Environmental contamination has become a global problem of increasing intensity due to the exponential growth of industrialization.One main debated issue is the metal contamination of rivers receiving Acid Mine Drainage (AMD) from active/abandoned mines.In order to assess the quality of lotic systems,diatoms are commonly used,as their assemblage modifies on the basis of changes in environmental parameters.Benthic diatom changes were analyzed along the metal-impacted Gromolo Torrent (Liguria,NW Italy) with the aim of understanding the effects of input from the abandoned Libiola Cu mine.The results support the hypothesis that metals from AMD lead to massive changes in diatoms,resulting in low biological diversity and in a shift of dominance,passing from the genera Cymbella and Cocconeis to more tolerant and opportunistic species,such as Achnanthidium minutissimum and Fragilaria rumpens.The high concentrations of labile metals,measured through Diffusion Gradients in Thin-films (DGT) immediately downstream of the two AMD inputs in the torrent,corresponded to a sudden decrease in the presence of diatoms,indicating the possible reaching of acute toxic levels.In particular,A.minutissimum dominated the mining area and was positively correlated with Cu and Zn;whereas F.rumpens bloomed downstream of this area,where the metal content was diluted,and was positively correlated with As and Pb.Finally,an important abundance of Nitzschia palea and teratological forms ofA.minutissimum and F.rumpens were observed downstream from the mine,indicating that metals may have an important impact on diatoms up to the torrent mouth.