Hydrodynamics regulates microbial degradation of microplastics by modulating bottom-up and top-down effects in a river-lake confluence zone

River-lake confluence zones, characterized by unique hydrodynamic conditions, are critical areas for pollutant transformation. Nevertheless, degradation of microplastics (MP) mediated by multi-trophic microbial communities remains poorly understood under such complex hydrodynamic disturbances. This study investigated the characteristics of multi-trophic microbiota of the microplastome and explored their roles in MP degradation across four distinct hydrodynamic zones, i.e., maximum velocity zone (Z1), flow buffer zone (Z2), flow deflection zone (Z3), and flow reestablishment zone (Z4), in a river-lake confluence. A pronounced spatial heterogeneity in MP abundance and available nutrients was revealed among the four flow zones, with Z3 exhibiting the most intense MP degradation. Additional microcosm experiments demonstrated that microbial MP degradation was primarily driven by the enriched degrading bacteria and fungi, facilitated by multi-trophic microbial interactions. Furthermore, in situ analysis revealed that both bottom-up and top-down effects occurred across all flow zones, with their intensity being positively correlated with the degree of MP degradation. Thereby, nutrient availability driven by hydrodynamics stimulated the growth of MP degrading bacteria and fungi through a bottom-up effect. The increase in the relative abundance of MP degrading bacteria, concurrent with enhanced protozoan predation on bacteria, suggested that this top-down control operated through the selective predation of protozoa on non-MP degrading bacteria. Across the entire river-lake confluence zone, directional flow fluctuations were identified as the paramount environmental factor through the causal effect model, explaining >50% of the variance in bottom-up and top-down effects. Our study demonstrates how hydrodynamics governs MP degradation via multi-trophic microbial interactions, advancing our fundamental understanding of MP fate in aquatic ecosystems.