Zonal and vertical distribution of radiolarians in the western and central Equatorial Pacific in January 1999

The zonal and vertical distribution of radiolarians was studied to document their relationship with their environment in the Equatorial Pacific in January 1999. Vertical plankton tows using a closing-type net with 63 μm mesh were made between the sea surface and 1000 m depth on board R/V Mirai at five stations located between 140°E and 170°W, i.e. in the western Pacific warm pool (WPWP) Region in the west and the Upwelling Region located in the east. The condition during the time of sampling was more or less La Niña, characterized by a zonal extension of the upwelling region at 160°E. Our survey yielded 261 radiolarian taxa including 114 Nassellaria, 120 Spumellaria and 27 Phaeodaria. Total radiolarian standing stocks increased from west to east, responding to the general increase in nutrients, chlorophyll-a, and diatom numbers. Three major radiolarian depth communities were identified: surface dwellers (0–120 m), subsurface dwellers (120–200 m), and intermediate-water dwellers (200–1000 m). When the vertical profiles were examined longitudinally, their abundance preference was characterized as follows: four taxa preferred the high-abundance water masses of the Upwelling Region: Lithomelissa setosa, Pseudocubus obeliscus, Stylodictya multispina, and Stylodictya validispina. On the other hand, Collosphaera tuberosa was most abundant in the WPWP.