Identification of hydrodynamic changes in rivers by means of freshwater mussels' behavioural response: An experimental investigation

The present work concerns the interaction between hydraulic processes and biological communities in rivers. In particular, the aim of this study is to investigate the interactions between flow dynamics and the freshwater mussels (FMs) to verify if the mussels' behavioural response to the hydrodynamic stress could be used to monitor natural extreme events in rivers. Although the influence of mussels on the kinematic characteristics of flow at the water–sediment interface was investigated by a certain number of studies, their behavioural response to flow, both in static and dynamic conditions, remains understudied. Laboratory experiments were performed in an artificial flume exposing Unio elongatulus to different values of flow discharge, both in steady and in unsteady conditions either with or without sediment transport. Mussels' behavioural responses were detected by using Hall sensor technology to measure gaping frequency, amplitude and duration, both in static conditions and under the effect of hydrodynamic stresses. Five categories of behavioural response were identified: Normal Activity (NA), Resting (Re), Transition (Tr), Adaptation (Ad) and Avoidance (Av). During NA (standard feeding and moving), FMs presented valve gaping, while during Re valves were kept constantly opened for water filtration. After a variation of flow discharge (ΔQ), FMs promptly reacted showing a transition from their normal behaviour, with constant gaping frequency (below 0.01 Hz), to higher valve gaping frequencies. The mean valves' gaping frequency increased as a function of ΔQ, and the highest values were reached in the presence of sediment transport. The mean valve opening amplitude was less sensitive to ΔQ. Its range of variation was very narrow with the highest values corresponding to the protrusion/retraction of the animals' foot to move or anchor to the substrate. The percentage of mussels responding to the discharge variation (Transition behaviour) increases with ΔQ confirming that mussels' behavioural response represents a promising tool for monitoring the occurrence of hydrodynamic stressors in fluvial systems

Flow characteristics in gravel bed rivers: an experimental investigation

The characterization of the kinematic characteristics of flow in gravel-bed rivers is one of the most important problems. In the present paper the streamwise velocity and turbulence intensity profiles are investigated on the basis of data collected in a straight laboratory gravel-bed flume. In order to take into account spatial heterogeneities, the double-averaged velocities are considered for the anal-ysis. Results show that, because of the presence of gravels, all the streamwise velocity profiles show an inflection point. Thus, the experimental profiles can be interpreted by the mixing layer scheme and considered as a composition of two parts of constant velocity separated by a confined interme-diate region (mixing layer) containing the inflection point. The thickness of the mixing layer varies passing from the banks to the channel axis

Gravel-bed rivers: roughness surface identification and effect on flow characteristics

Riverbeds are often covered with gravels. The characterization of the gravel bed is very complex, depending on many factors such as the gravels’ shape, morphology and spatial distribution. The presence of gravels determines a significant impact on flow field (Ferraro et al., 2016). The evaluation of flow velocity and turbulence characteristics in natural rough beds is a complex subject, still poorly understood despite the effort of several researchers, requiring both theoretical and experimental analyses. On one hand the bed roughness exhibits a significant spatial variability, on the other hand the presence of gravels determines a significant effect on flow velocity distribution, especially affecting it in the near-bed flow zone and determining the formation of coherent turbulence structures. How to reasonably quantify gravel bed surface roughness is still an open question and different methods have been proposed in literature (among others Nikora et al., 1998; Aberle and Nikora, 2006). It is important to understand the turbulent flow characteristics by resolving the spatial heterogeneities

Establishing mussel behaviour as a monitoring tool to measure climatic disturbances

Freshwater mussels (FM) are suitable biological indicators to assess environmental stressors. Behaviour is a good parameter to measure rapid mussel’s responses. In this sense, valvometric method, that exploits the Hall sensor (real-time remote monitoring tool), has been used to measure the behavioural responses of mussels (valve opening amplitude and valve opening-closure frequency). During the last two decades, these methodologies have been used to measure the presence of the pollutants in the waterbodies. However, studies that focus on physical disturbances in the environment related to climate changes are lacking. Since future climatic scenarios in Europe predict an increase in temperature and change in the hydrological conditions, the main objective of this study was to evaluate the influence of water temperature increase and water discharge rates (without and with sediment transport) in four endpoints of mussel's behaviour: normal activity, resting, transition and avoidance. Our results suggest, that on experiments with increasing temperatures and high variation in water discharge rates with sediment transport, mussels presented high valve opening-closure frequencies and amplitudes. In this sense, mussels could be used as biological sensors to monitor changes in thermal and hydrological conditions which are increasing with climate changes

Mussel behaviour as a tool to measure the impactof hydrodynamic stressors

Freshwater mussels (FMs) are useful bioindicators to detect environmental disturbances. However, studies that evaluate FMs suitability for monitoring the hydrodynamic stressors impact are lacking. Since future climatic scenarios predict an increase in frequency and intensity of extreme events, understanding how flood conditions affect freshwater organisms are crucial for their conservation. In this study, we performed experiments in an artificial flume to evaluate the eligibility for FMs behaviour for developing a tailored biological early warning system (BEWS). For this, we used the valvometric technique (Hall magnetic sensors) to measure the FMs valve gaping behaviour when subjected to increasing discharges/sediment transport mimicking the onset of floods. After analysing baseline behaviour in non-stressful conditions, we performed experiments in steady and transient conditions to verify the FMs’ response and the threshold that prompted it. Under steady conditions, FMs maintained a constant gaping frequency that characterizes their normal behaviour. The FMs promptly reacted to discharge variations with sediment transport, showing a transition from their normal behaviour to higher valve gaping frequencies. We demonstrated that FM transition behaviour is a useful tool to measure hydrodynamic stressors. A future step will be the application of this BEWS on natural ecosystems to assess possible hydrodynamic changes in real-time