The community structure and distribution patterns of zooplankton are crucial for understanding the functioning of marine ecosystems. This study investigates twilight zone zooplankton community and the influence of sea ridge on its vertical distribution using metabarcoding analysis of the V9 region of the 18S rRNA gene based on 44 zooplankton samples collected in the Ninety East Ridge of the Indian Ocean. The results showed that northern ridge is distinguished by high temperature, salinity, DO and Chl-a compared to southern ridge based on cluster analysis， exhibited different vertical pattern. Zooplankton such as Euphausiacea, Chaetognatha, Amphipoda, Trachymedusae, Siphonophorae contributed higher proportions in the northern ridge while Copepoda, Gastropoda and Decapoda contributed higher proportion in the southern ridge. In the northern ridge, Amphipoda and Siphonophorae dominated the eastern area; Copepoda contributed highest proportion in the central area and Euphausiacea and Chaetognatha dominated the western region. In contrast, in the southern ridge, Copepoda, Chaetognatha, Gastropoda dominated in the central area; Euphausiacea and Trachymedusae was found high proportion in the western part and Decapoda dominated in the eastern area. Throughout the water column, zooplankton community dominated by Copepoda species such as Pleuromamma robusta, Neocalanus cristatus, Metridia longa, Clausocalanus furcatus and Euphausiacea species such as Meganyctiphanes norvegica. Aidanosagitta neglecta, Eukrohnia bathypelagica belonging to Chaetognatha and Sergia hansjacobi belonging to Decapoda were occurred only in the 3000-2000m depth. The co-occurrence network topological properties showed southern ridge community was more stable. Salinity and dissolved oxygen were the most influential factors, explaining zooplankton spatial and vertical variation while temperature and chlorophyll-a contributed in less proportion.
 

twilight zone,zooplankton,sea ridge,Indian Ocean