Bringing European river quality into line: an exercise to intercalibrate macro-invertebrate classification methods Cathy BennettRoger OwenSebastian BirkAndrea Buffagni • Stefania ErbaNicolas MenginJohn Murray-BlighGisela Ofenbock • Isabel PardoWouter van de BundFranz WagnerJean-Gabriel Wasson

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 compa- rable 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