<p>As dams age or become obsolete, their removal is increasingly promoted to restore river connectivity, yet short-term microbial responses remain poorly documented. We evaluated the effects of the drawdown of the Enobieta reservoir (Artikutza valley, Navarre) using a before/after-control/impact (BACI) design. Epilithic biofilms were sampled at four downstream impact reaches and four control reaches before and after drawdown, and bacterial and fungal communities were characterized through next-generation sequencing. We hypothesized that, prior to drawdown, the reservoir would alter downstream microbial assemblages, with effects attenuating with distance, and that impact reaches would be enriched in taxa involved in the oxidation of reduced compounds (e.g., metals and ammonium). We further expected drawdown to reduce differences between control and impact reaches, with faster responses in bacteria than fungi. Before drawdown, bacterial assemblages in impact reaches differed markedly from controls, showing absence of cyanobacteria and higher relative abundance of Chloroflexi, Nitrospirota, Proteobacteria and Planctomycetota, likely linked to elevated metal and ammonium concentrations. After drawdown, bacterial communities converged toward control conditions, whereas fungal assemblages diverged further. These contrasting trajectories indicate that microbial groups can exhibit distinct short-term responses to reservoir decommissioning, highlighting the importance of integrating multiple biofilm components when evaluating ecosystem recovery.</p>