This work aims to evaluate the effects of different environmental factors (i.e., geographical, chemical, and hydrological) on benthic diatoms at 34 sites located in 13 watercourses of northern Italy, and to highlight possible misclassifications of the ecological status of watercourses, sensu Water Framework Directive, related to the normative index currently adopted in Italy (ICMi). The analysis of both the taxonomical and functional composition of diatom communities confirmed the presence of differences in terms of taxonomical richness, diversity, and taxa assemblages, associated to the altitude and the geological characteristics of the investigated watercourses. Moreover, the data analysis revealed differences due to chemical and hydrological alterations. Specifically, our results showed a clear link among these environmental perturbations and the communities' functional composition expressed through the use of ecological guilds. High abundance and richness of motile diatoms were detected in sites characterized by nutrient enrichment, while high abundance of low-profile diatoms was linked to hydrological alteration. In contrast, these anthropogenic perturbations were not detected by the ICMi, which ranked more than 90% of the analyzed samples in the highest quality class. This study stresses the need for a different approach in diatom data interpretation in order to achieve reliable information about the ecological status of watercourses.
Keywords: Diatoms; Ecological guilds; Hydrology; Pollution; River; Water framework directive.