Preliminary Estimation of Protist Predation Potential on Picoplankton in Summer of the Yellow Sea

Despite the predation on picoplankton by diverse protist groups in marine ecosystems, heterotrophic nanoflagellates (HNF) have mainly been used to estimate the predation effect of protists on heterotrophic bacteria. A preliminary investigation was conducted to estimate the predation potential of protist communities including HNF, heterotrophic dinoflagellates, and ciliates on picoplankton in natural assemblages of the Yellow Sea (YS) during summer. A higher density of picoplankton was observed in nearshore waters (station 35-03) than in off-shore waters (stations 35-11 and 35-13). Among the protists, naked ciliates were also distributed in high abundance near the shore. Specifically, small ciliate cells (20–50 µm) were dominant there. HNF showed a different distribution, with higher abundance in off-shore water than in nearshore water. Although predation and ingestion rates of protists on the picoplankton were relatively higher in the nearshore, its predation pressure was on the average of approximately 33% d −1 without a considerable change, despite the different water sampling depths and locations along 35° N. The lower variation in predation pressure by protist predators may have been caused by complementary predation effects between small ciliates and HNF. Further studies are necessary to understand the specific predation of protist groups in the YS.