Towards a Single Eutrophication Assessment: Identifying Drivers for an Integrated WFD-MSFD Eutrophication Framework in Portuguese Coastal Waters

The Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD) are the two main European policy instruments for assessing eutrophication in coastal waters, yet their differing assessment architectures often lead to inconsistent classification outcomes. This study provides a scientific comparison of WFD Ecological Status and MSFD Good Environmental Status (GES) classifications for Portuguese coastal waters across three assessment cycles. This is achieved by quantifying the coherence between Eutrophication assessments, by identifying the main drivers of divergence beyond chance, and evaluating where harmonization improved agreement, providing an evidence-based guidance to decision-making and policy regulation. Using officially validated national classifications, we analyzed the methodological drivers of divergence (without reprocessing raw monitoring data) and harmonized both outcomes into a common three-class scheme. Coherence was evaluated using a Discordance Index and Cohen’s kappa coefficient. Results showed that divergence was systematic rather than random, primarily driven by structural (spatial and temporal) misalignment, methodological differences in indicator integration, and contrasting statistical metrics. Both Directives consistently identify eutrophication hotspots associated with major river plumes but differ in how these signals are aggregated and translated into status classes. The study demonstrated that WFD and MSFD provide complementary but only partially aligned assessments, and that coherence improved with methodological harmonization.