The purpose of our study was to describe the dynamics of E. coli in the rivers of the Seine watershed. This watershed is characterized by a poor microbiological water quality due to an important population density, industrial activities and intensive agriculture. The main objectives of our study were to obtain field microbiological data describing the system and to understand the underlying mechanisms in order to build a model able to describe and predict the fate of the fecal bacteria at the scale of the whole watershed. This model was built in order to help management of microbiological water quality. <p>To be able to describe and modelize the fate of fecal bacteria at the scale of such a large and complex system, it was first necessary to identify and quantify the sources of contamination and to be able to describe the processes affecting these bacteria once discharged to the rivers. <p>The quantification of E. coli in the natural aquatic systems through the traditional culture-based techniques has been strongly questioned during the last years because they have a long response delay (24 to 48 h) and they do not take into account the viable but not culturable bacteria (VBNC) which nevertheless could represent a health thread. For this reason we developed in this study two alternative methods: the direct (without passing through a cultivation step) measurement of the â-D-glucuronidase activity and a fluorescent in situ hybridization with an E. coli specific probe coupled with a viability test. The first has the main advangate to give a very rapid evaluation of the E. coli content of the water while the second one was able to enumerate VBNC E. coli. <p>The study of the sources of contamination of the rivers revealed the predominance, in this anthropogenicised watershed, of the point sources (effluents from wastewater tretament plants). Diffuse sources soil leaching and surface runoff) may nevertheless be locally important and have also be quantified. The influence of the land use on these diffuse sources has been quantified demonstrating that soil leaching and surface runoff of grazed areas was mainly responsible of fecal contamination in rural areas.<p>The attachment of E .coli to particles in the river waters was studied as it can influence the fate of this kind of bacteria in the aquatic systems. A linear regression was found between the percentage of particle-associated E. coli and the SM content of the water. Settling velocities of particle-associated E. coli were determined. The mortality rate of E. coli was analysed following various perspectives: the river order, the attachment of E. coli to particles, the importance of the cultivability loss in the disappearance of these bacteria and the importance of protozoan grazing. The results confirmed the importance of grazing and showed that the abondance of free living E. coli bacteria decreased two times more rapidly than particle-associated E. coli. In the Seine river, we showed that the net loss of culturable E. coli was 1.5 time higher than the net loss of viable E. coli. Finally, no relationship was found between mortality rate and river order. <p>These results have been used to build two models CF-SENEQUE and CF-SiAM-3D which describe E. coli dynamics respectively for the whole Seine river watershed and for the Seine for the estuary. Comparisons model calcultaions with field data of fecal coliforms abundances showed that these models correctly simulated the longitudinal distribution of fecal colifoms in the main rivers of the Seine watershed and in the estuary. These models were used to test the impact on the microbiological water quality of various scenarii of wastewater management .<p> / Doctorat en sciences, Spécialisation biologie animale / info:eu-repo/semantics/nonPublished