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 parallel factor 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.
N2 - Plain Language Summary: Mesoscale eddies are ubiquitous circulation features in the ocean with horizontal scales on the order of 100 km and lifetimes of days to months. Their swirling motion can cause nutrients from deeper waters to be transported to the surface, stimulating phytoplankton biomass and resulting in the production of dissolved organic matter. However, these effects are difficult to quantify and proxies (biomarkers) are needed to monitor the impact of eddies at high resolution. In this work, we used the optical properties of the dissolved organic matter, especially the fraction capable of fluorescence (FDOM) as biomarker in two cyclonic eddies, one formed in an eastern boundary upwelling system and one formed offshore by winds/Island interaction. We identified four FDOM components, among which an indicator of cyclonic eddy productivity and two indicators of dissolved organic matter recycling, which also tracked nutrient transport in the offshore cyclonic eddy. Our study highlights that continuous FDOM data obtained with sensors could help to follow eddy development and influence on seawater biogeochemistry. N2 - Key Points:Four fluorescent dissolved organic matter (FDOM) components were studied in two cyclonic eddies (CEs) in the Eastern Tropical North Atlantic
Tryptophan‐like FDOM was an indicator of the CEs' productivity as it correlated with semi‐labile dissolved organic matter and microbial metabolic activities
Humic‐like FDOM was a by‐product of microbial respiration; its distribution within an offshore CE covaried with nutrient upwelling