Establishing reference conditions for European streams

The European Water Framework Directive stipulates that Member States have to assess the ecological status of a water body by comparing the present to the expected reference condition. In the AQEM project participating countries used the criteria from the Water Framework Directive to select reference sites for each stream type. The purpose of this study was to evaluate the suitability of these criteria and to validate the reference conditions chosen, by comparing the classification of sites before analysis with the final assessment of the sites using the assessment system which was developed within the AQEM project. Our study showed that not all criteria for reference conditions could be met for all stream types. This implies that 'true' reference sites could not be selected for all stream types within the AQEM project. In our study, the differences between ecoregions, countries, and stream types were interpreted in terms of human impact. Validation of reference conditions showed differences between classification of sites as reference sites based on criteria to be used in the field and directly interpreting environmental or biological data and the result of the final assessment system. In some countries it was not possible to select reference conditions, because most of the a priori criteria were met. For example, reference conditions for Dutch stream types were established using historical data and predictions based on data from other geographical regions. Using data from adjacent countries appeared to be a feasible method for establishing reference conditions. Reference sites taken from other geographical areas represented a higher ecological quality than the Dutch sites of good ecological quality. However, metric results showed a large overlap between good and high ecological status. Historical information, on the other hand, was not found to be useful in metric calculations because of the confounding differences in sampling methods. One strong advantage of using historical information is that rare species that are locally extinct but still occur in the same stream type in other geographical areas can be added to the expected reference conditions. When and which methods can be used for establishing reference conditions is illustrated in a decision tree.