Connectivity modulates local and beta diversity responses of macroinvertebrates to water-level fluctuations

Abstract While hydrological regimes and connectivity shape floodplain ecosystems, our understanding of how macroinvertebrate communities within these systems respond to long-term water-level fluctuations remains limited. This represents a critical knowledge gap, as these organisms underpin essential services that freshwaters provide for human well-being. Here, we assessed taxa richness, density, beta diversity, and community composition across three hydrological conditions (low, intermediate, and high-water levels) in sporadically (SCL) and permanently (PCL) connected lakes, using 14 years of monitoring in the Upper Paraná River. Richness and density displayed a curvilinear response, peaking at intermediate-water levels. Beta diversity varied among lakes with SCL contributing mostly to regional diversity during low-water levels, whereas PCL contributed more during high-water levels. Hydrology drives these patterns by regulating connectivity once floods facilitate dispersal and regional homogenization, while droughts isolate lakes, amplifying local environmental filters. Local abiotic (dissolved oxygen, conductivity, turbidity) and biotic (fish density, macrophyte taxonomic richness) predictors influenced community composition during droughts, whereas regional hydrology dominated under elevated water conditions. These results highlight the complementary roles of SCL and PCL in sustaining regional biodiversity, providing critical evidence for floodplain conservation under increasing climate change.