Coloured dissolved organic matter (CDOM) in Southern North Sea waters: Optical characterization and possible origin

The variability and origin of the Coloured Dissolved Organic Matter (CDOM) were studied in the Belgian coastal and adjacent areas including offshore waters and the Scheldt estuary, through the parameters: absorption at 375 nm, a(CDOM)(375), and the slope of the absorption curve, S. a(CDOM)(375) varied between 0.20 and 1.31 m(-1) and between 0.97 and 4.30 m(-1) in the marine area and Scheldt estuary, respectively. S fluctuated between 0.0101 and 0.0203 nm(-1) in the marine area and between 0.0167 and 0.0191 nm(-1) in the Scheldt estuary. The comparative analysis Of a(CDOM)(375) and S variations evidenced different origins of CDOM in the BCZ. The Scheldt estuarine waters showed decreasing a(CDOM)(375) values with increasing salinity but constant S value of similar to 0.018 nm(-1) suggesting a dominant terrestrial origin of CDOM. On the contrary, samples collected in the marine domain showed a narrow range of a(CDOM)(375) but highly variable S suggesting the additional presence of autochthonous sources of CDOM. This source was evidenced based on the sorting of the marine offshore data according to the stage of the phyto-plankton bloom when they were collected. A clear distinction was made between CDOM released during the growth stage characterized by high S (similar to 0.017 nm(-1)) and low a(CDOM)(375) and the decay phase characterized by low S (similar to 0.013 nm(-1)) and high a(CDOM)(375). This observation was supported by CDOM measurements performed on pure phytoplankton cultures which showed increased CDOM release along the wax and wane of the bloom but decreasing S. We concluded that the high variability of the CDOM signature in offshore waters is explained by the local biological production and processing of CDOM. (C) 2009 Elsevier Ltd. All rights reserved.