Using samples collected in the Makarov Basin (MB) and Chukchi Abyssal Plain (CAP) in September 2012, the vertical distributions and geographical differences of zooplankton in the 0—1 000 m layer were analyzed. According to the species composition and abundance, both the diversity and the abundance of zooplankton changed with depth. On average, zooplankton abundance was 265.0, 360.7, and 231.2 ind·m-3 at depths of 0—50 m, 50—100 m, and 100—200 m, respectively, while it was only 64.4 and 36.9 ind·m-3 at depths of 200—500 m and 500—1 000 m, respectively. Herbivorous species such as Oithona similis, Calanus glacialis, and Calanus hyperboreus gathered in the 0—200 m layer and dominated in number. Although abundance of omnivorous species such as Microcalanus pygmaeus, Oncaea spp., and Metridia longa also decreased with depth, their percentages in the total abundance increased. Despite similar species assemblages recorded in each layer, epipelagic abundance of the large copepod C. hyperboreus was higher and that of small copepods was lower at the CAP station than at the MB stations, whereas in the 200—1 000 m layer, total abundance was much lower in CAP. Abundance of zooplankton in the 500—1 000 m layer varied between 22.7 and 92.6 ind·m-3 in MB but it was only 1.6 ind·m-3 in CAP. Geographical differences of zooplankton abundance in the mesopelagic layer were in accord with the heterogeneity in the function of the biological pump. It was suggested that low zooplankton abundance in the mesopelagic layer of CAP was induced by the high density of C. hyperboreus in the epipelagic layer, which starts feeding before ice melting in the spring, consequently reducing the downward flux of organic matter formed mainly by ice algae.
