From expert judgement to supervised classification: a new approach to assess ecological status in lowland streams

The EC Water Framework Directive (WFD) clearly states that undisturbed reference states of aquatic ecosystems should be used to set standards for restoration. Across Europe defining biological reference status and setting boundaries for ecological status classes continues to represent a major challenge. In the present study we investigate if a paradigm exists among experts that can guide the development of assessment systems based on the normative definitions of ecological status classes of the WFD. Our main questions were: 1) Will experts from species abundance data and typology descriptors independently arrive at similar assessments of ecological status, and 2) Can the expert interpretation of ecological status be transferred into a statistical model allowing for a standardization of assessments from plant assemblages in lowland streams? We used a large dataset covering 1244 randomly distributed stream sites in Denmark and asked a group of experts to independently classify the sites using the WFD's normative definitions of ecological status. According to the combined expert group, no Danish stream sites belonged to the undisturbed reference state. For the remaining ecological status classes we found good concordance in the classification made by the five experts. From this we infer that a common paradigm does exist, which may guide the development of assessment methods for aquatic plants in lowland streams. We also found that the common view of the experts could be transferred into a supervised classification model that can serve as a classification tool for aquatic plant assemblages in lowland streams. We conclude that the combined use of experts and advanced multivariate statistics can provide a useful approach in the development of systems for assessment of ecological status in water types, where a reference network cannot be established.