A new macroinvertebrate-based multimetric index (I2M2) to evaluate ecological quality of French wadeable streams fulfilling the WFD demands: A taxonomical and trait approach

Following the Water Framework Directive (WFD) requirements, we designed a new multimetric index (I2M2) for the invertebrate-based ecological assessment of French wadeable streams. This index should be able to identify impaired reaches for 17 anthropogenic pressure categories potentially leading to water quality alteration or habitat degradation. Based on a national database, we defined an iterative procedure to select taxonomy- and trait-based metrics exhibiting the best trade-off between (i) high discrimination efficiency, (ii) low specificity and (iii) high stability in least impaired conditions. The I2M2, defined as the best combination of such metrics, has been composed by: (i) Shannon diversity index, (ii) original ASPT score, (iii) the relative abundance of polyvoltine taxa, (iv) the relative abundance of ovoviviparous taxa and (v) taxonomic richness. The I2M2 was tested against an independent data set. It exhibited good and robust pressure–impact relationship for all the pressure categories, correctly identifying in average 82% of reaches impaired by water quality alterations or habitat degradation. The I2M2 significantly improved the detection of impaired reaches by at least 17% for nitrogen compounds and up to 35% for organic micropollutants and clogging risk, when compared to the normalized French biotic index (IBGN). The I2M2 has been proposed for future use in the national biomonitoring of wadeable reaches in the context of the WFD implementation.