Assessing the hydromorphological status of transitional and coastal water bodies in the Basque Country, under the European Water Framework Directive

• An index was proposed to assess the hydromorphological quality. • Separate assessment of morphological and hydrodynamic components is proposed. • Coastal water bodies show a ’Good’ or ’High’ hydromorphological quality. • Transitional water bodies show, in general, a ’Moderate’ hydromorphological quality. Within the Water Framework Directive, the assessment of the hydromorphological quality of water bodies is relevant in the determination of their ecological status, and in the designation of heavily modified water bodies. Based on the Hydromorphological Quality Index (HQI) developed for Ireland, this work presents an adaptation of the approach applied to the transitional and coastal waters in the Basque Country, for the last consecutive 6-year periods (2011–2016 and 2017–2022). The proposed methodology determines the overall quality (HQI g ), and allows the morphological (HQI m ) and hydrodynamic regime (HQI h ) components to be assessed separately. Coastal water bodies showed a ’Good’ or ’High’ overall quality, while for most of the transitional water bodies it was ’Moderate’, except for Nerbioi interior, with ‘Poor’ quality in 2011–2016, and Oka Exterior and Deba, with ’Good’ quality in both periods. The approach is consistent with previous expert judgment assessments and could be useful in designing restoration scenarios.