Effects of sampling on benthic macroinvertebrate assemblages in assessmentof biological quality of running water

In running waters, a picture of the macroinvertebrate community of a statio n is generally obtained by adding individuals sampled in different mesohabi tats. Mesohabitats can be defined as combinations of a substrate type and a range of current speed. Simulations allowed us to study the variation in t he community picture of a station when 8 mesohabitats were taken into accou nt although eleven were sampled. Results were replaced in the framework of the use of the Indice Biologique Global Normalise (IBGN). Taxonomic structure described with indices varied greatly with the combinat ion of mesohabitats sampled. Taxonomic richness was the more stable index c onsidering the potential range of this index. In contrast, abundance was th e more variable index. Between these extremes, diversity or dominance indic es had an intermediate range of variability. The variability of a structure index seemed to increase when this index integrates taxa abundances, which depend on mesohabitats sampled. Taxonomic richness is integrated in IBGN i n the form of classes; this expression reduces the effect of richness varia bility. Even if the sampling protocol was respected, the uncertainty on the IBGN can reach 3 units. Solutions have been considered to optimize the ass essment of running water biological quality.