Bringing European river quality into line: an exercise to intercalibrate macro-invertebrate classification methods

The Water Framework Directive (WFD) requires intercalibration to be performed to ensure that ecological status, as defined by the boundary values of national biological assessment systems, is consistent with the definitions outlined in the WFD and comparable between Member States (MS). This article describes an intercalibration of 17 national river macro-invertebrate assessment methods from the Central and Baltic regions of Europe. We explore the hypothesis that intercalibration should be successful if ratios of the observed biota to that expected in reference condition are used to compare assessments of different national assessment systems. National boundaries expressed as ecological quality ratios (EQRs) were converted to values of a common multi-metric for the purpose of comparison. Twelve MS for the High/Good boundary and nine MS for the Good/Moderate boundary (and four MS who subsequently harmonised their boundaries) were within ±0.05 EQR units of the intercalibration boundaries and were deemed to be of comparable ecological standard. The use of a reference-based approach was deemed to be successful given that all the critical pre-requisites for intercalibration were satisfied. The boundaries derived from this intercalibration represent the first common interpretation of the ecological status of rivers based on macro-invertebrate assessment methods across Europe.