Seasonal variation in community assembly mechanisms of benthic fauna and phytoplankton in an urban–rural interface river

Unraveling the mechanisms that drive changes in community composition and biodiversity is a central focus of freshwater ecological research. However, the role of these mechanisms in shaping seasonal variations in benthic fauna (Ben) and phytoplankton (Phy), particularly in urban-rural interface (URI) rivers, remains poorly understood. In this study, we compared the community structure, spatial patterns, coexistence modes, and assembly mechanisms of Ben and Phy across different seasons in the Yongding River, China. Our results revealed significant seasonal variations in biodiversity and community composition for both Ben and Phy. Because community similarity was significantly positively correlated with geographic distance, the distance-decay pattern was not observed. Environmental factors were found to play a more dominant role than spatial factors in shaping community composition and biodiversity. In particular, Ben richness was predominantly shaped by water quality conditions, whereas Phy richness was mainly regulated by climatic variables. Community coexistence patterns indicated stronger interspecific synergies within both groups, with Phy forming a more stable and complex network structure compared to Ben. Furthermore, the community assembly processes differed: Ben was primarily shaped by deterministic processes, while Phy was predominantly influenced by stochastic processes. Although both processes jointly influenced community dynamics across seasons, their relative contributions varied. These findings offer new insights into the community dynamics and driving mechanisms of Ben and Phy in URI rivers, advancing our understanding of their spatiotemporal patterns and ecological processes. This research also provides a theoretical foundation for river restoration efforts.