Comment on bg-2022-49

Abstract

Mesoscale eddies play essential roles in modulating the ocean's physical, chemical, and biological properties. In cyclonic eddies (CE) nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophic bacterial activities (biomass production and respiration) and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To address this gap, we investigated the horizontal and vertical variability of phytoplankton and heterotrophic bacterial activity along ~900 km zonal corridor between the coast of Mauretania and the Cape Verde Islands in the eastern tropical North Atlantic (ETNA). We additionally collected samples from a CE along this transect at high spatial resolution. Our results show cascading effects of physical disturbances induced by a CE on phyto- and bacterioplankton biomass and metabolic activities. Specifically, the injection of nutrients into the sunlit surface resulted in enhanced autotrophic plankton abundance and activity as indicated by Chlorophyll a (Chl-a) concentration, DOM exudation, and primary productivity (PP). However, the detailed eddy survey revealed an uneven distribution of these parameters with, for example, the highest Chl-a concentrations and PP rates near and just beyond the CE’s periphery. The heterotrophic bacterial activity was similarly variable. Optode-based bacterial respiration (BR) and 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 studied here that was closely related to variability in autotrophic production. This was supported by a significant positive correlation between concentrations of semi-labile organic carbon (SL-DOC; the sum of dissolved hydrolyzable amino acids and combined carbohydrates) and BR measurements. Bacterial growth efficiency (BP/(BR+BP)) was variable (1.4–10.5 %) within the CE and carbon exudation was not always sufficient to compensate the bacterial carbon demand (BR+BP; 28.3–114.5 %). We have additionally estimated the metabolic state in our samples, which showed that the CE carried a strong autotrophic signal (PP/(BR+BP) > 1). Overall, our results show that submesoscale (0–10 km) processes lead to highly variable metabolic activities of both phototrophic and heterotrophic microbes, which has implications for biogeochemical models estimating oceanic carbon fluxes. Additionally, we revealed that the CE not only traps and transports coastal nutrients and carbon to the open ocean but also stimulates phytoplankton growth generating freshly produced organic matter during westward propagation. This organic matter may fuel heterotrophic processes in the open ocean and may help to explain the often-observed net heterotrophic metabolic state of these environments.