolic activities. The injection of nutrients into the
sunlit surface resulted in enhanced autotrophic pico- and
nanoplankton abundance and generally increased autotrophic
activity as indicated by chlorophyll a (Chl a) concentra-
:jon, primary production (PP), and extracellular release rates.
However, the detailed eddy survey also revealed an uneven
distribution of these variables with, for example, the high-
est Chl a concentrations and PP rates occurring near and just
beyond the CE’s periphery. The heterotrophic bacterial ac-
tivity was similarly variable. Optode-based community res-
piration (CR), bacterial respiration (BR) estimates, and bac-
terial biomass production (BP) largely followed the trends
of PP and Chl a. Thus, a submesoscale spatial mosaic of het-
erotrophic bacterial abundance and activities occurred within
the CE that was closelv related to varilability in autotrophic
production. Consistent with this, we found a significant pos-
itive correlation between concentrations of semi-labile dis-
solved organic carbon (SL-DOC; here the sum of dissolved
hydrolysable amino acids and dissolved combined carbohy-
drates) and BR estimates. Extracellular release of carbon as
indicated by primary production of dissolved organic carbon
(PPpoc) was variable with depth and laterally and not al-
ways sufficient to compensate the bacterial carbon demand
{BCD: BR + BP), with PPpoc accounting for between 28 %
and 110% of the BCD. Bacterial growth efficiency (BGE:
BP/BCD) ranged between 1.7 % and 18.2 %. We estimated
the metabolic state to establish whether the CE was a source
or a sink of organic carbon. We showed that the CE carried
a strong autotrophic signal in the core (PP / CR > 1). Our re-
sults suggest that submesoscale (0-10km) processes lead to
highly variable metabolic activities in both photoautotrophic
and heterotrophic microorganisms. Overall, we revealed that
the CEs not only trap and transport coastal nutrients and
organic carbon to the open ocean but also stimulate phyto-
plankton growth, generating freshly produced organic mat-
ter during westward propagation. This drives heterotrophic
processes and may contribute to the previously observed net
heterotrophy in open Atlantic surface waters.
1 Introduction
Mesoscale eddies (10-100km) are ubiquitous in the ocean.
affecting upper-ocean biogeochemistry and ecology. For ex-
ample, upwelling of nutrients inside eddies can enhance pri-
Published by Copernicus Publications on behalf of the European Geosciences Union.
