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International audience; The European Union (EU) Water Framework Directive depends, for effective implementation, on Member States (MSs) agreeing to a concept of the unimpacted “reference” state, which will then provide the “expected” value in Ecological Quality Ratio (EQR) calculations. Reference assemblages of organism groups will, in turn, vary, due to geological, hydrological, climatic, physicochemical and biological factors. Member States tackle this by establishing “types” which share common characteristics. However, for the purposes of ensuring consistent application, broad transboundary types were also established within five Geographical Intercalibration Groups (GIGs, referred to here as “regions”) as part of the EU’s intercalibration exercise. In this paper, we evaluate these types using river diatom assemblages and also provide reference threshold values for two common metrics used in pan-European diatom assessments. A database was assembled, representing 14 EU Member States from Ireland and Portugal in the West, to Estonia and Cyprus in the East, in order to explore biogeographical patterns in assemblages unaffected by anthropogenic pressures. Multivariate analyses were used to examine this pattern and its relationship with geographic, typological and abiotic parameters. After taxonomic harmonisation, NMDS ordination of samples indicated weak differences in assemblage composition among regions. ANOSIM analyses, in turn, indicated that MS was the best factor to group similar samples whereas alkalinity, recognised as the primary environmental variable structuring diatom communities, although significant, explained less variability in the dataset. This, we believe, reflects the importance of methodological factors other than taxonomy (e.g. counting protocol, sample season) that may be constant within a MS but which vary between MSs. When two diatom metrics, the TI and IPS, were applied to the data, differences in the distribution of the metric scores between MS were generally not statistically significant even though some differences between regions were apparent. A trend of increasing values of TI (decreasing values of IPS) was observed in the sequence: Nordic < Alpine < Mediterranean < Central-Baltic < Eastern Continental regions. Additionally, some differences were observed among types within the Mediterranean and Nordic regions, though not for other regions. The data used in this exercise provides us with a region and, in some cases, a type specific benchmark dataset against which national reference data can be compared.

International audience; In 2009, seventeen analysts participated in a pan-European diatom ring-test (intercalibration), analyzing nine samples from seven countries following the European standard EN 14407. The objective of this exercise was to agree on practical conventions on diatom identification to facilitate future intercalibration work and to assess the extent to which national differences in sample analysis (counting protocol and identification conventions) contribute to variability in EU-level comparisons of diatom-based methods. Differences in the reported taxa lists were large, but not a major source of variation in values of a common metric (the phytobenthos Intercalibration Common Metric, ICM). Therefore, every country can apply its own identification conventions for national assessments, and still be fairly confident that the ICM reflects the national classification of its streams. Part of the index variation was due to differences in counting protocols and care should be taken when handling broken valves, girdle views and small taxa. More work at both national and European level is needed to provide a harmonized way of using diatoms for ecological status assessments in the future.

International audience; The European Union (EU)'s Water Framework Directive (WFD) requires that all Member States participate in intercalibration exercises in order to ensure that ecological status concepts and assessment levels are consistent across the EU. This paper describes one such exercise, performed by the countries in the Central/Baltic Geographical Intercalibration Group stretching from Ireland in the west to Estonia in the east and from the southern parts of Scandinavia to the northern regions of Spain and Italy (but excluding alpine regions, which were intercalibrated separately). In this exercise, methods used to measure ecological status of rivers using benthic diatoms were compared. Ecological status is estimated as the ratio between the observed value of a biological element and the value expected in the absence of significant human impact. Approaches to defining the 'reference sites', from which these 'expected' values were derived, varied from country to country. Minimum criteria were established as part of the exercise but there was still considerable variation between national reference values, reflecting typological differences that could not be resolved during the exercise. A simple multimetric index was developed to compare boundary values using two widely used diatom metrics. Boundary values for high/good status and good/moderate status set by each participant were converted to their equivalent values of this intercalibration metric using linear regression. Variation of ±0.05 EQR units around the median value was considered to be acceptable and the exercise provided a means for those Member States who fell significantly above or below this line to review their approaches and, if necessary, adjust their boundaries.