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

Abstract

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.

Dissolved Organic Matter Fluorescence as a Tracer of Upwelling and Microbial Activities in Two Cyclonic Eddies in the Eastern Tropical North Atlantic

“…During the cruise M160, humic‐like FDOM components C468 and C398 correlated significantly with DIN, and physical (temperature, oxygen) and biogeochemical (DOC) parameters highlighting well the upwelling occurring in the CE. In contrast, those correlations were weaker (for C398) and even inverse (for C468) during cruise M156 and the humic‐like FDOM components distribution did not follow the upwelling (Figure 2 and Figure S4 in Supporting Information ; Devresse et al., 2022). We propose two hypotheses regarding those differences that are depicted in Figure 10: The Mau eddy trapped coastal water enriched in humic‐like FDOM during his formation.…”

“…The first CE (Mau eddy) was sampled in summer 20 km offshore the Mauritanian Coast during M156 along two zonal transects (19.1°W−18.2°W at 18.3°N and 18.5°W−17.1°W at 18.6°N) and one meridional transect (19.4°N−18°N at 18.4°W–18.1°W). In addition, we sampled water along an 18°N transect, a typical coast‐to‐open ocean trajectory of eddies in this region (Devresse et al., 2022; Schütte et al., 2016; Figure 1). The second CE was sampled in winter during the cruise M160, in the vicinity of Brava Island (Brava eddy).…”

“…Analyses of microbial metabolic activities are described in detail in Devresse et al. (2022). Briefly, PP was determined from 14 C incorporation according to Gargas (1975) and Nielsen (1952).…”

“…In the subtropical gyres of the World's oceans, inorganic nutrients (e.g., nitrate or phosphate) limit microbial activities of dissolved organic matter (DOM) production and degradation (Alonso‐Sáez et al., 2007; Moore et al., 2013; Romera‐Castillo et al., 2016). Eddies, through their passages, might suppress those limitations and enhance microbial activities (Devresse et al., 2022; Ewart et al., 2008). The phytoplankton response to nutrient is nonlinear, with a lag phase with a lag phase of 1–3 days between uptake of nutrients and plankton bloom (e.g., Collos, 1986; Wetz & Wheeler, 2004).…”

“…For this reason, we combined different approaches to define eddy structures as the area influenced by eddies. For the Mau eddy, we used the same border as described in details in Devresse et al (2022), and for the Brava eddy, we used surface temperature, salinity, and Chl-a data through high-resolution sampling to approximate eddies' periphery as the area influenced by eddies. During M156, the Mau eddy center was located at 18.69°N−18.05°W, on 22 July 2019, with a core radius of 40.5 ± 5.7 km and a mean azimuthal velocity of 19.9 ± 0.7 cm s −1 .…”