Spatial distribution of benthic macroinvertebrate assemblages at mesohabitat level in the lower Ebro River (NE Spain) and the influence of colonization by macrophytes

Macroinvertebrate assemblages at 2 reaches in the lower Ebro River affected by the recent macrophyte colonization of riverbed was studied in order to examine the spatial distribution of the community in relation to physical conditions and biological aspects. Each mesohabitat was delimited according to measurements of water velocity, depth and the visual characterization of transversal-longitudinal water profiles and was mapped in situ using geographical information systems (GIS). An stratified random sampling was performed for invertebrates at each mesohabitat ocupied partially by macrophytes, using a surber sampler (250µm pore, 50x50 cm), at the same time near-bed flow velocity and depth averaged velocity  were measured. Also periphyton (chlorophyll-a, total organic matter and diatom assemblages) and oxygen and nutrient concentrations in sediment were analysed . Invertebrates were sorted and identified to genus or species level. Ephoron virgo (Ephemeroptera: Polymitarcyidae) was used as a bioindicator of habitat perturbation since it could be negatively affected by habitat constraints as a consequence of the macrophyte colonization. A significant influence of the mesohabitat scale to the benthic community is expected, and within mesohabitats the presence or absence of macrophytes can also be determinant.