Growing evidence highlights the diverse biogeochemical processes that stimulate biological productivity in the oligotrophic Kuroshio. Although biological productivity stimulated by upwelling nutrients has been proposed by experimental approaches and intensive oceanographic observations, subsequent increases in zooplankton standing stocks and productivity have not been observed in the downstream Kuroshio. Here, we report geographical variations in standing stocks, productivity, and taxonomic composition of zooplankton communities in the Kuroshio based on oceanographic observations. Less saline water in the surface layers extended toward the Kuroshio Current, corresponding to high nutrient and chlorophyll a concentrations. Zooplankton standing stocks and taxonomic compositions exhibited significant differences between the coastal and Kuroshio waters, but no clear pattern was observed from the upstream to the downstream Kuroshio. Based on multivariate analysis, copepod species compositions were segregated between the coastal and Kuroshio waters, but some coastal species appeared abundantly in the Kuroshio waters. Significant positive correlations were found between nutrients, chlorophyll a concentrations, and zooplankton standing stocks, and the mixed ratio of less saline waters. We suggest that the geographical distribution patterns of zooplankton standing stocks and community structure along the Kuroshio current are affected by the advection of coastal communities in less saline waters than by locally stimulated biological productivity.

In Tosa Bay, euphausiids (krill) play a crucial role as prey for various marine organisms, ranging from pelagic fish to higher predators, such as cetaceans, and monitoring their distribution is essential for understanding current and future trends in fishery resources. However, information on their distribution and ecology is lacking. The present study investigated the species-specific horizontal and vertical distributions of euphausiids over a large area of Tosa Bay during the day and night at 42 stations using nets in autumn (24–31 October 1993). The season is considered to have the highest species diversity of euphausiids. A total of 27 species belonging to six genera were identified, including both adults and juveniles. Among the euphausiid species, four were mesopelagic and 23 were epipelagic. The 10 dominant species, with their densities and frequencies of occurrence, respectively, were as follows: Euphausia tenera (9.9 ind. m^-2, 16.2%), Euphausia recurva (7.8 ind. m^-2, 12.7%), Hansarsia microps G.O. Sars (6.4 ind. m^-2, 10.5%), Euphausia mutica (6.2 ind. m^-2, 10.2%), Euphausia nana (5.7 ind. m^-2, 9.3%), Pseudeuphausia latifrons (5.0 ind. m^-2, 8.2%), Stylocheiron carinatum (4.9 ind. m^-2, 8.1%), Euphausia similis (4.4 ind. m^-2, 7.1%), Hansarsia gracilis (2.0 ind. m^-2, 3.3%), and Stylocheiron affine (1.6 ind. m^-2, 2.6%). Euphausiid densities (61.1 ind. m^-2) were relatively higher in the offshore areas and lower in the coastal areas. For the dominant species, excluding E. nana, the densities were significantly higher in offshore areas than in the coastal areas. In contrast, E. recurva, E. similis, and S. affine were more abundant in the eastern and central areas of the bay than in the western area. In contrast, temperature and salinity in the surface layer showed no significant differences between the coastal and offshore regions, or among the western, central, and eastern areas. As Tosa Bay possesses a broad continental shelf, seafloor topography, mainly water depth, may act as a limiting factor for vertical distribution, thereby influencing horizontal distribution. The findings suggest a low density of deep-water-preferring mesopelagic species in the Tosa Bay. At night, the 10 dominant species were divided into two groups based on their vertical distribution: Group 1, consisting of E. similis, H. microps, H. gracilis, S. affine, and S. carinatum, was distributed below the 100-m layer, whereas Group 2, including E. nana, E. mutica, E. recurva, E. tenera, and P. latifrons, occurred above the 75-m layer. The grouping patterns were inferred to be associated with differences in feeding habits among the species.