Production and degradation of fluorescent dissolved organic matter in surface waters of the eastern North Atlantic Ocean

The distribution and fate of coloured dissolved organic matter (CDOM) in the epipelagic Eastern North Atlantic was investigated during a cruise in the summer 2009 by combining field observations and culture experiments. Dissolved organic carbon (DOC) and nitrogen (DON), the absorption spectra of CDOM and the fluorescence intensity of proteins (Ex/Em 280/320nm; F(280/320)) and marine humic-like substances (F(320/410)) were measured in the upper 200m. DOC and DON showed higher concentrations in the top 20m than below, and DOC increased southwards, while DON decreased. F(280/320) and F(320/410) showed maxima near the deep chlorophyll maximum (at about 50m), suggesting that these fluorophores were linked to phytoplankton production and the metabolism of the associated microbial community. The coloured and fluorescent fractions of DOM showed low levels south of the Azores Front, at about 35°N, likely due to the accumulated photobleaching of the waters transported eastwards by the Azores current into the study area (at 20°W). Twelve culture experiments were also conducted with surface water (5m) to assess the impact of microbial degradation processes on the bulk, coloured and fluorescent fractions of DOM. After 72h of incubation in the darkness, 14±9% (average±SD) of the initial DON was consumed at an average rate of 0.24±0.14μmol l ^-1 d ^-1 and the protein-like fluorescence decayed by 29±9% at a net rate of 0.06±0.03 QSU d ^-1. These rates were significantly lower south of the Azores front, suggesting that DOM in this region was of a more recalcitrant nature. Conversely, the marine humic-like fluorescence increased at a net rate of 0.013±0.003 QSU d ^-1. The close linear relationship of DON uptake with F(280/320) consumption (R ²= 0.91, p <0.0001, n=12) and F(320/410) production (R ²= 0.52, p <0.008, n=12) that we found during these incubation experiments suggest that the protein-like fluorescence can be used as a proxy for the dynamics of the labile DON pool and that marine humic-like materials can be produced as a by-product of microbial DOM degradation.