Chironomid assemblage of a Lower-Rhone dike field: relationships between substratum and biodiversity

The effect of microhabitat on the chironomid (Diptera) community of a large river was tested by trapping floating pupal exuviae after emergence of adu lts. The spatial distribution of the 33 species or groups of species that w ere identified was strongly explained by mean particle size, porosity and h eterogeneity of substratum. A study of the size, means of respiration, mode of life and feeding habits of the species showed that the substratum-fauna l relationships could be explained in part by biological traits of the spec ies present. Two gradients could be defined on the basis of habitat variabl es: a substratum porosity gradient that was significantly correlated with s pecies richness and diversity, and a second gradient related to particle si ze that was significantly correlated with biological categories.