Mieux comprendre les effets écologiques des variations artificielles des débits au travers de la reproduction des insectes aquatiques

Global changes and the increasing electricity demand are driving a societal transition toward renewable energy sources. Hydropower represents the largest source of low-emission electricity worldwide but has deleterious effects on aquatic ecosystems, whose nature and extent depend on the mode of hydropower plant management. Hydropeaking causes (sub)-daily variations in discharge, and consequently in flow velocity and water level downstream of hydropower plants. These artificial variations disturb aquatic organisms adapted to the natural flow regime. Stream insects, keystone organisms in freshwater food webs, have complex life cycles including multiple life stages, with transitions between terrestrial and aquatic environments (e.g. emergence, oviposition) for many taxa. The effects of hydropeaking on stream insects have been mainly studied for the aquatic larval stage. Oviposition by the adult stage and egg development may also be disturbed by artificial variations of hydraulics conditions, which may cause changes in oviposition habitat availability and egg desiccation. This thesis uses Baetis spp., a genus of mayflies with selective oviposition behaviour, as a study model to assess the effects of artificial hydraulic conditions variation caused by hydropeaking on the oviposition habitat and behaviour and the response of egg masses to dewatering. Chapter 1 presents an empirical study conducted on two rivers during the flight period of stream insects to investigate the oviposition habitat selectivity of Baetis females under different flow regulation contexts. Field observations show less oviposition habitat selectivity by Baetis females in the hydropeaked river compared to an unregulated river, in terms of substrate size and flow velocity. Chapter 2 uses mesocosm experiments to assess the sensitivity of Baetis egg masses to dewatering. This work identifies a tolerance threshold of approximately 6 h of dewatering, beyond which egg hatching percentage declines drastically. Chapter 3 is based on the results of Chapter 1 and on the outputs of a 2D hydrodynamic model of the lower Ain River (a hydropeaked river) to evaluate the availability of Baetis oviposition habitat at the large river segment scale (38 km). The results reveal significant variation of oviposition habitat availability in response to variation of discharge, with, in some cases, more than 85 % of oviposition habitat available during peak flows being dewatered after a hydropeaking event. Focusing on reproduction, a key biological function, this thesis uses multiple approaches to deepen our understanding of the constraints imposed by artificial flow variations on organisms with complex life cycles, such as stream insects. Affecting at large scale the availability of oviposition habitat, these disturbances may act as a population bottleneck preventing the completion of the life cycle locally and creating extensive ‘’sink’’ areas within metapopulations. The empirical estimates obtained through fieldwork and laboratory studies provide a basis for potential new indicators to quantify the impacts of flow management and the effectiveness of mitigation measures. This work opens numerous outlooks and questions for future research that will provide a better understanding of the limits of resistance and resilience of aquatic organisms to human activities and provide new benchmarks for sustainable management of aquatic ecosystems.