Abstract
Biodiversity in benthic diatom communities was determined to assess the ecological state and to evaluate changes in water quality in time and space in the prealpine part of the river Töss (Switzerland). One site upstream and three sites downstream from the inlet of a communal waste water treatment plant (WWTP) were analyzed during 1996/97. In total, 67 diatom species were detected and the relative abundance of each taxon determined. The Shannon Diversity Index and the Saprobic Index were calculated and related to the newly developed Swiss Trophic Diatom Index. Dominant species belonged to the genera Achnanthes, Amphora, Cymbella, Diatoma, Gomphonema, Navicula, and Nitzschia. The species composition and the abundance of the diatom species present varied over the seasons, but changes in the chemical parameters or the flow regime had little effect. The Shannon Index gave evidence of biodiversity variations over the seasons and some differences between sampling sites. The Saprobic Index oscillated within a small band, indicative for oligo‐mesosaprobic conditions. It could not identify significant changes in water quality between sampling sites and over the seasons. The Swiss Trophic Diatom Index differentiated better between sampling sites and seasons and gave values corresponding to a β‐mesosaprobic state. Preliminary data from 2005 and 2006 give no evidence of a change in water quality during the last decade.