The spatial heterogeneity of a river bottom: a key factor determining macroinvertebrate communities

We examined the relationships between mesohabitat heterogeneity and benthic macroinvertebrate assemblages in a fourth-order stream (Mortagne, North-Ea stern France). The heterogeneity of the mesohabitat mosaic was described ar ound each sampling point for eight different surfaces from 0.5 to 4 m radiu s. Environmental descriptors integrated both the composition and configurat ion heterogeneity of the substrate mosaic. Faunal data were analysed in ter ms of biocenotic indices and taxonomic composition. The maximum number of s ignificant Spearman correlations was obtained for a mosaic with a diameter of 2 m around the sampling point and significant correlations were both the highest and the most numerous for this surface. Relationships were establi shed between community structure and substrate heterogeneity. The faunal ri chness was higher in a heterogeneous environment composed of numerous subst rates, an elevated patchiness and with high perimeters. Such a mosaic poten tially offers a great number of niches for invertebrates. A reduced distanc e between two types of substrate favours exchange of species. At the opposi te, a very homogeneous mosaic offers a low variety of niches and shelters f ewer taxa. Furthermore, in a homogeneous environment we observed that one o r two particular taxa dominated the community, probably because competition with taxa coming from neighbouring patches was reduced. The community comp osition mainly depended on the characteristics of the mesohabitat sampled. Our results showed that the neighbouring environment around such mesohabita ts also had a significant influence. Further research must be conducted to specify the influence of the mosaic heterogeneity on biological and ecologi cal traits of invertebrates.