Operational comparability of ecological status assessment in Iberian transboundary rivers

The European Union Water Framework Directive (WFD) requires member states to assess the ecological status of surface waters using biological quality elements (BQEs). Although the WFD includes intercalibration procedures to harmonise class boundaries, differences between national assessment systems may affect the comparability of ecological status classifications, particularly in transboundary rivers. This study evaluated the consistency of Portuguese and Spanish ecological status assessments in 32 water bodies across Iberian transboundary rivers in the Minho, Lima, Douro, Tagus and Guadiana basins, using data collected between 2019 and 2021. Comparisons included fish, macrophytes, benthic macroinvertebrates and diatoms. Biological assemblages, ecological quality ratios (EQRs) and ecological quality classifications were analysed. Results differed markedly among BQEs. Fish showed no significant differences in assemblage composition or EQR values between countries and a moderate positive correlation in ecological quality classifications. Macroinvertebrates displayed weak compositional separation, no differences in EQR values and a weak, nonsignificant positive correlation between countries. Macrophytes showed significant compositional differences despite similar EQR distributions and a weak, nonsignificant positive correlation in ecological quality classifications. Diatoms showed the strongest divergence, with clear assemblage separation, higher EQR values in Spain and a very weak, nonsignificant negative correlation between countries. Final ecological classifications also showed a weak, nonsignificant positive correlation between countries. These findings show that formal intercalibration alone does not guarantee operational comparability in shared river systems. Cross-border assessment consistency depends strongly on the BQEs considered and on methodological differences between national monitoring systems, highlighting the need for improved harmonisation of monitoring procedures and taxonomic practices.