Distribution and characterisation by fluorescence of the dissolved organicmatter within the central Channel waters.

Intensity and spectral characteristics of the fluorescence of water samples collected along the transect Cherbourg-Isle of Wight during four cruises f or excitations at 370, 313 and 270 nm have been investigated within the fra mework of the FluxManche II program. Seasonal and spatial differences appea red to reflect on the one hand, variations in terrestrial inputs, and on th e other hand, waters masses structuring. The observed linear variation of t he fluorescence intensity with salinity indicates a gradual dilution of the continental humic material in going from the coasts to the central part of the English Channel. The largest signal was observed for the English coast al waters in agreement with important terrestrial inputs from the Solent ri ver. While the fluorescence intensity was not found to be correlated with t he DOG, it shows a good correlation with the nutrients. This result could i ndicate both a supply from terrestrial inputs and an almost simultaneous au tochtonous regeneration. As similarly reported for waters in the eastern pa rt of the French coastal zone, excitation at 313 nm gives evidence for the presence of two classes of fluorescent dissolved organic matter. These two fluorescent components are indicative of the simultaneous presence of conti nental humic substances and substances whose assignment to marine humic sub stances or to heterotrophic related substances are still an open question. As compared to the eastern part of the Channel along the French coasts, exc itation at 270 nm indicates a lower content of protein-like compounds which may be related to a lower biological activity along the transect or may be due to the fact that the samplings were made out of the phytoplanktonic bl oom period (April-May 1995). (C) Elsevier, Paris.