INVESTIGATION OF INTERACTION BETWEEN FLOW AND FRESHWATER MUSSELS IN RIVERS IN A CONTEXT OF CLIMATE CHANGE

Freshwater mussels have been acknowledged as valuable indicators for environmental disturbances, there is a limited understanding of their efficacy in monitoring the effects of hydrodynamic stressors and their role in surface processes and sediment budgets in rivers. Given the projected increase in extreme events due to future climatic scenarios, it is crucial to comprehend the impact of flood conditions on freshwater organisms. This understanding is essential in determining whether the behavioral responses of mussels can serve as an early warning system for flow events in rivers and clarify fundamental concepts in fluvial geomorphology.The present study aims to investigate the interplay between flow dynamics and freshwater mussels, specifically those belonging to the order of Unionidae. Laboratory experiments were conducted in an artificial flume at the Hydraulic Engineering Laboratory at the University of Palermo, involving two types of experiments to explore the relationship between hydrodynamics and freshwater mussels, as well as the effects of their presence on bed erosion processes. In the first type of experiments, two groups of sixteen mussels (Unio elongatulus) were subjected to different levels of flow discharge, both in steady and unsteady conditions, considering both gravel bed and sand bed conditions. The use of these two substrates facilitated the analysis of the mussels' response to sudden changes in flow discharge with or without sediment transport, as well as the impact of mussels' movement in the sand bed environment. The steady conditions involved maintaining a constant flow rate, while the unsteady conditions entailed an abrupt change in flow rate. Different observation periods were utilized to assess the behaviour of Freshwater Mussels (FMs) throughout the day in laboratory conditions and their immediate response following the sudden variation in discharge.Valvometry technique and Hall sensor technology were used to detect the mussels' behavioural responses, measuring the frequency, amplitude, and duration of valve gaping under steady and unsteady conditions. The mean frequency and amplitude of valve gaping were analysed and compared during the different tests. The behaviours of the FMs, the percentage of mussels exhibiting each behaviour, and the duration were also evaluated. Statistical analyses, i.e., T-tests or Wilcoxon tests, were conducted after a Shapiro-Wilk test, to assess the data distribution and to compare the FMs’ response to the different conditions examined. Six distinct categories of behavioural response were identified: Normal Activity (NA) and Resting (Re) during steady conditions, Transition (Tr), Normal Activity after Variation (NAaV), Adaptation (Ad), and Avoidance (Av) during unsteady conditions. During steady conditions, FMs displayed valve gaping while engaging in normal activity, such as feeding and moving. During resting, the valves were held constantly open for water filtration. Following a variation in flow discharge, the mussels promptly responded with a transition from their normal behaviour to higher valve gaping frequencies, indicating a Transition behaviour. The mean valve gaping frequency increased in proportion to the flow discharge variation, with higher values observed in the presence of sediment transport and higher flow rates. In steady conditions, the mean frequency was lower than 0.01 Hz and the mean valve opening amplitude was below 20%. In unsteady conditions the values of the FMs’ mean frequency increased as the value of the discharge variation, being higher than 0.01 Hz, reaching values about 0.02 Hz.A pilot installation was also realized, as part of activities conducted in the ambit of a PRIN2017 project, at the Adunata Bridge along the Paglia River (Orvieto, Umbria). Thirteen Unio mancus’ response were analysed also by the valvometry technique. In particular in the present work the data collected on 31st of March 2022, day in which a flood event occurred, have been analysed. The field data collected demonstrated an increase in the mean frequency of the FMs’ valve gaping as the value of the flow rate during the flood event intensified, reaching the value of 0.02 Hz, as in the laboratory tests. Moreover, during the peak of the flood event a high percentage of FMs showed Transition behaviour, corroborating the findings obtained in the laboratory experiments. The statistical analysis conducted to compare the laboratory data to the field data indicated a no difference in terms of mean amplitude with the FMs’ response over the gravel bed in the laboratory condition.Additional experimental analyses were conducted in the laboratory flume to examine the effects induced by the presence of FMs on local scouring processes downstream of a fixed bed by considering three different FMs’ spatial distribution on the bed and different values of flow discharge. The results showed that the presence of FMs reduced the erosion processes downstream of the fixed bed, reducing the depth of the erosion peaks. The global eroded volumes for the entire length of the sand bed in the runs without FMs increased with rising flow rates. Similarly, the deposited volumes were lowest in the run with the lowest flow rate, while in the other runs, they were comparable. In the run with FMs positioned close to the fixed bed, the eroded volumes strongly decreased in comparison to the case of the absence of FMs. In conclusion the analyses in the present work have demonstrated that invertebrates are not only crucial to the ecological functioning of freshwaters (Wallace & Webster, 1996) but also serve as important sediment engineers with small-scale effects that may be significant at much larger scales.