Does subsurface interstitial space influence general features and morphological traits of the benthic macroinvertebrate community in streams?

The benthic macroinvertebrate community and substratum characteristics were assessed at 17 study sites differing in the amount of interstitial space i n the subsurface interstitial zone (0-15 cm deep). The invertebrate communi ty was described in terms of density, taxonomic richness, Shannon diversity and morphological traits (size, flexibility and body shape of the inverteb rates). The amount of interstitial space in the subsurface zone that is des cribed by porosity was significantly and negatively related to percentage o f fine sediment, but not to stream slope, particle heterogeneity and partic le size. Invertebrate density was significantly and positively related to t he amount of interstitial space whereas taxonomic richness was slightly and negatively related to fine particulate organic matter content. Relative ab undance of intermediate body size (5-10mm), intermediate body flexibility ( 10-300 degrees), and flattened or streamlined shape were significantly and positively related to the amount of interstitial space, contrasting with re lative abundance of small body size (<5 mm), and cylindrical or spherical s hape that were significantly and negatively related to this variable. These results highlight the importance of interstitial space, both as a factor p artly controlling community structure and as an evolutionary factor acting on macroinvertebrate ability to penetrate the substratum. In addition to fl ow, the substratum selects the invertebrates through their different abilit ies to use the interstitial habitat.