Both deterministic (e.g. species-environment interactions) and stochastic processes (e.g. random birth and death events) shape communities, but it remains poorly understood, which environmental conditions promote stochasticity. Here, we investigated interactive effects of nutrient availability and community size on stochasticity in order to predict how eutrophication and biomass loss shift the balance between predictable and random community dynamics. For this, we used freshwater bacterial communities in a microcosm experiment, where communities were diluted to varying sizes and exposed to low, intermediate, and high nutrient concentrations. Stochasticity was estimated with null modelling and as beta-diversity among replicate communities. At low nutrient concentrations, deterministic processes dominated, especially in smaller communities, which had the lowest diversity and abundance. Whereas, higher nutrient concentrations increased stochasticity. In contrast to theoretical predictions, this was particularly the case in larger communities with the highest diversity and abundance, likely due to stochastic initial growth. The findings underline how nutrient availability and community size jointly influence stochastic assembly processes, with important consequences for bacterial diversity and ecosystem functioning under environmental change. This study shows that nutrient availability and community size jointly determine whether freshwater bacterial communities are shaped more by deterministic or stochastic processes, with low nutrients favouring deterministic assembly and high nutrients promoting stochasticity, especially in larger, more diverse communities.