SUMMARY The methods used to indicate the biological state of streams are often based on taxonomic composition, and the abundance of species or other taxa. This ‘taxonomic structure’ varies among ecoregions and cannot be applied to wider geographical areas. Therefore, we assessed the species traits of benthic macroinvertebrates from semi‐natural reference sites as a potential benchmark for large‐scale biomonitoring. Our purpose was to assess the stability of community structure, based on the representation of taxa and of traits, across large gradients of geology (sedimentary to granitic), altitude (65–1982 m), geographical coordinates (0° 48′ W to 7° 20′ E and 42° 52′ to 48° 44′ N), stream order (1–5) and slope (0.5–60‰). We used invertebrate abundance data from the 62 most natural French stream sites available. These abundance data served to weight the occurrence of ‘biological’ traits, such as reproductive characteristics, mobility, resistance forms, food, feeding habits, respiration, and ‘ecological’ traits, such as preferences for temperature, trophic level, saprobity, biogeographic distribution, longitudinal zonation, substratum and current velocity. Multivariate analyses of taxonomic composition demonstrated a clear site gradient from lowlands to uplands and from calcareous to granitic geology. In contrast, community structure based on both biological and ecological traits was stable across environmental gradients. The frequency distribution of biological traits indicated that the stream benthos of the ‘reference sites’ had a mixture of categories which confirmed theoretical predictions for temporally stable and spatially variable habitats. A mixture of ecological trait categories also occurred at our reference sites. Thus, semi‐natural benthic macroinvertebrate communities are functionally diverse. Moreover, we included an initial application of these traits to a case of slightly to moderately polluted sites to show that the impact of humans significantly changes this natural functional diversity. Future studies should focus on the potential for various biological and ecological traits to discriminate different human impacts on the benthic macroinvertebrates of running waters, and on the integration of this functional application into a general ‘reference‐condition’ approach.

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A scale of sensitivity of mollusc species to biodegradable pollution was proposed in a previous paper (Mouthon 1996). Using these results as a basis, and introducing supplementary elements in a correspondence analysis, we have been able to formulate a scale of sensitivity of mollusc genera and families. A quantification of information lost when using genus or family-level rather than species-level identification was carried out by calculating the Euclidean distances between each species and the gravity centre of the genus and family to which it belongs. The results obtained showed that the genus-level identification can constitute an acceptable compromise for using molluscs as water quality indicators in biomonitoring. Sensibilité comparée des espèces, genres et familles de mollusques aux pollutions biodégradablesMots-clés : mollusques, niveau de détermination, pollution, cours d'eau. Une échelle de sensibilité des espèces de mollusques aux pollutions biodégradables a été proposée dans un précédent article (Mouthon 1996). A partir de ces résultats et grâce à l'introduction dans l'analyse des correspondances d'éléments supplémen-taires, représentés par les moyennes des effectifs des espèces appartenant au même genre, puis à la même famille, nous avons pu en déduire une échelle de sensibilité des genres et des familles de mollusques. Une estimation de l'importance de l'information perdue lorsque l'on identifie les mollusques à la famille ou au genre au lieu de l'espèce a été effectuée grâce au calcul de distances euclidiennes entre chaque espèce et le centre de gravité du genre ou de la famille auxquels elle appartient. Les résul-tats montrent que, si l'on veut intégrer les mollusques aux indicateurs de qualité d'eau, lé genre pourrait constituer un compromis acceptable.

This methodology has been applied to the Chalaronne river, a water body receiving effluents from a waste water treatment plant. Biocenotic analysis, based on abundances of taxa, allows discrimination of upstream from downstream stations since 17.9 % of the variability is dedicated to it. Functional analysis, based on biological traits, gives a better discrimination of stations, since the value is equal to 23.1 %.This study shows that functional structure observed upstream is characteristic of resilience. On the contrary, functional structure observed downstream is characteristic of resistance.A functional approach is interesting for two reasons : first as a mean to discriminate stations efficiently, and secondly it is promising in study of biological functionalities of ecosystems.