LAUSR

Abstract Benthic macroinvertebrate assemblages have been of major importance in the biomonitoring of rivers for many decades. To more closely fit with new regulatory purposes, standardized methods of macroinvertebrate field sampling have changed over time both in terms of sampling effort and criteria of habitat type sampling priority (e.g. according to a decreasing gradient of “bottom surface coverage” versus “substratum favorability”). Such methodological heterogeneity has rendered the exploitation of biomonitoring data far more complex because they are indeed not fully comparable over time and may induce bias in long term series studies. In this context, we assessed the influence of variation in field sampling strategy on (i) the taxonomical characterization of benthic macroinvertebrate assemblages (297 taxon abundances, taxonomic richness and diversity, EPT-related metrics), (ii) the response of sensitivity descriptors (ASPT, SPEAR), (iii) the bio-ecological trait description of assemblages and (iv) the resulting ecological diagnostic of river reaches, based on 10,074 sampling events across France. Six sampling strategies were compared, corresponding to three levels of sampling effort (4, 8 and 12 individual samples) and different criteria for substratum selection; e.g. marginal (=