Comparison of radiolarian vertical distributions in the Okhotsk Sea near the Kuril Islands and in the northwestern North Pacific off Hokkaido Island

The vertical distribution of radiolarians was investigated by using a closing-type net with 63 μm mesh in the Okhotsk Sea (Station NUʹ2, near Bussolʹ Strait) down to 1000 m depth and in the northwestern North Pacific (Oyashio region, Station X) down to 500 m depth. Both stations were sampled in September 1995. Radiolarian standing stocks were high between 50 and 200 m at both stations, and generally decreased with depth. The depth-integrated living total radiolarian standing stock from 0 to 500 m at Station NUʹ2 was more than three times higher than that at Station X. The maximum standing stocks of polycystine radiolarians occurred in the 50–100 m depth interval, but there were also significant abundance peaks of phaeodarian radiolarians in the 100–200 m interval at both stations. The radiolarian production is likely to be associated with not only phytoplankton production in upper waters, but also with microbial production in intermediate waters. A total of 79 taxa were encountered: 25 Spumellaria, 41 Nassellaria, and 13 Phaeodaria. Five radiolarian groups were obtained based on R-mode cluster analysis: Group A: lower intermediate water dwellers (300–1000 m), Group B: taxa with different distributions in the Okhotsk Sea (300–1000 m) and in the Oyashio region (0–100 m), Group C: upper intermediate water dwellers (200–500 m), Group D: surface-subsurface water dwellers (0–200 m), and Group E: taxa with pronounced 200 m.