Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic
Authors: Devresse Quentin , Becker Kevin W. , Bendinger Arne , Hahn Johannes , Engel Anja 
Published: November 2022

Abstract. Mesoscale eddies modulate the ocean's physical, chemical, and biological
properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate
primary production by phytoplankton. Yet, how this locally enhanced
autotrophic production affects heterotrophy and consequently the metabolic
balance between the synthesis and the consumption of dissolved organic
matter (DOM) remains largely unknown. To fill this gap, we investigated the
horizontal and vertical variability in auto- and heterotrophic microbial
activity (biomass production and respiration) within a CE that formed off
Mauritania and along the ∼ 900 km zonal corridor 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 their
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) concentration, primary production (PP), and extracellular release
rates. However, the detailed eddy survey also revealed an uneven
distribution of these variables with, for example, the highest Chl a
concentrations and PP rates occurring near and just beyond the CE's
periphery. The heterotrophic bacterial activity was similarly variable.
Optode-based community respiration (CR), bacterial respiration (BR) estimates,
and bacterial biomass production (BP) largely followed the trends of PP and
Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial
abundance and activities occurred within the CE that was closely related to
variability in autotrophic production. Consistent with this, we found a
significant positive correlation between concentrations of semi-labile
dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable
amino acids and dissolved combined carbohydrates) and BR estimates.
Extracellular release of carbon as indicated by primary production of
dissolved organic carbon (PPDOC) was variable with depth and laterally
and not always sufficient to compensate the bacterial carbon demand (BCD:
BR + BP), with PPDOC 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 results suggest that
submesoscale (0–10 km) 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
phytoplankton growth, generating freshly produced organic matter during
westward propagation. This drives heterotrophic processes and may contribute
to the previously observed net heterotrophy in open Atlantic surface waters.