Abstract : Organic matter was studied in the turbidity maximum zone (TMZ) of the Seine Estuary during 8 tidal cycles from April to October in 2001, 2002 and 2003, covering a salinity range from 0 to 27. The hydrological conditions were quite varied (extremely wet in 2001, unusually dry in 2003). A particularly striking feature is the high organic matter content in the suspended solids (SS) of the Seine estuary (4-5%). By determining micro-organism activity and organic carbon partitioning, either linked to particles or in dissolved forms, and estimating the TMZ water volumes, together with SS, we extrapolated these activities and stocks to the whole TMZ. Carbon metabolism in the TMZ and fluxes upstream of the TMZ were compared on the dates of field surveys, and the routes and fate of carbon in the TMZ were quantified in order to learn about the trophic status of this estuarine zone in terms of autotrophy vs. heterotrophy. The upstream total organic carbon (TOC) fluxes (48% of particulate organic carbon (POC), 52% of dissolved organic carbon (DOC) on average) varied fourfold between the surveys, reaching the highest value of 280 TC d-1 during the wet summer of 2001; and the lowest value of about 70 TC d-1 in August 2003. Whereas nearly all of the DOC flux entering the TMZ reaches the coastal marine zone, mostly (at least 85%) in a refractory form, the POC accumulates in the TMZ of the estuarine channel, particle exportation being negligible. In the TMZ, biodegradation of DOC was, on average, much less (only a 2% decrease in the BDOC/DOC ratio between the TMZ upstream and downstream fluxes) than biodegradation of POC (11%). A simplified model of the TMZ (LIFT- Lumped Idealisation of the ecological Functioning in estuarine Turbidity maximum) was constructed for investigating the dynamics of organic matter on a seasonal scale. The agreement between observation and calculation allowed us to run sensitivity tests using new constraints; reductions of the upstream fluxes of phytoplankton and organic carbon showed that the high content of organic matter, originating mostly from the domestic effluents of the Paris conurbation, can explain the high organic content of the Seine estuary TMZ.