Characterization of diatom assemblages in mid-altitude streams of NW Italy

Following the European Water Framework Directive, this study aims to be the first step to (i) identify diatom type assemblages in unpolluted streams in NW Italy, and (ii) find which ecological factors explain most of the variation. To achieve this, we collected physical, chemical and benthic community data from four streams in NW Italy from 2001 to 2004, for a total of 72 samples. All sampling sites were between 200 m a.s.l. and 800 m a.s.l., but differed in the dominant lithological substrate, i.e. alluvial or siliceous. Relationships between diatom communities and environmental factors were examined using canonical correspondence analysis (CCA), while Indicator Species Analysis was used to define characterizing species and accompanying species of three environmental groups identified by CCA: (1) high water quality and medium saline content, (2) high water quality and low saline content, (3) poor water quality. The diatom assemblages of the three groups of sites differed significantly, as shown by the Multi-Response Permutation Procedure. There were strong correlations between diatoms and environmental factors, especially chlorides (also highly correlated with sulphates and carbonate hardness), nitrate concentration and conductivity. The group 1 assemblage was typical of the alluvial Alpine streams with medium saline content and was characterized by mostly oligosaprobic/beta-mesosaprobic taxa such as Cymbella affinis, Diatoma ehrenbergii and Staurosira pinnata. The species assemblage found in the siliceous Alpine rivers with good water quality make them suitable reference sites for a benthic diatom community.