Microbial and photochemical reactivity of fluorescent dissolved organic matter in a coastal upwelling system

We observed significant changes in the dissolved oxygen content and the fluorescence of humic substances and dissolved aromatic amino acids after 24 h light and dark incubations in the coastal upwelling system of the Rı´a de Vigo under a wide variety of meteorologic and oceanographic conditions. Respiration rates were inversely correlated with the net production of humic fluorescence in the dark at a net rate of 20.027 6 0.003 mg equivalents of quinine sulphate per mmol of O2, suggesting that marine humics are a by-product of the bacterial respiration of dissolved organic matter (DOM). On the contrary, humic fluorescence consumption in the light minus dark incubations was positively correlated with the net production in the dark, indicating a rapid photodegradation of recently produced marine humic substances. Parallel incubation experiments demonstrated that daily photodegradation rates and residual humic fluorescence levels followed a seasonal pattern characterized by a marked autumn maximum. Finally, a significant linear correlation between the gross primary production (Pg) and the net production of aromatic amino acids fluorescence in the light pointed to the rapid consumption of dissolved protein-like materials at a net average rate of 21.4 6 0.2 ppb equivalents of tryptophan per day, which accumulates in the water column only when Pg exceeds 80 6 20 mmol kg21 d21