Microbial immigrants arriving with influent wastewater may influence activated sludge(AS)ecosystems.However,the extent to which immigration impacts AS communities is still debated.To explore the intensity of immigration impact,we used sequencing technology to track the raw wastewater and AS communities from a membrane bioreactor plant over a 12-month period.We first distinguished core populations from peripheral ones in both raw wastewater and AS based on their occurrence frequency and abundance.The results showed that core OTUs(≥80％occurrence frequency)made up a large fraction(＞90％)of total sequences,while peripheral OTUs composed the majority of all detected OTUs but merely occupied a few sequences.A significant difference in core communities between the influent and AS was found,as well as between the compositions of core and peripheral populations.Additionally,the persistent functional bacteria of AS,although not numerically dominant,accounted for 96.24％of the total sequences related to nutrient turnover,suggest-ing the presence of a small number of longstanding and core functional bacteria in the AS ecosystem.Importantly,64％of the 5188 OTUs in AS,which accounted for 91.51％of the se-quences,exhibited positive growth rates,which suggested that their apparent abundances were due to growth within the plant,not from immigration.Taken together,these results demonstrated that the impact of influent populations on core AS communities was limited.Overall,this work provides quantitative insights into the impact of immigration,which is expected to advance our understanding of the AS community assembly.