First report on catchment scale distributions and multidimensional connectivity pattern of microbial communities...

Microbial communities are vital for biogeochemical cycling of major nutrients, yet the catchment-scale dynamics of their composition, assembly, and connectivity remain poorly understood, particularly in fragile trans-Himalayan rivers, limiting predictions of their biogeochemical response to environmental change. To address this, we examined the distribution, assembly processes, and environmental drivers of bacterial communities across the aquatic-terrestrial interface (water, sediment, soil) of the Gandaki River, collecting 78 samples from the upstream glacial terminus (Tibet, China) to the Nepal-India border. Multidimensional connectivity (longitudinal, lateral, and vertical) was quantified using a Bayesian source-tracking approach. Results showed that alpha diversity increased significantly from water to sediment to soil, while community composition was markedly habitat-specific. Assembly processes revealed a stark ecological contrast: dispersal limitation dominated soil communities (54%), whereas ecological drift governed water communities (42%); sediments exhibited a blend of these stochastic forces. Homogeneous selection (23-39%) provided a consistent deterministic filter across all habitats. Community drivers were habitat-specific: water communities were predominantly influenced by ionic concentrations, temperature, and DOC; nitrogen content, pH, and EC in sediment; and nitrogen content and TOC in soil. Critically, source tracking revealed substantial longitudinal (45%), lateral (37%), and vertical (30%) connectivity, establishing the Gandaki River as a highly interconnected meta-ecosystem. We demonstrate that this high connectivity, coupled with habitat-specific assembly, creates a sensitive and predictable microbial template. Our findings provide the first catchment-scale evidence that microbial community patterns serve as integrative indicators of ecosystem connectivity and environmental filtering, establishing a critical baseline in vulnerable Himalayan river systems.