Abstract:

We systematically analyzed the newly collected radiolarian assemblages data from 34 surface sediment samples that span the northern, middle and southern Okinawa Trough to better understand the regional species compositions and distribution range. Our study provided the first detailed regional characterization on the modern distribution patterns of radiolarian species under varying temperature, nutrient, and water depth. The results showed that diverse subtropical species dominated the radiolarian assemblages in all samples.The most abundant species was Tetrapyle octacantha Müller group (23%), followed by Spongodiscus resurgens and Euchitonia furcata. Variation in the radiolarian assemblages in different parts of the Okinawa Trough could reflect the differences in the overlying water masses. In general, the relative abundance of warm-water species decreased with increasing latitude. However,the distribution pattern of the T.octacantha group showed an opposite trend with high abundance in the western continental slope of the northern Okinawa Trough. The northern Okinawa Trough near the East China Sea continental shelf is strongly affected by the Changjiang diluted water,resulting in large seasonal variations of temperature and salinity. The T.octacantha group has been found to have stronger tolerance to relatively lower temperature and thus is able to become the dominant species in the northern Okinawa Trough. In this study,the depth-related distribution of radiolarians in the Okinawa Trough was also investigated. The lower abundances of Cycladophora davisiana and Actinomma leptodermum, which are the mid-water species in the Okinawa Trough,may be partly related to the impact of higher mid-water temperature. Another factor that may affect the distribution pattern of deep-water dwellers in the Okinawa Trough is the inhibition effect of shallow straits on the Pacific deep water intrusion.

Key words: radiolarians, Kuroshio, temperature, surface sediment, Okinawa Trough