Responses of Oligochaeta and Chironomidae to restoration of connectivity in a river–floodplain stretch of the Upper Danube River

Abstract Introduction In the Upper Danube River, a key goal of river–floodplain restoration is to enhance hydrological connectivity and reduce aquatic habitat isolation and terrestrialization. To evaluate restoration effectiveness, it is crucial to understand the functional responses of freshwater biodiversity and the role of connectivity and environmental factors. Objectives In this study, we compared oligochaetes and chironomids across control and impacted sites before, short‐term, and long‐term after restoration in a river–floodplain stretch of the Donau‐Auen National Park. Methods We applied a Before‐After‐Control‐Impact (BACI) design to analyze the effects of restoration‐induced connectivity changes on the taxonomic and functional composition and diversity of these groups. A graph theoretical approach and partial least squares regressions were used to assess the impact of connectivity changes on taxonomic and functional diversity. Results Our BACI analysis showed positive effects of restoration on oligochaete diversity, but long‐term terrestrialization processes outweighed restoration impacts. Traits such as stream zonation preference, current preference, body size, dispersal strategy, drift propensity, and adult lifespan exhibited short‐term restoration effects for both groups, returning to pre‐restoration conditions in the long term. For oligochaete functional diversity, connectivity was influential shortly after restoration, while environmental factors became more significant over time. Conclusion Our results highlight both the potential for short‐term improvements and the challenges of maintaining restoration‐driven benefits over time.