Fluxes and seasonal changes in composition of organic matter in the English Channel

Dissolved and particulate organic matter (DOM and POM) have been investigat ed along a transect between Cherbourg and the Isle of Wight, In addition, t he relative contribution of different sources of POM have been assessed by the use of lipid biomarkers (e,g. fatty acids). Seawater samples were colle cted at two depths (subsurface and above the bottom) at five stations locat ed on the transect during five cruises (from September 1994 to July 1995). Particulate organic carbon (POC) and dissolved organic carbon (DOC) concent rations vary between 30-530 mu g l(-1) and 0.5-2.7 mg l(-1), respectively, for all the cruises. Fluxes of POM and DOM have been estimated at 0.6 x 10( 12) g yr(-1) and 6.5 x 10(12) g yr(-1) of carbon, respectively. General flu xes of water and therefore of DOC and POC are oriented eastward. However, a round the Isle of Wight a west-ward oriented flux exists due to a gyre loca ted in the area. The major DOC and POC fluxes occur in the central part of the Channel where the water column is deepest. Seasonal variations of diffe rent sources of POM (algal, bacterial and terrigenous) have been examined f or the five cruises. The fresh algal organic fraction is relatively importa nt in September in coastal waters with a predominance of diatom species on the English side, whereas it has a low or undetectable contribution during winter months. The bacterial fraction generally varies in concert with the algal component. It is low during the winter period and more important in b loom or post-bloom conditions, as for example in May. Terrestrial organic m atter is restricted to coastal areas in September, and is present at low le vels in May and July. Nevertheless, in November and February, terrigenous i nputs have been clearly identified for the whole transect even in central w aters. (C) 1999 Elsevier Science Ltd. All rights reserved.