Connectivity in rivers: Can it be assessed by tracing microorganisms?

River connectivity is a key indicator of a river's function in exchanging matter and energy. Human activities increasingly challenge this connectivity. Most studies assess river connectivity from physical or chemical perspectives, limiting the ability to characterize it at the ecosystem level. River microorganisms exhibit strong macroscopic continuity and are highly sensitive to environmental changes, making them a promising indicator for characterizing river connectivity. To explore the feasibility of using microbial community composition to characterize river connectivity, this study analyzed the connectivity of environmental characteristics, bacterial community, and eukaryotic microbial community in the Nu River and their driving factors using Bayesian classification and generalized additive models. The results show that microbial-based longitudinal connectivity is more sensitive than environmental connectivity in indicating environmental heterogeneity in the Nu River. Specifically, bacterial community connectivity is more sensitive than eukaryotic microbial community connectivity due to its greater diversity and transferability. Water-based longitudinal connectivity is more indicative than sediment-based connectivity. Bacterial transport at the water-sediment interface is primarily driven by sediment release. During the low-flow period, human activities have a notable impact on the connectivity of bacterial community in water. A distinct negative correlation exists between the connectivity of bacterial community and anthropogenic land use. In the high-flow period, however, the influence of natural factor is prominent. In contrast, during the low-flow period, the connectivity of eukaryotic microbial community in water is predominantly influenced by natural factor. Conversely, during the high-flow period, human-induced factor plays a dominant role. Our study provides a novel perspective for evaluating river connectivity and offers a scientific foundation for effective river management strategies.