Legacy land use and seasonal dynamics jointly drive spatial and temporal β-diversity of aquatic insects in eastern Amazonian streams

Land-use changes are major drivers of freshwater biodiversity loss, yet the persistence of its legacy effects and their interactions with hydrological seasonality remain poorly understood, particularly in tropical regions. In this study, we investigated how spatial and temporal variation shape the beta diversity patterns of aquatic insects belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT) in streams exposed to different degrees of anthropogenic alteration in eastern Amazonia. Specifically, we tested the hypotheses that (i) spatial beta diversity is primarily explained by environmental gradients associated with land-use differences among streams, (ii) seasonal beta diversity varies between dry and rainy periods, and (iii) more altered streams exhibit greater temporal dissimilarity across years. We sampled 28 streams during six campaigns conducted between 2022 and 2025, totaling 3943 individuals distributed among 50 genera. Seasonal patterns revealed exclusive genera in both dry and rainy periods, evidencing temporal turnover. Spatial beta diversity was explained by land-use and land-cover variables, including remnants of old-growth forests, indicating a legacy of historical landscape transformations. More altered streams showed higher nestedness, reflecting genus loss. Temporally, we observed alternating gains and losses of genera across sampling campaigns, with greater dissimilarity in streams under stronger anthropogenic impact. By integrating spatial and temporal β-diversity analyses across a multi-year dataset, our results demonstrate that both current and historical land use jointly determine the structure and resilience of Amazonian stream communities. This dual-scale approach provides a framework for detecting cumulative anthropogenic pressures in tropical ecosystems.