Relationship Between Zooplankton Community Niche Characteristics and Environmental Factors in a Typical Mountain Stream River: A Case Study of the Upper Cao'e River, Zhejiang Province

This study investigated zooplankton community structure, niche characteristics, and their relationships with environmental factors in the upper Cao'e River, a mountain stream system in southern Zhejiang Province. Twenty-one sampling sites encompassing diverse habitat types were established, and field surveys were conducted during spring and autumn 2024. The zooplankton community was dominated by protozoans and rotifers, with 81 species identified across four taxonomic categories. While density was primarily driven by protozoans and rotifers, copepods contributed most to biomass. Both density and biomass exhibited significant seasonal variation, being significantly higher in autumn than in spring, with lake/reservoir habitats showing the highest values. Environmental analysis revealed that water temperature, chlorophyll-a (Chl.a), Secchi depth (SD), total nitrogen (TN), and total phosphorus (TP) significantly influenced zooplankton community dynamics and niche partitioning. Spring communities were primarily shaped by SD and Chl.a, whereas autumn communities showed stronger associations with Chl.a and TP. Niche breadth analysis categorized species into broad, moderate, and narrow ecological niches. Broad-niche species (Nauplius) demonstrated high environmental adaptability, while narrow-niche species (Difflugia acuminata) exhibited specific habitat requirements. Niche overlap analysis indicated intensified interspecific competition during autumn. The alternating current-stillwater habitats in the upper Cao'e River support diverse zooplankton assemblages, contributing to the ecological complexity, biodiversity maintenance, and stability of this riverine ecosystem.