Spatial and seasonal changes of radiolarian standing stocks in the Sea of Okhotsk

The distribution of living radiolarians was investigated at 24 plankton stations collected with opening/closing nets from five depth intervals down to a depth of 1000 m in combination with a hydrographic CTD survey in the Sea of Okhotsk during summer 1998 and spring 1999. The radiolarian standing stock varied seasonally and regionally from 10 to 1775 skeletons/m(3) and was highest during summer. Nassellarians exhibited highest densities in summer, a period characterized by high productivity of heterotrophic organisms, whereas in spring, the season of maximum diatom production, phaeodarians dominated. Spumellarians were rare in both seasons. Regional differences in the radiolarian standing stocks were observed during both seasons. The radiolarian standing stock was highest during summer in the vicinity of the Sakhalin continental shelf, but pronounced regional variations in the radiolarian standing stock were also noted during spring. The seasonal and regional differences in the radiolarian standing stock are strongly related to the seasonally changing productivity regimes and the regional differences in the food availability. A total of 58 radiolarian taxa in spring and 64 taxa in summer were recorded. Of these eleven taxa and taxonomic groups (Antarctissa (?) sp. 1, Cycladophora davisiana, Challengeron sp. aff. C neptuni, Peridium sp. 1, the Plagoniidae, Protocystis tridens, Dictyophimus hirundo, Rhizoplegma boreale, the Spongodiscidae, Ceratospyris borealis and Lophospyris sp. 1) made up an average of 89% of the spring and 80% of the summer assemblages. Most polycystine taxa inhabit confined depth intervals independent of their seasonally or regionally varying standing stock. A distinct depth relation of the polycystine radiolarian standing stock, specific radiolarian assemblages and taxa remains consistent during both seasons. The maximum polycystine radiolarian standing stock occurs generally at intermediate depth below the dichothermal layer. The depth habitat of specific radiolarian taxa and assemblages shows a close relationship with the Sea of Okhotsk water masses and allows definition of four assemblages: surface water, subsurface cold water, intermediate water and deep water assemblage. This study shows that specific radiolarian taxa and assemblages are strongly related to water mass structure and to different water masses of the Sea of Okhotsk and thus represent a potential tool to reconstruct changes in water mass distribution and structure during the late Quaternary in the Sea of Okhotsk. (C) 2002 Elsevier Science Ltd. All rights reserved.