Logo: to the web site of Uppsala University

Understanding the mechanisms that shape microbial community stability and assembly is essential for predicting ecosystem responses to environmental change. In this thesis, the influence of abiotic factors (e.g., nutrients, salinity, and temperature) and biotic interactions (e.g., grazing) on freshwater bacterial communities was investigated. Using a combination of in situ and laboratory experiments, community responses were assessed through measures of growth, biomass production, and community composition. Community dissimilarity and null modelling were further used to quantify the relative roles of deterministic (predictable) and stochastic (random) assembly processes. Results showed that repeated inputs of nutrients and dissolved organic matter led to greater changes in bacterial responses than single disturbances. Furthermore, communities with a disturbance history responded less strongly to a subsequent perturbation than those without such history, while the role of stochastic processes increased, particularly when time between disturbance events was short. In addition, nutrient enrichment in larger communities promoted stochastic assembly, likely by reducing competitive exclusion under high-resource conditions, thereby allowing more species with similar fitness to coexist and increasing the role of random colonization and drift. Top-down control by grazing promoted deterministic assembly despite increased variability among communities. Overall, this thesis demonstrates that the balance between stochastic and deterministic processes can shift with environmental context, disturbance regimes, and biotic interactions. These findings highlight the importance of considering multiple interacting drivers when studying microbial community dynamics. Improving our understanding of these processes is critical for predicting how freshwater ecosystems will respond to ongoing environmental change, with implications for ecosystem functioning, water quality, and resource management.