The paradox of expert judgment in rivers ecological monitoring

A great investment has been done in the last decades in the development of numerical and qualitative assessment methods to classify the ecological quality of water bodies. Yet, in spite of all attempts to avoid subjectivity, expert judgment is still used at numerous steps of the ecological classification and is considered by some authors as indispensible for management purposes. Thus, the aim of this study is to test the hypothesis that expert judgment, when done by the adequate experts (limnologists/river ecologist) with experience in the study area (i.e., natural conditions and expected communities), could be as good as quantitative indices and measures (i.e., result in the same classification), but quicker and with lower cost. For that we compared the classifications (on 13 aspects of rivers ecosystems) attributed by two experts to 20 sites (10 each) located in their study areas, with the classifications of ecological quality based on biological indices (for invertebrates and diatoms), hydromorphology and water chemistry, calculated by an independent team. Our results show that assessments made by experts and those calculated through indices (biological quality and hydromorphology) are globally very similar (RELATE test; Rho = 0.442; p