New Insights on Dissolved Organic Matter Cycling in the Cape Verde Frontal Zone From Its Optically Active Fraction

Abstract

Absorption and fluorescence spectroscopy have been used to gain new insights on the dynamics of Dissolved Organic Matter (DOM) in the Cape Verde Frontal Zone (CVFZ), a highly dynamic area comprising the thermohaline Cape Verde Front (CVF) and the Cape Blanc Giant Filament (CBGF), which exports organic matter produced over the shelf to the adjacent open ocean. A full‐depth hydrographic box embracing the CVF and the CBGF was occupied in summer 2017 to examine the impact of these hydrographic structures and local‐scale remineralization processes on DOM cycling using the distributions of the colored (CDOM) and fluorescent (FDOM) fractions of DOM as tracers. In the surface layer, we observed contrasting optical properties between the stratified, productive tropical waters south of the CVF with higher signals of optically active substances with higher molecular weight, and subtropical waters North of the CVF with lower signals and lower average molecular weight, pointing to lower microbial production and more intense or sustained photochemical degradation. In the ocean interior, although the mixing of intermediate and deep waters masses of contrasting origins and large‐scale mineralization were the main factors controlling the distributions of the bulk and colored fractions of DOM, local‐scale net accumulation of refractory humic‐like and net consumption of labile protein‐like components were observed in the mesopelagic layer. In bathypelagic waters, the optical properties of DOM allowed to characterize a bottom nepheloid layer (BNL) close to the Mauritanian coast, characterized by prominent CDOM and FDOM signals that were not noticeable in dissolved organic carbon (DOC) profiles.