It is generally argued that epilithic diatoms and macroinvertebrates are highly sensitive to changes in water quality, while only a few studies have been conducted on their response to physical disturbance. The main purpose of this research is to investigate whether these two communities respond to physical river disturbance like banks, Substrate and flux alteration induced by human action. We sampled 12 high-altitude streams in the Gran Paradiso National Park, a protected area where the main human activities are limited to hiking and high-altitude pasture. The sampled stretches of the rivers were characterized with respect to their geomorphological features and possible human modifications. Water uptake for power supply, riverbed and bank modifications represented the main human activities which could determinate alterations in community structure and composition. Habitat structure was evaluated using the Southern European River Habitat (RHS-SE), while the biological status of the river has been estimated by means of the following biotic and diatomic indices: IBE, BMWP, ASPT, Ephemeroptera. Plecoptera and Trichoptera families (EPT) for macroinvertebrates, EPI-D, IBD and IPS for diatoms. A general concordance between chemical parameters and structure of biological communities was observed. On the other hand, the RHS-SE highlighted the main geomorphological features and detected sonic slight physical alterations, as quantified by the indices Habitat Modification Score (HMS) and Habitat Quality Assessment (HQA). In Short, our findings demonstrate that morphological alterations. even if slight, affect biological diversity and the presence of some specific taxa, while biological indices are independent of the level of physical modifications. Copyright (C) 2008 John Wiley & Sons, Ltd.
