Radiolarian distribution in surface sediments of the Philippine Sea and adjacent areas and its response to environment

In order to understand the distribution pattern of radiolarians in the Philippine Sea, this article, based on a unified method for sample processing and analysis, made analysis and comparison of radiolarians for 44 surface sediment samples taking from the Philippine Sea and its adjacent regions. A total of 500 radiolarians species are identified, suggesting a very high species diversity. The community structure and abundance of radiolarians in the surface sediments of the Philippine Sea vary greatly, suggesting complex regional ecological or sedimentary environments. The abundance of radiolarians dominated by Nassellaria is also very high in the northern South China Sea, indicating that the northern South China Sea is rich in nutrients and high in biological productivity. However, the radiolarian abundance, dominated by Spumellaria, is relatively low in the Okinawa Trough. It is speculated that the submarine environment of the Okinawa Trough is not so conducive to the accumulation and preservation of radiolarian shells. 8 warm water species group living in the euphotic layer and 162 environmental variables at different depths of the 0～200 m water layers are selected for RDA analysis. The results show that the distribution of these warm water species in the Okinawa Trough is significantly positively correlated with the summer temperature in 125 m of water depth, probably owing to the influence of the summer Kuroshio subsurface water. The distribution of warm water species in the northern South China Sea is mainly affected by winter silicate of 75 m and summer phosphate of 200 m. It means that high-concentration silicate is more conducive to the production of Nassellaria. In the Philippine Sea, however, environmental factors mainly in the subsurface water affect the distribution of warm water species, such as winter salinity of 75 m, 200 m annual dissolved oxygen content and summer temperature of 125 m. In addition, the RDA analysis results of 66 environmental variables at different depths of the medium-deep water (1000～3000 m) of the Philippine Sea and 5 cold water species living in this layer show that the northern Philippine Sea is mainly positively correlated with the silicate concentration of 1000 m. This may be related to the fact that the silicate-rich intermediate-deep water mass of the North Pacific moving southward into the Philippine Sea. The distribution in the central and southern part of the Philippine Sea is mainly negatively correlated with the concentration of silicate at 1000 m, and is significantly positively correlated with dissolved oxygen at 2000 m. It may be related to the Circumpolar Deep Water with high dissolved oxygen content and low silicate entering from the southern end of the Philippine Sea, and part of the water upwardly enter the intermediate layer of the Philippine Sea.