Effect of diatom abundance and biogenic silica availability on the population growth of tintinnid ciliates at Suruga Bay

The effects of diatom abundance and biogenic silica (BSi) deposition on the population growth of tintinnid ciliates were investigated using incubations of sub-surface waters (10 m depth) of Suruga Bay, Japan in April and November 2018. Seawater for incubation was pre-filtered using a 200-µm mesh to remove mesozooplankton grazers. Initial concentrations of dissolved inorganic nitrogen were similar in both seasons, but silicic acid and phosphate were higher in April than in November. This allowed us to observe the growth of planktonic communities under varied silicic acid availability and explore the effects of BSi accumulation and food availability on the growth of tintinnids. Phytoplankton blooms dominated by diatoms formed in both, April and November incubations, but their biomass was higher in April than in November, demonstrating the effects of nutrients, particularly silicic acid, on diatom growth. The BSi concentration increased in both incubations but was higher in April than in November, confirming the effects of diatom abundance and size on BSi deposition. Tintinnid biomass also increased in both incubations but was higher in April than in November, showing that the biomass of tintinnid was affected by the BSi concentration. Species composition of diatoms affected the species composition of tintinnids by providing selected material for the construction of their lorica. Moreover, the results showed that the food supply mainly provided by pico-nanoflagellates via microbial loop and detritus may limit tintinnids’ population growth in natural planktonic ecosystems and this demonstrates tintinnids’ role as top predators in the microbial loop