Th e main objective of the Water Framework Directive (WFD) is to achieve good ecological status for surface waters in Europe by 2015. Th e ecological status has to be defi ned based on near-natural reference conditions. Benthic invertebrates and diatoms are among the key biological elements recommended by the Directive to assess ecological quality of water bodies. Th e purpose of this study is to identify species associations of these biological elements that are characteristic of the diff erent stream types occurring in Luxembourg and that distinguish degraded from reference conditions. In general, the results reveal that diatoms and invertebrates can be considered as complementary indicators with more diatom species being characteristic of small size stream types and more benthic invertebrate species being associated with larger stream types. Among invertebrates, Trichoptera, Hydrachnidia, Ephemeroptera and Diptera show high affi nities for most stream types. Plecoptera, Oligochaeta appear as useful indicators for some particular types. If only reference sites are selected (all river types considered), the number of indicator species drops from 55 to 24 for diatoms and from 81 to 48 for benthic invertebrates. Moreover, for the larger stream type, no reference site was found at all. Th is trend is likely to be a consequence of the multiple anthropogenic pressures that have aff ected large parts of European lowland rivers for decades. Our results suggest that Trichoptera, Hydrachnidia, Diptera, Ephemeroptera and Oligochaeta could be considered as best candidate groups for a tiered-taxonomic resolution approach where only taxa which have narrow and specifi c ecological requirements would be identifi ed to fi ner levels. In Central Europe, however, since taxonomic soundness and easy recognition are required, only Trichoptera and Ephemeroptera are the groups to be recommended at the present time.
The management of aquatic ecosystems requires stream-type specific assessment systems, which enable the partition of natural spatial variability and consequently an optimisation of monitoring and assessment programs. Two different approaches are generally used to construct such typologies. In a top-down approach, environmental variables are chosen on previous knowledge of the general topography of the landscape. In a bottom-up approach, analyses of aquatic communities are used for grouping the streams. In the present study, a typology for LuxembourgÕs streams is defined with the top-down approach i.e., with abiotic criteria and thus criteria different from those necessary for the assessment of biological conditions (i.e., biotic assemblages). This typological determination is based upon a combination of classification and ordination procedures. A particular emphasis was put on variable characterisation. The results show that six different water stream types were identified using our databases. Typological descriptors are stream dimension, elevation as well as geology. Organic charge, slope and land use rather characterise sites within groups. So, types are defined with criteria independent from any anthropogenic influence or pressure. Such a typology is expected to be relevant for analysing aquatic biota in order to build up a model for the biomonitoring of streams.
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