Origin and transport of organic matter across the Seine estuary: Fatty acid and sterol variations

Suspended matter (SPM), particulate organic carbon (POC), dissolved and particulate fatty acids (FA), and sterols (ST) have been analyzed in the Seine estuary to assess the sources and transport of organic matter. Two sampling sites have been chosen: the first at Buoy 26 near the Normandie Bridge, in the estuarine mixing zone (October 28th 1993) and the second at La Bouille (November 4th 1993) in the upper fluvio-estuarine zone, which is only exposed to tidal pulse influence. At each site, samples were collected every 1h30 during an entire tidal cycle. In surface waters, in the particulate phase, C16, C18 and C20 polyunsaturated FA, branched C15 and C17 FA, long chain C26–C32 FA and the faecal ST coprostanol indicate, respectively, planktonic, bacterial, terrestrial, and sewage organic matter inputs. Diatoms were the predominant planktonic contributors at the time of our study, but other species may have been present especially in the fluvio-estuarine zone. Terrestrial plant and sewage particulate organic matter are transported seaward at La Bouille and are trapped in the ETM in which a bacterial imprint is also noticeable. A punctual event such as a recent diatom bloom decay, was also possibly the cause of the important concentrations of dissolved FA and ST during the flood at La Bouille. At Buoy 26, dissolved FA were also decoupled from particulate FA. Due to the presence of ubiquitous compounds, no precise origin can be attributed to this dissolved fraction. In ETM, organic matter is better characterized in the particulate phase.