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

Abstract

Mesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post−upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with PARAFAC analysis, two humic−like, and two protein−like components. Humic−like FDOM components correlated to optode‐based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g. temperature) and to dissolved inorganic nitrogen. The tryptophan−like FDOM components correlated with the carbon and nitrogen content of semi−labile DOM, phytoplankton biomass, community respiration and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies.

Water aging and the quality of organic carbon sources drive niche partitioning of the active bathypelagic prokaryotic microbiome

“…The protein-like components C3 and C4 have been attributed to tryptophan and tyrosine, respectively (Catal a et al 2015b). A good correlation between the tryptophan-like FDOM component and the carbon (carbohydrates) and nitrogen (amino acids) content of semi-labile DOM was recently found (Devresse et al 2023), supporting the use of FDOM properties to trace the lability of OM. In addition, there is a tight covariation of the two labile C3 and C4 FDOM components between surface and bathypelagic waters (Ruiz-Gonz alez et al 2020), suggesting that sinking particles arriving to the deep ocean release these components.…”

Distribution of polysaccharidic and proteinaceous gel−like particles in three cyclonic eddies in the Eastern Tropical North Atlantic

“…During the cruise M156, 25 stations were sampled; with 14 of them inside the Mau eddy (Devresse et al, 2022). During the cruise M160, 26 stations were sampled with 4 stations inside the Brava eddy (Devresse et al, 2023) and 5 inside the Sal eddy (Figure 1). Sampling was conducted in the epi−mesopelagic layer (0−800m during M156 and 0−400m during M160), including samples from the surface mixed layer, the Chl−a maximum, and the shallow oxygen minimum zone (OMZ;<50 μM kg −1 between 0−200 m) when present.…”

“…Analysis of eddy statistics from an eddy detection and tracking algorithm (Le Vu et al, 2018) established Mau, Brava and Sal CEs to be 1.5, 1.5, and 4 months old, respectively. The Mau and Brava CEs were extensively described in (Devresse et al, 2022(Devresse et al, , 2023. Briefly, during M156, the Mau CE was sampled at ∼ 200km off the Mauritanian Coast.…”

“…In M160, the Brava and Sal CEs were sampled near the Brava and Sal Islands respectively (Figure 1). Peripheries of Mau, Brava (Devresse et al, 2022(Devresse et al, , 2023, and Sal (Supplementary Figure S2) CEs were defined as the area influenced by eddies, based on sea surface temperature, salinity, dissolved inorganic nitrogen (DIN) and Chl−a data through high−resolution sampling. During cruise M156, just beyond the eddy periphery at St. E3, a front was observed with surface temperature and salinity (not compensated by density) different from the adjacent stations.…”

“…Analyses of semi−labile dissolved organic matter are described in detail in Devresse et al (Devresse et al, 2022(Devresse et al, , 2023. Briefly, we determined high−molecular−weight (HMW > 1 kDa) dissolved combined carbohydrates (dCCHO) and dissolved hydrolyzable amino acids (dHAA) as the main biochemical components of DOM (Carlson, 2002).…”