Distribution of polycystine radiolarians in the northern South China Sea in September 2005

We studied the distribution of polycystine radiolarians in the northern South China Sea (SCS) in September 2005 in order to document the relation between radiolarian assemblages and environmental parameters. Vertical plankton tows were collected at depth-intervals from 0 to 250 m at thirteen stations, using a closing net (62 µm mesh). The highest abundance of polycystine radiolarians generally occurred at depths of 0–75 m, i.e., above the chlorophyll-a maximum and thermocline. In contrast, the maximum polycystine abundance occurred below the chlorophyll-a maximum at station E105 (75–150 m), possibly as a result of down-welling of warm waters, and the maximum fell within the chlorophyll-a maximum at station E702, as a result of upwelling of intermediate cold waters.During our sampling period, the southwestern monsoon caused upwelling in the northern SCS (the Taiwan Bank region), as shown by the high abundance of Tetrapyle octacantha, Acanthodesmia vinculata, Cladococcus cervicoris, Octopyle octospinosa, Spongaster tetras, Streblacantha circumtexta and Spongotrochus glacialis. The patterns of distribution and the abundances of the species Botryocyrtis scutum, Pterocorys hertwigii, Collosphaera tuberosa and Didymocyrtis tetrathalamus tetrathalamus suggested that western Equatorial Pacific waters did not yet influence the region at the time of sampling; the first two species were dominant in the northern SCS during our sampling. The species Cornutella profunda and Cyrtopera laguncula were rare at depths of 150–250 m in the tropical region of the SCS, and assemblages in northern and southern SCS were very similar. The relative abundance of B. scutum and P. hertwigii is an indicator for seasonal changes in monsoonal circulation.