Potential phytoplankton-bacteria mutualism decreases microbial food web stability in river-reservoir systems

Microbial interactions form food webs and are crucial for regulating food web stability. A structurally stable food web is a prerequisite for aquatic ecosystem functioning. Dam construction and reservoir operation significantly alter environmental conditions, in which hydrodynamic, trophic, and vertical variations affect all trophic levels of the hierarchy via top-down and bottom-up forces, with profound effects on microbial food web structure and stability. However, little is known about which external environmental drivers primarily influence the shaping of microbial food web structure, and how phytoplankton-heterotrophic bacteria interactions affect the stability of the entire microbial food web in river-reservoir systems. Here we explored multi-trophic interactions among phytoplankton, heterotrophic bacteria, and protists along trophic, longitudinal (hydrodynamic), and vertical gradients in five reservoirs of the upper Yangtze River. Hydrodynamic conditions explained more variation in microbial food web stability than trophic and vertical conditions. Food web stability declined along trophic, longitudinal, and vertical gradients, accompanied by reductions in α-diversity, K-strategist bacteria abundance, and food web complexity, together with an increase in the proportion of positive phytoplankton-heterotrophic bacteria associations. Potential phytoplankton-heterotrophic bacteria mutualism was intensified by an increase in trophic state, a reduction in water flow velocity, and a shift from bottom to upper layers, which had an adverse effect on microbial food web stability by reducing predation pressure. Our findings highlight the importance of hydrodynamic variations in shaping microbial food web structure and underscore the pivotal role of phytoplankton-heterotrophic bacteria interactions in affecting food web structural stability, providing critical insights for river-reservoir system management.