DOM fluorescence, a tracer for biogeochemical processes in a coastal upwelling system (NW Iberian Peninsula)

Abstract

Fluorescence of dissolved organic matter (FDOM) was determined for the first time during a complete seasonal cycle in a coastal upwelling system (Ría de Vigo, NW Spain). Specific excitation/emission wavelengths for the refractory humic substances and the labile aromatic amino acids were detected and quantified. The distribution of the fluorescence of marine humic substances (FDOMM) correlated significantly with nutrient salts (r = 0.62, p < 0.001) and dissolved oxygen (r < –0.71, p < 0.001), after removal of the effect of water masses mixing. It suggests that humic substances are a by-product of organic matter mineralisation processes in shelf bottom waters. It was estimated that about 10% of the organic carbon degraded in the water column was transformed into humic substances, which were produced in the ría at a rate of about 0.1 ppb eq (parts per billion equivalents) quinine sulphate d–1. The distribution of the fluorescence of dissolved aromatic amino acids (FDOMT) correlated significantly with the distribution of particulate proteins (r > 0.57, p < 0.001). FDOMT can be used to trace sites of net accumulation of labile DOM in (1) the photic layer (at a rate of about 0.5 ppb eq tryptophan d–1), where labile DOM is produced from phytoplankton exudation and/or lysis and (2) the bottom nepheloid layer (at a rate of about 0.2 ppb eq tryptophan d–1), due to an in situ enhancement of the microbial activity and/or a release from sediment pore waters.