The Western Pacific Warm Pool (WPWP) contains the warmest surface waters exceeding 28.5°C. It is the Earth's heat engine and influences global climate change. This region is excessively oligotrophic, possibly induced by a strong pycnocline isolating surface waters from subsurface nutrient pools.

However, it is unclear how radiolarians, a major group of marine microzooplankton, respond to the typically warm oligotrophic condition of WPWP.

Recently, a research team led by Prof. LI Xinzheng from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) provided new insights into the radiolarian assemblages in the western WPWP and environmental determinants that controlled species composition.

The study was published in Deep Sea Research Part I: Oceanographic Research Papers.

The researchers identified three major radiolarian faunas in surface sediment samples of the western WPWP: the North fauna, South fauna, and Ethmodiscus-rich fauna.

They found co-occurrences of highly abundant radiolarians and Ethmodiscus diatom fragments in the Ethmodiscus-rich fauna. For this phenomenon, they proposed two hypotheses: radiolarian assemblages coexist in the upper column during Ethmodiscus blooms, or assemblages represent co-preservation in a sediment thanatocoenose.

"Primary production is the strongest factor controlling the species composition of the northern and southern faunas in the WPWP waters. North fauna shows an adaption to lower primary production, while South fauna prefers higher ones," said Prof. LI.

According to the results, researchers suggested that primary production, likely associated with food availability, is a major determinant of radiolarian assemblages in the oligotrophic WPWP. Radiolarians are mainly heterotrophic plankton, feeding on copepods, microalgae, and cyanobacteria, and the latter two are producers of primary production. Besides heterotrophic species, some radiolarians are also reported to maintain photosymbionts.

Some radiolarian taxa have higher abundance in more productive waters, probably indicating their heterotrophic nutrition with heavier dependence on food availability. Several other species, reported to harbor obligate photosymbionts, are abundant in less productive waters, suggesting their reduced food dependence on microplankton.

"Results of this study highlight radiolarians may serve as potential proxies of surface primary production in warm stratified waters," said Prof. LI.

This work was supported by the Global Change and Air-Sea Interaction Project granted by the Ministry of Natural Resources of China, and the Senior User Project of RV KEXUE.