Caractéristiques hydro-géomorphologiques des microhabitats d'Unio crassus (Ardenne, Belgique)Hydro-geomorphological characteristics of Unio crassus microhabitats (Ardenne, Belgium

<p>Well-oxygenated riverbed is essential for the proper ecological functioning of watercourses and the clogging of interstitial spaces by fine sediments considerably reduces the ecological quality of aquatic habitats. This phenomenon is linked to several parameters: the availability of fine sediments in catchments, soil erosion and land use, and the dynamics of the bed load. Within the framework of two European LIFE projects (LIFE19 NAT/BE/000093 and LIFE 19 NAT BE000054), which aim, among other things, to reintroduce endangered freshwater mussels (<em>Margaritifera margaritifera</em> and <em>Unio crassus</em>) into gravel-bed rivers of the Ardennes region, the clogging/unclogging extent of substrates and the sediment dynamics are monitored. Several monitoring devices were set up.Fine sediment traps were installed in the counter-slope of pools which is known to be a favourable habitat for mussels. This method makes it possible to quantify the amount of fine particles that can infiltrate the subsurface layer. Scour chains were placed on riffles to quantify the thickness of the active layer, which corresponds to the thickness of the bed that can be unclogged during floods. Grainsize surveys were also carried out on riffles to characterise the flow competence of rivers. Finally, the dynamics of the bed load was monitoredthanks to pebbles marked with PIT-tags. At the end, these results alongside biomonitoring and physico-chemical data will enable the identification ofthe most appropriate sites for the reintroduction of freshwater mussels.</p><p> </p>

<p>Since decades, rivers have undergone numerous alterations that have led to disturbances in flow regimes, sediment transport, and water quality to name a few, leaving freshwater fauna facing among the highest risk of extinction. The thick shelled river mussel (<em>Unio crassus</em>) is a European endangered freshwater mussel. As an “umbrella” species, its preservation benefits the whole river ecosystem. However, it remains uneasy to establish a direct causal link between a specific factor of alteration and a decline in the mussel populations. Indeed, in Belgium, this species is unevenly distributed, and its decline is equally uneven. This study explores the link between the species and its fluvial habitat to identify which parameters of the abiotic and biotic environment are responsible for the current distribution of the thick shelled river mussel in Belgium. To test the ability of environmental variables to predict the presence of the species, hydro-geomorphological characteristics of the streams (e.g., return period of the bankfull stage, substrate grain size, land cover, local slope, stream power at bankfull stage…), physico-chemical parameters (water chemistry and pollutants), and host fishes’ abundance of the mussel were compiled. A regularized linear regression technique was used to select the relevant variables to the presence of the species. After selection, logistic regression was performed on the selected parameters to determine which variables have the greatest impact on the species distribution. Our analysis indicates local slope as the most impacting parameter for the presence-absence of the thick shelled river mussel in the studied rivers. Other parameters could be significant, especially at a more local scale. The ecological significance of river longitudinal profile in the existing distribution patterns of the species could be useful to river managers to optimise species habitat and helps establishing target actions to be undertaken to improve their conservation.</p>

<p>Physical microhabitat descriptors are important to predict species habitat and therefore, are critical to river management as a mean to identify and manage habitats of aquatic fauna. Flow velocity, water depth and substrate constitute the fundamental microhabitat of benthic macroinvertebrates. The thick shelled river mussel (<em>Unio crassus</em>) is an endangered European freshwater mussel. The species and its habitat are protected in Europe by Council Directive 92/43/EEC (Annexes II and IV) on the conservation of natural habitats and of wild fauna and flora. Therefore, Member States must establish conservation strategies for this species. In this context, microhabitats of the thick shelled river mussel are mapped in several Belgian rivers, along with the use of the microhabitat by the species. For each microhabitat descriptor i.e., bathymetry, flow velocity, and substrate grain size, a habitat suitability index is computed. Habitat suitability index is defined as the ratio of the species’ use of a microhabitat to its availability. Optimal microhabitat is defined as the microhabitat having to highest frequency of use by mussels. Each shell is measured to detect size-related differences in microhabitat use. The hydromorphological preferences of the threatened mussel are thus identified which will allow the development of habitat suitability criteria for the adult life stage of the species.</p>