Typology and type specific reference conditions for medium-sized and large rivers in North Rhine-Westphalia: Methodical and biological aspects

According to the ‘EU Water Framework Directive’ the assessment of the ecological quality of streams and rivers should be based on type specific reference conditions. In Central Europe, stream sections suited to serve as reference sites for the high ecological status of rivers are rarely found, due to long lasting anthropogenic impact on virtually all rivers. We present a method for the description of reference conditions for medium-sized and large rivers, which has been developed and tested for rivers in North Rhine-Westphalia (Germany). Present data on the fauna proved to be sufficient for the description of river types, at least on a rough scale. However, even the fauna of the least deteriorated Central European river sections is very different to the ‘historical’ fauna. Therefore, the biological reference conditions are only partially based on investigations in the present fauna. It is indispensable to use additional sources to describe ‘reference communities’, particularly historical literature data. Furthermore, considerations on only slightly altered habitat-conditions in rivers and on landscape patterns under pristine conditions are taken into account.