Towards the assessment of E-flows: a fish-based approach for the Tiber River basin (Central Italy)

Estimating ecological flows (EFs) in rivers is crucial for meeting the Water Framework Directive (WFD) 2000/60/EC goals. To develop an EF model, it is essential to use biological indicators to reliably record the impacts of human-induced alterations on aquatic ecosystems. The adaptability of fish to annual flow variability makes them a significant biological quality element (BQE). Consequently, using fish as bioindicators is a powerful tool for assessing the impact of changes in the hydrological regime on the natural communities within a river system. The new index of the ecological status of fish communities (NISECI) is a methodology developed in Italy that uses fish communities as BQEs to assess the ecological status of rivers, as directed by the WFD. We aimed to apply the NISECI to define the transition from the minimum vital flow (MVF), currently in force in Italian legislation, to the EF, as per the WFD. Specifically, we defined NISECI values for 26 stretches of river located within the Tiber River basin hydrographic network and characterized the physicochemical and hydromorphological conditions. Fish and environmental data were collected between July 2024 and May 2025. For summer flow rates (the low-flow season), we equated the EF to the MVF at locations where WFD objectives had been met (i.e. good ecological status). In the other cases, we predicted an increase in flow rate depending on the deviation from good ecological status. In line with this holistic multidisciplinary approach, we based variation in the quantity, quality, and timing of water discharge envisioned by the EF concept on the natural flow duration curves calculated for the Tiber River basin.