Zooplankton behavioral segregation and vertical positioning in the stratified summer Ulleung Basin: Diel variation in functional composition across ecological boundaries

Understanding the diel vertical migration strategies of zooplankton is essential for evaluating their ecological roles in stratified marine systems. This study examined the vertical distribution and behavioral segregation of zooplankton in the summer-stratified Ulleung Basin through high-resolution sampling at six-hour intervals across five discrete depth layers. A strong pycnocline acted as an environmental boundary, separating layer-optimized DVM (loDVM) taxa from migratory groups that distinct size-dependent strategies. loDVM taxa, such as Acartia pacifica and Metridia pacifica , consistently occupied either upper or lower layers, constrained by thermal and metabolic thresholds, with minimal diel displacement. In contrast, migratory taxa showed distinct body size-dependent patterns: larger taxa undertook extensive vertical excursions across multiple hydrographic layers, whereas smaller taxa displayed more restricted movements centered around the deep chlorophyll maximum (DCM) layer. Notably, the timing of vertical shifts varied across size classes, creating temporal asynchronies in access to productive foraging zones and reducing direct competition. Functional trait analysis further revealed trophic differentiation: particle and current feeders dominated the surface and DCM layers, while omnivorous-detritivores and filter feeder prevailed in deeper waters. This behavioral and trophic partitioning established two functionally distinct ecological zones: an upper production-consumption zone and a lower processing zone critical for carbon transport. Our results demonstrate that vertical segregation of zooplankton arises not only from physical stratification but also from coordinated behavioral and trophic strategies, highlighting their role in mediating energy transfer and supporting the biological carbon pump in the East Sea.