Tidal Time-scale Changes in the Composition of Particulate Organic Matter Within the Estuarine Turbidity Maximum Zone in the Macrotidal Seine Estuary, France: the Use of Fatty Acid and Sterol Biomarkers

During low flow and spring tide conditions of September 1995, the organic character of suspended particulate matter (SPM) was investigated in the estuarine turbidity maximum (ETM) zone located within the inner Seine River estuary, France. The appearance of the ETM was observed 2–4 h before low tide, during ebb resuspension into the water column. During the flood resuspension, a second ETM was observed but only in bottom waters. This suggests separation of the surface (fresh waters) from the bottom layer (brackish/marine waters) at maximum flood velocities. Suspended particulate matter composition was relatively constant in terms of organic carbon (2%) and only a slight decrease was observed during the high SPM load. Lipids (i.e. fatty acids and sterols) were used as biomarkers to assess different sources of organic matter including terrestrial inputs, phytoplankton, bacteria and domestic sewage. Given specific assumptions, source-specific biomarker concentrations were used to trace quantitatively temporal changes in the relative and absolute importance of source-derived organic material. Terrestrial matter was a very important part of ETM organic matter [35–58% particulate organic carbon (POC) in ebb ETM at low tide; 10–20% POC at high slack tide], whereas sewage organic matter was a relatively minor (<4% of POC) component. Although, the proportion of phytoplankton matter in the high SPM load was low, phytoplankton was a predominant component of SPM during the flood slack (12–31% of POC). This prominence of SPM was associated with the oxygen saturation due to the diatom bloom in the estuary.