MEPS 297:33-50 (2005)  -  doi:10.3354/meps297033

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 (FDOM_M) 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 (FDOM_T) correlated significantly with the distribution of particulate proteins (r > 0.57, p < 0.001). FDOM_T 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.

KEY WORDS: Dissolved organic matter · Fluorescence · Humic substances · Amino acids · Seasonal cycle · Coastal upwelling · NW Iberian Peninsula