Protistan community structure and the influence of a branch of Kuroshio in the northeastern East China Sea during the late spring

Abstract

The northeastern East China Sea is an ecologically important marine ecosystem influenced by warm water derived from the Kuroshio Current. However, relatively little is known about the spatial variation of protist communities and their regulating factors from this region’s ecosystem during the spring season. Here, we investigated protistan community structures using a complementary approach combining 18S rRNA gene amplicon sequencing and light microscopy cell counts at nine stations from the northeastern East China Sea to the west of Jeju Island. The vertical profiles of physicochemical properties revealed that the Jeju Warm Current water mass, flowing from the southeast towards the northwest, created a thermohaline front dividing the region in two. These two regions had similar planktonic biomass, but the protistan communities differed significantly: dinoflagellates accounted for higher proportions of the protistan communities in the warm and saline waters, particularly at stations E35, E44, and E45, while the relative abundances of diatoms and picochlorophytes were higher in the low-density water of the western stations (E32 and E42). Furthermore, higher species richness and Shannon Diversity Index values in the warm and saline waters suggests that the Jeju Warm Current, a branch of the Kuroshio, increases protistan taxonomic diversity in the northeastern East China Sea during the late spring. Seed populations of harmful algal bloom-causing species were discovered in the warm and saline water originating from the Kuroshio, which is particularly important as it indicates that these waters could introduce harmful species that may spread to the Yellow Sea and Korea Strait. Taken together, the study suggests that potential changes to the current systems in the region could dramatically alter the structure of its protistan community.