Nous mettons ce document à disposition sous cette forme dans le but d'assurer la plus grande diffusion de nos travaux scientifiques et techniques, en particulier auprès des étudiants. Le copyright et tous les droits restent la propriété des auteurs et des éditeurs. Nous prions chaque personne utilisant ces informations de respecter les droits et copyright des auteurs et éditeurs. Dans la plupart des cas, ces travaux ne peuvent être réexpédiés sans la permission explicite du détenteur des droits. Key words : Suburban watercourses, urban inputs, coarse sediments, hyporheic zone, Oligochaeta This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Hydrobiologia. 2 AbstractThe Chaudanne stream received urban inputs discharged through combined sewer overflows (CSOs). The water quality was not severely impaired, with pollution mainly of organic origin.Oligochaete assemblages were studied in the coarse surface sediments and the hyporheic zone and sampled at 4 sites on 7 occasions during 2000 and 2001. The seasonal distribution of oligochaete assemblages was analyzed by a PCA, the oligochaete species being assigned to functional traits (FTrs). Site 1, located upstream of the CSOs, was characterized by FTrs 1 and 2 (species indicating permeability and those intolerant to water pollution). Below the CSOs, high densities of oligochaetes occurred in the benthic layer of sites 3 and 4, with a predominance of FTr3 (species with tolerance to water pollution). At site 4, FTr4 species (indicative of sludge conditions) constantly predominated in the hyporheic system, but predominated in the benthos only during low stream discharges associated with peaks in CSOs. FTr3 was related to amounts of the oxidized forms of nitrogen, high stream discharges and probably to groundwater upwellings and the sludge tolerant species group (FTr4) was associated with high NH 4 + contents. We are now testing the relevance and generalisation of this new approach.3

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