Effects of a Small Dam on Macroinvertebrate Communities and Environmental Variables in a Headwater Stream.

Three reference sites and at six impacted sites located upstream and downstream respectively of a small dam located in the Simbrivio Stream (Tiber basin, Central Italy) were investigated over 1-yr period. Data on physicochemical, hydro-morphological and trophic variables together with species composition, density, diversity and trophic structure of macroinvertebrate communities were collected to assess possible dam effects. Major changes in environmental variables occurred within the first three sites downstream the dam. In particular: change of flow regime, increase of epilithon biomass and, during spring period, decrease of the coarse and fine particulate organic matter associated with benthic samples. Water temperature, conductivity, pH, dissolved oxygen and nutrient concentrations (ammonium and soluble reactive phosphorus) were not affected by dam presence. In spring, the dam has a clear effect on the invertebrate fauna, with an alteration of the species composition and a reduction of taxa richness of downstream sites, particularly in relation to stoneflies, caddisflies and mayflies (EPT taxa). Seven taxa collected upstream were absent in all downstream sites. On the contrary, in autumn, the results showed an increase of total and EPT taxa of downstream communities. The analysis of environmental variables patterns along the study sites suggested that the principal factors affecting the biota were alteration of hydrological conditions, epilithic development and substrate stability.