The validation of common European class boundaries for river benthic macroinvertebrates to facilitate the intercalibration process of the Water Framework Directive

The Water Framework Directive (WFD) requires that all the water bodies in Europe be protected and enhanced to achieve Good Ecological Status by 2015. The intercalibration of the biological monitoring results of Member States has to be carried out in relation to classification tasks to guarantee a common understanding of ‘Good Ecological Status’ at a European level. An intercalibration exercise was carried out within the framework of different Geographical Intercalibration Groups (GIGs), each composed of a group of countries that share similar river types and have a discrete geographical range. By means of the intercalibration exercise, common European boundaries were proposed for river invertebrate assessment methods within each GIG. The purpose of this study was to validate the boundaries proposed for the formal intercalibration exercise. A benchmark data set was used, which comprised data, collected in different European countries that satisfied WFD requirements. The data set included a set of reference sites and provided evidence of a high degree of comparability among countries. The STAR Intercalibration Common Metric index (STAR_ICMi) was calculated for benchmark samples and was selected as the index in which national assessment boundaries were expressed. It was applied for the intercalibration exercise in two GIGs. For the STAR_ICMi, the coefficient of variation was also calculated, demonstrating a comparable variability with indices that are based on species level identification. A fixed percentile of reference samples STAR_ICMi values was selected as the boundary between High and Good status. The range from this fixed percentile to the lowest possible value was divided into four equal parts to obtain the remaining class boundaries. The resulting High/Good and Good/Moderate boundaries were compared to the boundaries proposed by GIGs and proved to be in line with those defined in the intercalibration exercise. Even if, for the intercalibration exercise, some procedures to check the Member States (MS) data sets where put into practice, it is the responsibility of each state to guarantee the WFD compliancy of their reference conditions and methods. Accordingly, the process of validation explained in this article, or similar ones, can be an important step forward and demonstrate the comparability of the actual boundaries.