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 Estuarine Turbidity Maximum (ETM) zone of the Seine River estuary, for two different hydrodynamic regimes in February and September 1995. During ebb resuspension, occurrence of the ETM was documented two to four hours before ebb. The discharge regime greatly influence the position and importance of the ETM. It occurs more prominently during high flow than during low flow. Suspended particulate matter (SPM) and particulate organic carbon (POC) maxima, as well as surface-bottom concentration gradients, decrease when river flow is enhanced. During flood resuspension a second peak was observed, but only for bottom waters. This indicates decoupling of surface (fresh waters) and bottom layers (brackish/marine waters) at the beginning of the flood. Organic matter content in SPM remained almost unchanged (2 to 3%) and only a slight decrease was noticed during the ETM. Recent particulate organic matter sources such as terrestrial plant, phytoplankton, bacteria and domestic sewage were differentiated by the use of lipidic biomarkers (fatty acids and sterols). In the ETM, terrestrial plant, sewage and bacterial inputs are at maximum. On the other hand, phytoplankton abundance is minimum in ETM but it may be important during slack periods. This is the case in September, when evidence is given for a diatom bloom during high tide slack. © 1998, Kluwer Academic Publishers.