Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic

[摘要] Mesoscale eddies modulate the ocean's physical, chemical, and biologicalproperties. In cyclonic eddies (CEs), nutrient upwelling can stimulateprimary production by phytoplankton. Yet, how this locally enhancedautotrophic production affects heterotrophy and consequently the metabolicbalance between the synthesis and the consumption of dissolved organicmatter (DOM) remains largely unknown. To fill this gap, we investigated thehorizontal and vertical variability in auto- and heterotrophic microbialactivity (biomass production and respiration) within a CE that formed offMauritania and along the ∼  900 km zonal corridor betweenMauritania and the Cape Verde islands in the Eastern Tropical North Atlantic(ETNA). Our results show how the physical disturbances caused by the CEaffected the biomass distribution of phyto- and bacterioplankton and theirmetabolic activities. The injection of nutrients into the sunlit surfaceresulted in enhanced autotrophic pico- and nanoplankton abundance andgenerally increased autotrophic activity as indicated by chlorophyll  a (Chl  a ) concentration, primary production (PP), and extracellular releaserates. However, the detailed eddy survey also revealed an unevendistribution of these variables with, for example, the highest Chl  a concentrations and PP rates occurring near and just beyond the CE'speriphery. 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 andChl  a . Thus, a submesoscale spatial mosaic of heterotrophic bacterialabundance and activities occurred within the CE that was closely related tovariability in autotrophic production. Consistent with this, we found asignificant positive correlation between concentrations of semi-labiledissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysableamino acids and dissolved combined carbohydrates) and BR estimates.Extracellular release of carbon as indicated by primary production ofdissolved organic carbon (PP DOC ) was variable with depth and laterallyand not always sufficient to compensate the bacterial carbon demand (BCD:BR  +  BP), with PP DOC 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 ora sink of organic carbon. We showed that the CE carried a strong autotrophicsignal in the core (PP  /  CR  >  1). Our results suggest thatsubmesoscale (0–10 km) processes lead to highly variable metabolicactivities in both photoautotrophic and heterotrophic microorganisms.Overall, we revealed that the CEs not only trap and transport coastalnutrients and organic carbon to the open ocean but also stimulatephytoplankton growth, generating freshly produced organic matter duringwestward propagation. This drives heterotrophic processes and may contributeto the previously observed net heterotrophy in open Atlantic surface waters.

[学科分类] 大气科学