Hydrological seasonality shifts zooplankton metacommunity assembly mechanisms in the Chishui River, China

Hydrological regimes of river ecosystems are increasingly altered by climate change and other human-induced stressors, yet how these alterations affect aquatic metacommunity assembly remains unclear. Here, we investigated seasonal zooplankton dynamics in the subtropical Chishui River in southern China by sampling 80 sites across the dry and wet seasons. Using the morphology-based approach, we identified 146 zooplankton taxa, including protists, rotifers, cladocerans, and copepods, with protists accounting for nearly half of the total taxa and being dominated by ciliates. By integrating physicochemical, climatic, and land-use data with neutral community models and co-occurrence network analysis, we found a clear seasonal shift in assembly mechanisms: stochastic dispersal dominated during the hydrologically connected wet season, whereas deterministic environmental filtering prevailed during the dry season. Unexpectedly, higher α-diversity was associated with lower cohesion-based network stability, suggesting that seasonal community assembly processes may influence the diversity-cohesion relationship. During the dry season, deterministic assembly processes were more prominent, and community composition was associated with nutrient-related variables. These findings suggest that maintaining natural flow variability and reducing nutrient inputs may help sustain habitat heterogeneity and seasonal metacommunity dynamics in river ecosystems.