Traits of benthic macroinvertebrates in semi-natural French streams: an initial application to biomonitoring in Europe

1. 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 degrees 48' W to 7 degrees 20' E and 42 degrees 52' to 48 degrees 44' N), stream order (1-5) and slope (0.5-60%). 2. 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. 3. 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. 4. 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. 5. 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.