Historical Contingency Drives Freshwater Microbial Community Assembly Across Successional Time

Microbial community assembly is governed by the interplay among historical contingency, environmental selection, and dispersal, yet their relative importance and temporal dynamics remain poorly resolved, particularly in freshwater ecosystems. Most previous studies have examined these drivers in isolation, limiting our ability to predict microbial community trajectories under changing environmental and dispersal regimes. Here, we conducted a 60-day full-factorial reciprocal transplant microcosm experiment using freshwater bacterial communities, manipulating historical source communities, environmental media, and the intensity of external immigration (ambient vs. enhanced). Bacterial community dynamics were tracked at early (day 13) and late (day 60) successional stages using 16S rRNA gene amplicon sequencing. We found that relative dispersal limitation strongly influenced community assembly during early succession, with higher immigration rates increasing alpha diversity (e.g., ASV richness F = 23.45, p < 0.001) and altering community composition. However, this effect weakened over time, indicating a transition toward dispersal saturation. In contrast, the influence of historical contingency persisted throughout the experiment and became the strongest single driver of community composition at the late successional stage (R ² = 13.34%), exceeding the explanatory power of environmental selection (R ² = 5.02%). Communities sharing the same historical source consistently followed distinct assembly trajectories, regardless of environmental medium or immigration rate. Together, our results demonstrate a time-dependent shift in the mechanisms governing freshwater microbial community assembly, from early relative dispersal limitation to late-stage historical dominance driven by priority effects. These findings highlight the critical and lasting role of historical contingency in shaping microbial community structure and suggest that community assembly is governed by a dynamic interplay of mechanisms, with historical contingency playing a greater, lasting role that leads to strong path dependency.