A Dedicated Alarm Criterion by Using Freshwater Mussels' Valve Movement as Biological Early‐Warning System to Identify Impacts of Flow Discharge Variations in Fluvial Ecosystem

Understanding the impact of hydrodynamic variations, induced by climate change or anthropogenic pressures on aquatic habitats is necessary for effective freshwater conservation. In this work, an ecosystem‐impact approach is applied by using freshwater mussels (FMs) as biological indicators of perturbations of aquatic environment. The valvometry technique is used to investigate FMs' response in different substrate compositions and environments. Laboratory flume experiments were performed with Unio elongatulus over two substrates (gravel and sand), and in situ pilot installation with stuck Unio mancus was realized in Paglia river (Umbria, Italy). FMs' response to flow discharge variation was analyzed in terms of average gaping frequency (Fa) and gaping amplitude (Aa). For the field case, the analysis concerned valvometry data recorded during a moderate flood on 31 March 2022. Both laboratory and field‐based experiments showed that FMs promptly react to flow perturbation increasing values of Fa and decreasing values of Aa as the percentage of flow discharge variation, ΔQ/Qbp (being ΔQ the flow discharge variation and Qbp the flow discharge before the perturbation), increased. Based on the data, a threshold condition was derived which can be used as an ecosystem alarm criterion based on mussel behavior. This could help practitioners, academic ecologists and controlling agencies in decision‐making processes. In this view, the paper also presents, to our knowledge for the first time, a [Fa, ΔQ/Qbp] benchmark graph which could be helpful in guiding the selection of the ecosystem alarm criterion in clear water, constituting a base for future development in natural conditions with suspended materials.