Efficacy of basin-wide river restoration in a changing climate: contrasting responses of fish and macroinvertebrates in the Dommel catchment

In the European Union, the European Water Framework Directive (WFD) has prompted water authorities to implement restoration measures and monitor biological indicators such as fish and macroinvertebrates. In this study, we examined the effects of longitudinal connectivity and morphology restoration together with water quality improvement on the long-term development (2002–2024) of fish and macroinvertebrate assemblages in the lowland Dommel river catchment in the Netherlands (1500 km2). Despite the gradual improvement in connectivity, morphology, and water quality, contrasting responses were observed between the two indicator groups examined. The proportion of sites with high or good ecological status based on period-average EQR scores for macroinvertebrates increased from 6% in 2002–2009 to 25% in 2017–2024. Positive indicator taxa such as Gammarus and Calopteryx were associated with well-connected, re-meandered or non-channelised reaches and higher oxygen concentrations. In contrast, fish ecological quality ratios declined with the proportion of sites with High or Good status dropping from 9% in 2002–2009 to just 2% in 2017–2024. In addition, GAMMs showed fish density declines throughout the catchment after years with dry summers, indicating that the overarching effects of droughts can mask potential positive effects of restoration. These results demonstrate that biological indicators might reflect changes at different spatial and temporal scales, likely due to contrasting dispersal abilities, life-cycle durations, and sensitivities to fragmentation and drought between the indicator groups. Moreover, this study highlights that achieving both WFD targets and long-term ecological recovery requires addressing broad-scale stressors such as climate change.