Assessment of the effect of hydrological variations on macrobenthic communities in pools and riffles of a Mediterranean stream.

Changes in the benthic macroinvertebrate community in the Arente River (a tributary of the Crati river in the Region of Calabria, Italy) were monitored monthly for 1 year through stratified sampling by habitat (pools and riffles) together with general descriptors of communities structure and the extended biotic index (EBI) to assess the potential development of community alteration following strong hydrological increases. Results suggest that flow peaks and substratum instability remarkably affect population abundance and composition of macroinvertebrate assemblages in both macro-habitats whereas their ordination is due to chemical parameters analyzed only at low water periods. Organism and taxonomic densities as well as taxonomic richness all showed an appreciable reduction simultaneously with flow increases both in pools and riffles. Early colonizers (organisms with high dispersal and drifting tendency) dominate during hydrological perturbation and post-perturbation phases. Both EBI and environmental quality values more frequently show the development of river disturbance in pools than in riffles during and after flow changes. Regression analysis clearly shows that hydrological and substrate parameters are better predictors of community parameters changes (higher r(2) values) in pool habitats than in riffles habitats suggesting that depositional environments seem more suitable than erosive ones in monitoring macrobenthic community changes and, possibly, alterations due to hydrological variations.