Natural disturbances and connectivity shape the seasonal variability of aquatic macroinvertebrate communities across Europe

Understanding the joint influence of natural disturbance, spatial connectivity and biogeography on biodiversity is essential to forecast its responses to climate change. Macroinvertebrate communities in drying river networks constitute an ideal study system to understand the interplay of these ecological processes. We analyze the taxonomic and functional structure of macroinvertebrate communities sampled across 126 reaches with perennial and intermittent streamflow, surveyed in six drying river networks (DRN) across Europe, six times over one year. Drying frequency decreased community richness and functional diversity of communities, whereas spatio-temporal connectivity increased community richness in intermittent reaches. Communities experiencing a high drying frequency increased the proportion of taxa with K-strategies and drying resistance traits. Communities experiencing a long drying duration compensated by high spatio-temporal connectivity had more taxa with a r-strategy and high dispersal ability. Perennial communities varied from taxa-poor communities of r-strategists in spring and autumn and taxa-rich communities of K-strategists in summer and had a constant functional diversity throughout the year. When drying frequency increased, communities showed a similar pattern except in autumn when they shifted towards species-poor communities of K-strategists. Functional diversity then peaked in summer. Community trait structure and in particular optimal drying resistance traits changed across biogeographical scales. It opposed communities from mountainous DRN (with more r-strategies and high dispersal ability) to non-mountainous DRN (with more K-strategies). Drying frequency, drying duration, and spatio-temporal connectivity drive divergent community structures, suggesting the presence of an ecological threshold that explains the variability of disturbed ecosystems across broad spatial scales. These factors also shaped seasonal community variations, particularly after summer, with intermittent communities influenced by stochastic recolonization events in spring and autumn. Spatial-temporal connectivity proved crucial for maintaining diversity in communities subjected to intense drying. Lastly, the effectiveness of drying resistance traits was dependent on the biogeographical and environmental conditions of drying river networks.