Linking nutrient availability and community size to stochasticity in microbial community assembly

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. This knowledge is crucial in order to predict how, for example, 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 dispersion among replicate communities. At low nutrient concentrations, deterministic processes dominated, especially in smaller communities, which had the lowest diversity and abundance. In contrast, higher nutrient concentrations increased stochasticity, particularly in larger communities with the highest diversity and abundance, likely due to stochastic initial growth. These 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.