Okhotsk Sea ostracods in surface sediments: depth distribution of cryophilic species relative to oceanic environment

Abstract

The depth distribution of benthic ostracods in surface sediments of the southern Okhotsk Sea, in the marginal northwestern Pacific, reflects environmental factors such as water depth, water-mass properties and sea-ice transport. Ostracods are much more abundant on the continental shelf than on the continental slope, with only a few specimens present on the lower slope. Many species in this study area are identical to cryophilic species in the northern Japan Sea, which lies adjacent to the southern Okhotsk Sea. Comparison of the depth-distribution patterns of shallow-water (e.g. Johnnealella nopporensis and Munseyella hatatatensis) and deep-water (e.g. Argilloecia toyamaensis) species in the northern Japan Sea reveals two relationships: (1) the depth-distribution range of J. nopporensis, M. hatatatensis and other species in the Okhotsk Sea (upper shelf to upper continental slope; 50–500 m) is greater than in the Japan Sea (upper to middle shelf; 40–120 m); (2) the abundances of A. toyamaensis and other species on the continental slope of the Okhotsk Sea are lower than on the Japan Sea slope. The presence of a few shallow-water species such as M. hatatatensis on the upper continental slope of the Okhotsk Sea is probably the result of ice-rafting. The low abundance of A. toyamaensis and other species in the Okhotsk Sea reflects both lower dissolved-oxygen values compared to the northern Japan Sea and the shallow depth of Soya Strait that connects the two seas.