One of the objectives of the European Water Framework Directive (WFD; EU, 2000) is to attain a good status for all surface waters in the European Union by the end of 2015. To this end, the ecological status of natural surface waters should be assessed, based on a number of biological quality elements which depend on the category of surface water (rivers, lakes, transitional waters or coastal waters). For each of these elements, member states must choose or develop a classification method, taking into account a set of parameters depending on the quality element and on the surface water category (EU, 2000). The assessment system must be differentiated among the types of water bodies within a category. The method must be in agreement with an Ecological Quality Ratio (EQR) showing relative proportion of the index compared to the reference conditions. This EQR ranges from zero to one, respectively corresponding to a bad and a very good ecological status. This interval is divided into five classes reflecting bad, poor, moderate, good and high ecological status (EU, 2000). For the categories rivers and lakes, one of the relevant biological quality elements is the “benthic invertebrate fauna” (EU, 2000), commonly referred to as macroinvertebrates. For this quality element, the parameters “taxonomic composition and abundance”, “ratio of disturbance sensitive to insensitive taxa” and “diversity” should be taken into account. In Flanders, Belgium, the Belgian Biotic Index (BBI; De Pauw and Vanhooren, 1983), based on macroinvertebrates, has been applied in routine monitoring schemes by the Flemish Environment Agency (VMM) since 1989, confirming the reliability and robustness of this biological quality index. However, with respect to the application of this index for rivers and lakes within a WFD context, not all technical requirements are met by the BBI. The abundance, which is one of the relevant parameters imposed by the WFD, is not taken into account in the BBI calculation, and this index is not explicitly based on a ...