The European reference condition concept: A scientific and technical approach to identify minimally-impacted river ecosystems.

One objective of the European Union (EU)'s Water Framework Directive (WFD: Directive 2000/60/EC) is for all European surface waters to achieve 'good status' by 2015. In support of this objective, the EU has facilitated an intercalibration exercise to ensure harmonized definitions of the status of water bodies, reflecting the deviation of their properties (mainly biotic assemblages) from a minimally disturbed state, termed the "reference condition". One of the major challenges of the WFD has been to find common approaches for defining reference conditions and to define the level of anthropogenic intervention allowed in reference sites. In this paper we describe how river reference sites were selected in the Central-Baltic region of Europe. A list of pressure criteria was provided and 14 Member States (MSs) categorized each criterion according to the method (i.e. measured, field inspection, etc.) used for reference site screening. Additionally, reference land-use and water-chemistry thresholds were agreed among countries in order to base reference site selection on objective criteria. For land-use criteria, a reference threshold and a rejection threshold were established. Sites with all criteria below the reference threshold were considered to be reference sites; sites having most criteria below the reference threshold and only some parameters between the reference and rejection threshold were "possible reference sites". These sites were retained only after carefully checking the cumulative effects of the pressures using local expertise, and a posteriori water-chemistry evaluation was necessary. In general, the most widespread method for defining a reference site was the measurement of pressures, followed by field inspections and expert judgment. However, some major pressures (e.g. hydromorphological alteration) were evaluated in a number of different ways (e.g. measured, field inspection, expert judgment). Our meta-analyses reveal a need to reinforce standardization in the application of pressure criteria by Member States. The pressure criteria identified in this exercise should be refined and tested with biological data to help in the further validation of minimally disturbed sites (i.e. the WFD "reference condition") and to provide a firm foundation for ecological status assessment. This in turn would ensure that there is pan-European comparability when evaluating the achievement of environmental objectives.