Beyond the Cascade Dams: Scale-Dependent Effects on Zooplankton Stability in River Ecosystems

Cascade dams are widely recognized as major drivers of environmental change in river ecosystems and are associated with alterations in biodiversity and community stability. However, the factors associated with community stability at different spatial scales remain unclear. This study compared zooplankton community synchrony and stability at local and regional scales and examined the factors associated with stability across these scales. We analyzed how variation in water quality and heavy metal factors is related to zooplankton diversity, synchrony, and stability using field sampling, community analysis, and structural equation modeling. Our results showed that cascade dams were associated with complex shifts in community stability and synchrony, and that these shifts varied notably between spatial scales. At the local scale, significant differences in community stability were observed between upstream, reservoir, and downstream sections of individual dams. Community stability was positively associated with diversity but showed no clear association with environmental variables. Improved water quality was associated with higher species richness and diversity, which in turn were associated with higher community stability and lower synchrony. At the regional scale, community stability generally increased from upstream to downstream, a pattern similar to that observed in undammed control rivers. Environmental variables were associated with both community structure and stability, with water quality showing positive associations and heavy metal factors showing negative associations. Overall, our findings highlight the importance of considering spatial scale when evaluating river ecosystem stability. Large-scale cascade dam impacts may be associated with reduced stability through changes in water quality conditions.