Metabarcoding dissection of harmful algal bloom species in the East China Sea off Southern Zhejiang Province in late spring.

The coastal region of the East China Sea (ECS) is a famous "hotspot" for harmful algal blooms (HABs) in China. We hypothesize that such frequent occurrences of diverse HABs in the ECS are determined by the presence of unique HAB species in this region. In this project, we identified 3966 amplicon sequence variants (ASVs) representing 35 classes in six protist phyla/divisions. Among the 237 annotated protist species, we identified 58 HAB species, of which 23 HAB species had never been previously reported in the ECS. Many HAB species also displayed unique spatial distribution patterns in the ECS. Notably, we identified three HAB species Prorocentrum donghaiense, Lebouridinium glaucum and Noctiluca scintillans in the site S05-1 with substantially elevated abundance, suggesting that this sampling site was experiencing a multiple-species HAB event. This study was the first attempt in applying ASV-based metabarcoding analysis in studying protist and HAB species in the ECS.