Riverine systems support both high levels of biodiversity and important human activities. The management of these systems in the face of anthropogenic and climate change pressure requires fundamental knowledge of their functioning. Microbial communities in rivers carry out many important functions, such as decomposition and biogeochemical cycling; however, lotic microbial community assembly processes are not well understood. Cross-ecosystem provision of microbial taxa, such as those originating in soils and groundwaters may in part explain observed patterns of lotic microbial community composition. Though the influence of groundwater on lotic biodiversity and functioning has long been considered, its influence on microbial communities has not been well characterized. This project endeavors to investigate how surface water-groundwater mixing affects lotic fungal community structure. We propose a controlled microcosm study to allow for the testing of mechanistic hypotheses. Specifically within the context of microbial community coalescence, we plan to assess the influence of the cohesion of initial communities on the resultant (mixed) community following a mixing event. We will also investigate the potential for whole-community mutual invasibility of surface and groundwater communities. This work will contribute to the development of microbial-specific ecological theory and will provide insight into lotic microbial biodiversity patterns.