Do nutrient standards support ecological status? A two-step diagnostic framework for evaluation

High nutrient concentrations remain a major pressure on European surface waters, yet nutrient standards intended to support Good Ecological Status (GES) under the EU Water Framework Directive (WFD) vary substantially among Member States and may not support ecological objectives. We developed a diagnostic framework that can be applied to evaluate whether nutrient standards will support biological status using pan-European monitoring data, illustrated here using total phosphorus (TP). First, national TP boundaries were compared with pan-European warning values derived from: (i) stressor-response binary logistic regression models linking nutrients to biological condition, and (ii) smoothed Member State boundary estimates generated using generalised additive models. Second, nutrient and biological classifications were compared using confusion-matrix diagnostics. Phytoplankton classifications were used for lakes and phytobenthos for rivers. Classification agreement (Cohen's kappa) and the balance between commission and omission errors were used to evaluate classification performance and bias. Overall, 24% of lake and 50% of river TP boundaries exceeded warning values. Agreement between nutrient and biological classifications was generally modest (median κ = 0.39 for lakes and 0.31 for rivers), while classification errors showed a consistent positive bias. Country explained more variation in classification performance than typology, and positive bias increased where TP boundaries exceeded warning values. Together, the exceedance of warning values, the accompanying positive bias in classification (nutrient classification passing while biological classification fails), and the dependence of these patterns on country rather than typology indicate that some European TP standards may not consistently support GES and warrant further evaluation. The framework provides a transparent and transferable approach for evaluating nutrient standards and supports more evidence-based revision of nutrient criteria under the WFD.