PARTICULATE ORGANIC-MATTER COMPOSITION IN THE ESTUARINE TURBIDITY MAXIMA (ETM) OF THE SEINE RIVER ESTUARY

Organic matter (OM) was investigated in particles collected in the Est uarine Turbidity Maximum (ETM) zone of the Seine River estuary, for tw o different hydrodynamic regimes in February and September 1995. Durin g ebb resuspension, occurrence of the ETM was documented two to four h ours before ebb. The discharge regime greatly influence the position a nd importance of the ETM. It occurs more prominently during high flow than during low flow. Suspended particulate matter (SPM) and particula te organic carbon (POC) maxima, as well as surface-bottom concentratio n gradients, decrease when river flow is enhanced: During flood resusp ension a second peak was observed, but only for bottom waters. This in dicates decoupling of surface (fresh waters) and bottom layers (bracki sh/marine waters) at the beginning of the flood. Organic matter conten t in SPM remained almost unchanged (2 to 3%) and only a slight decreas e was noticed during the ETM. Recent particulate organic matter source s such as terrestrial plant, phytoplankton, bacteria and domestic sewa ge were differentiated by the use of lipidic biomarkers (fatty acids a nd sterols). In the ETM, terrestrial plant, sewage and bacterial input s are at maximum. On the other hand, phytoplankton abundance is minimu m in ETM but it may be important during slack periods. This is the cas e in September, when evidence is given for a diatom bloom during high tide slack.