Consideration of mass effect processes in bioindication allows more accurate bioassessment of water quality

Bioassessment is widely used to measure ecological integrity of natural habitats following anthropogenic disturbances and modifications. Traditionally, bioassessment has been based exclusively on species-environment interactions, i.e. niche processes. However, dispersal processes, in particular mass effect, could mask the influence processes lead erroneous conclusions about ecosystem health. To circumvent this problem, we identified 40 diatom species with distributions driven primarily by effect propose an alternative version Biological Diatom Index (IBD2007) excluding these species. We tested environmental responses both original IBD modified (IBDmod) a benthic dataset from France, collected between 2007 2013 including 9487 samples 3913 spatially distinct localities. Our results indicate better relationship IBDmod scores conditions, compared IBD2007 scores, leading more accurate determination river status, especially conditions moderate nutrient enrichment. This study supports idea that may result biased evaluation water quality. It advocated process considered other diatom-based indices, extension, any biotic index.