Establishing expectations for pan-European diatom based ecological status assessments

► Data from unimpacted (“reference”) streams from throughout Europe showed relatively little biogeographical variation. ► “Member State” was the factor which explained most of the variation in the dataset. ► This may reflect an accumulation of methodological issues which obscured patterns of variation between countries. ► Use of water quality metrics such as the Indice de Polluosensibilité and Trophieindex, however, enabled target values to be established for reference samples in different regions of Europe.