New insights on dissolved organic matter cycling in the Cape Verde frontal zone (CVFZ) from ITS optically active fraction

Abstract

The CVFZ is a highly dynamic ocean margin characterised by two well-defined oceanographic features: the thermohaline Cape Verde Front (CVF), that separates the subtropical from the tropical waters of the Northeast Atlantic, and the Cape Blanc Giant Filament (CBGF), which exports the organic matter produced in the Mauritanian upwelling to the open ocean. During an oceanographic cruise in the summer of 2017, we sampled a hydrographic box encompassing the CVFZ. Full-depth profiles of absorption spectra of coloured (CDOM) and excitation-emission matrices of fluorescent (FDOM) dissolved organic matter were collected and examined to produce an assortment of CDOM indices and 5-FDOM PARAFAC components. In epipelagic waters, we identified distinctive optical signatures in open ocean waters north and south of the CVF and in coastal waters transported by the CBGF that provide new insights on the origin (terrestrial vs. marine), photochemical and microbial reactivity (lability vs. recalcitrance), and molecular properties (molecular weight, aromaticity) of dissolved organic matter (DOM). In mesopelagic waters, we optically characterised the water masses of opposing origin that meet in the CVFZ: North and South Atlantic Central Water, Subpolar Mode Water, Mediterranean Water, Antarctic Intermediate Water and Labrador Seawater. In bathypelagic waters, we recognised the bottom nepheloid layer close to the Mauritanian coast as a massive source of optically active DOM that is not noticeable in dissolved organic carbon (DOC) distibutions