A key issue in the implementation of the Water Framework Directive is the classification of streams and rivers using biological
quality parameters and type-specific reference conditions. Four groups of stream types were defined in NE Spain on the basis
of 152 diatom samples by means of detrended correspondence analysis and classification techniques. Diatom analysis was restricted
to epilithic taxa, and the sites included gradients ranging from near-natural streams to sites with poor ecological quality.
The main gradient shows a clear separation of sites in relation to the degree of human influence: polluted streams (mainly located
in the lowlands) differ from streams in mountainous areas and in the Pyrenees. A second gradient is related to physiographical
features. Headwater streams can be distinguished by their catchment geology. The type-specific diatom taxa for the stream
types studied were determined by using indicator species analysis (IndVal). The type-specific taxa from near-natural streams are
coincident with the indicator taxa for high ecological status. Human impact reduced the typological heterogeneity of the diatom
community composition. Overall, the diatom communities in NE Spain exhibit a regional distribution pattern that closely corresponds
with that observed in river systems elsewhere. Physiographical differences are only evident in undisturbed sites, while
nutrient enrichment and other human disturbances may mask the regional differences in the distribution of diatom communities.