Radiolarian assemblages controlled by ocean production in the western equatorial Pacific

The Western Pacific Warm Pool, covering much of the western Pacific equatorial region, contains the warmest surface waters exceeding 28.5 degrees C. A thorough investigation is necessary to assess radiolarian composition in such unique oceanographic conditions. In this study, we analyzed 80 surface sediment samples collected from the western equatorial Pacific. On the basis of principal coordinates analysis, three major radiolarian assemblages in sediment samples are identified: assemblages from northern and southern sediments, and an Ethmodiscus-rich fauna. Co-occurring highly abundant radiolarians and Ethmodiscus diatom fragments are reported for the Ethmodiscus-rich fauna. To explain this phenomenon we propose two hypotheses: that radiolarian assemblages coexisted in the upper column, or that assemblages indicate co-preservation in a sediment thanatocoenose. Net primary production, sea surface temperature, and phosphate, best explain the species composition of the northern and southern faunas in the oligotrophic warm western equatorial Pacific waters. Dominant species with higher proportions in the southern fauna, including Dictyocoryne muelleri and D. profundum groups, Spongaster tetras, and Stylodictya multispina, may sensitively respond to net primary production, or rather food availability. The higher abundances of the Tetrapyle and Phorticium groups in the northern fauna indicate their preference for cooler and inorganic nutrient-rich waters, and decreased dependence on food source associated with net primary production, possibly due to their obligate photosynthetic associations.

Automated summary

This study analyzes surface sediment samples collected from the western equatorial Pacific and identifies three major radiolarian assemblages. It suggests that net primary production, sea surface temperature, and phosphate are important factors that affect the species composition of the assemblages. The southern assemblage is more sensitive to net primary production and food availability, while the northern assemblage prefers cooler and inorganic nutrient-rich waters.