Human impacts on biodiversity are well recognized, but uncertainties remain regarding patterns of diversity change at different spatial and temporal scales. Changes in microbial assemblages are, in particular, not well understood, partly due to the lack of community composition data over relevant scales of space and time. Here, we investigate biodiversity patterns in cyanobacterial assemblages over one century of eutrophication and climate change by sequencing DNA preserved in the sediments of ten European peri-Alpine lakes. We found species losses and gains at the lake scale, while species richness increased at the regional scale over approximately the past 100 years. Our data show a clear signal for beta diversity loss, with the composition and phylogenetic structure of assemblages becoming more similar across sites in the most recent decades, as have the general environmental conditions in and around the lakes. We attribute patterns of change in community composition to raised temperatures affecting the strength of the thermal stratification and, as a consequence, nutrient fluctuations, which favoured cyanobacterial taxa able to regulate buoyancy. Our results reinforce previous reports of human-induced homogenization of natural communities and reveal how potentially toxic and bloom-forming cyanobacteria have widened their geographic distribution in the European temperate region.