Combined effect of light exposure and microbial activity on distinct dissolved organic matter pools. A seasonal field study in an oligotrophic coastal system (Blanes Bay, NW Mediterranean)

A harmonic analysis of three years of data collected with fortnight to monthly frequency in the oligotrophic Bay of Blanes (NW Mediterranean) revealed that the water column mixing-stratification cycle dictated the seasonal build-up of the bulk and different coloured fractions of dissolved organic matter (DOM). Dissolved organic carbon (DOC) accumulated at late summer, reaching the annual maximum by early September, half a month later than water temperature. The seasonal cycle of the protein-like fluorescence (F(280/350)) was in phase with DOC, suggesting that reduced heterotrophic activity, due to severe P limitation, is likely the reason behind the late summer accumulation of these materials. The absorption due to conjugated carbon double bonds of DOM (aCDOM(254)) reached the annual maximum by early August, concomitant to the seasonal maximum of microbial activity, suggesting that biological production prevailed over photo-degradation of these compounds. On the contrary, the fluorescence of humic-like substances absorbing in the UV-A region (F(340/440)) presented the annual maximum in early February, coinciding with the seasonal maximum of autotrophic biomass, and the minimum in early August, because of the prevalence of photo-degradation over microbial production. The optical properties of DOM allowed distinguishing between three DOC pools of contrasting origin, photo- and bio-reactivity, in the oligotrophic NW Mediterranean Sea.