2-D Hydrodynamic Modelling of eDNA Dispersion and Occurrence in a Restored Alpine River Reach

Two-dimensional (2-D) hydrodynamic models based on high resolution bathymetric data from Airborne Lidar Bathymetry (ALB) surveys and cross-profile surveys are widely used to describe spatial flow variability in river systems and are particularly important in restored and widened river reaches, where flow patterns are highly heterogeneous. In parallel, environmental DNA (eDNA) has emerged as a cost-effective and non-invasive tool for biodiversity monitoring. However, linking eDNA signals to hydraulic processes in complex river geometries remains a key challenge, limiting its application for assessing restoration success.This study investigates the relationship between hydrodynamic conditions and fish eDNA dispersion and occurrence in a restored alpine river reach. The study site is located on the Inn River between Stams and Rietz in Tyrol, Austria, where as part of river restoration measures, bank protections were removed to widen the river and side arms were created to provide hydraulically diverse conditions for habitats. A controlled eDNA source, consisting of a cage containing dead eels (Anguilla anguilla), was placed in a side arm of the restored reach. Water samples were collected at multiple locations downstream and across the river and filtered on site. The eDNA was extracted from the filters and analysed for fish DNA using metbarcoding and general fish primers. Additionally, all samples were screed for eDNA of eel using qPCR and the total amount of overall fish eDNA from was determined for each sample. The spatial distribution and occurrence of eDNA from fish is analysed in relation to simulated hydraulic parameters, including flow velocity and water depth, derived from a 2-D hydrodynamic model. The results provide insights into how local flow conditions influence eDNA transport and dilution in restored river sections. This improved process understanding supports the use of eDNA as a monitoring tool for river restoration projects and contributes to the ecological assessment of restored reaches in line with the European Water Framework Directive.