Vertical partitioning of siliceous plankton in the stratified waters around Japan

Vertical distributions of siliceous plankton groups in the seas off Japan were studied to clarify their ecological preferences. Radiolarian standing stock maxima were absent in the layers where silicoflagellate and diatom standing stock maxima occured. The vertical distribution patterns of silicoflagellate species were consistent vertically, whereas their geographic distributions differed, likely reflecting current systems. Two silicoflagellate taxa ( Octactis speculum and O. octanaria var. pulchra ) showed strong correlations with temperature. In contrast, diatoms displayed two distinct vertical distribution types: Group 1 (above pycnocline) and Group 2 (below pycnocline). Based on their vertical and geographic distributions, large diatoms such as Stephanopyxis palmeriana , Coscinodiscus asteromphalus , C. wailesii group, C. oculus-iridis , C. perforatus , and Stellarima stellaris are identified as a”shade flora“. Members of the family Rhizosoleniaceae exhibit an effective strategy for adapting to oligotrophic conditions. Several taxa can serve as environmental proxies: Thalassiosira eccentrica as nitrate concentration in the surface, Bacteriastrum delicatulum as low temperature and high phosphate, and Phaeoceros spp. and Hyalochaete spp. as nitrite and ammonium. Comparison among siliceous plankton suggests that increasing stratification leads to vertical partitioning among siliceous plankton. Under weak stratification, silicoflagellates and diatoms dominate surface waters, whereas stronger stratification causes silicoflagellates and large centric diatoms to accumulate near the pycnocline, followed by other diatoms. Living radiolarian standing stocks are regulated by siliceous phytoplankton and peak below their abundant layers. This study demonstrates clear vertical partitioning among siliceous plankton groups. • Radiolarians are vertically partitioned from siliceous phytoplankton. • Silicoflagellates dwell with respect to primarily nutrients and secondly temperature. • Some diatoms can cope with less nutrients and lights. • Some diatoms vertically migrate with increasing stratification.