Background and aims – Diatom-based water quality management is increasing, and specific indicators are required for the assessment of priority substances such as metals. We tested a variety of features of diatom communities, in a river exhibiting a gradient of zinc contamination (the Riera d’Osor, Spain), to determine the most relevant ones. Key results – Community composition changed over time of exposure, and with the intensity of metal contamination. Species richness was significantly lower at the most contaminated sites. Species composition was more even under background and low exposure levels, but low metal inputs selected for varieties of Cocconeis placentula (var. placentula, euglypta and lineata). Small taxa (Eolimna minina, or Achnanthidium minutissimum and A. pyrenaicum) dominated in the most contaminated sites, and deformed diatom cells were found abundant. Conclusions – Although species composition clearly responded to varied levels of zinc pollution, combining cell size classes or total biovolume of the community and percentages of deformities allowed reliable assessment of the presence, and intensity, of contamination. These descriptors present the major advantage of being independent of regional taxonomic peculiarities, thus providing robust assessment irrespective of the area studied

Silvia Corcoll kindly mapped the Riera d’Osor watershed. The study was partially by the European project Keybioeffects (MRTN-CT-2006-035695) and the Spanish project FLUVIALMULTISTRESS (CTM2009-14111-C02-01)