How good is good ecological status? A test across river typologies, diatom indices and biological elements

Good ecological status is the environmental objective of EU water legislation to be achieved by all European water bodies. However, good ecological status varies depending on national criteria, typology approach, and classification systems used. Since nutrient enrichment is the main cause of river degradation, it is important to establish nutrient criteria that consistently support good ecological status across these influential factors. This study analyzes good ecological status, depending on the typology, classification system and biological element used, and it discusses potential implications of the results for river management. We used a database of 425 sites from northern Spain, corresponding to 11 river types of the Spanish typology derived from physiographic data, or to the four river types resulting from NORTIdiat predictive model, derived from regional diatom reference assemblages. PERMANOVA analysis found significant differences among diatom assemblages across the four river types derived from the NORTIdiat system. Among the classification systems currently in use, or of potential use in the area, the upper P-PO₄^3- threshold, established as the P₉₅ of the class distribution for good ecological status, both NORTIdiat (50.7 μg l^-1) and the Multimetric Diatom index (MDIAT; 26.4 μg l^-1) were close to proposed thresholds for good status at the EU level. However, this value was much higher for the Specific Polluosensitivity Index (IPS; 118.1 μg l^-1). Nutrient thresholds for good status also varied among bioindicators, since the predictive invertebrate-based model NORTI classified 67% of samples with high P-PO₄^3- content in good ecological status, whereas the NORTIdiat classified only 33% of them in good status. Results suggest that current nutrient criteria used to establish good ecological status should be revised, accounting for biological specificity and response of biological elements, to provide a more ecologically coherent approach to preserving or restoring good ecological status.