<p>We explored seasonal changes in community assembly processes shaping periphytic diatom distribution in hilly streams of the Danube River basin. Particularly, we were interested (1) whether compositional stochasticity of diatom community changes with increasing hydrological disturbance in spring compared to favorable conditions in autumn; (2) how the relative strength of environmental filtering and dispersal-related processes varies across seasons; and (3) whether the relative importance of these processes depends on dispersal abilities of diatom guilds (low-profile, high-profile, motile, and planktic). The compositional stochasticity increased in autumn, indicating that diatom assemblages were more prone to ecological drift in benign hydrological conditions. Variation partitioning showed that environmental filtering remained consistently more important than dispersal-related processes across seasons. The stronger taxonomic homogenization observed for low-profile guild in spring suggests that this guild was influenced by mass effects during high-flow period. Thus, low-profile diatoms were strongly driven by dispersal, while high-profile and motile diatoms were more controlled by the environment. However, the increased role of mass effects was mediated by high abundance and low disturbance sensitivity of low-profile diatoms, rather than their dispersal ability. Overall, our results highlight the important role of hydrology in the balance between deterministic and stochastic processes structuring lotic diatom metacommunities.</p>