Diversity and annual variation of phytoplankton community in Yesso scallop (Patinopecten yessoensis) farming waters of North Yellow Sea of China

Abstract Yesso scallop ( Patinopecten yessoensis ) is one of the most important maricultured scallops in northern China, which was first introduced from Japan in 1980s. However, with the rapid development of aquaculture, the frequent occurrence of Yesso scallop's high mortality in summer has led to huge production losses. In the present study, seawater samples were collected from Yesso scallop farming waters in Changhai, Dalian, China from November 2016 to August 2017. The composition and annual variation of phytoplankton community were determined by the high-throughput sequencing of 18S rDNA. The phytoplankton community in the seawater has been changed from diatom dominated type to dinoflagellate dominated type under the grazing pressure during the years. The structure of phytoplankton community fluctuated annually and more dramatically in summer compared with that in other seasons. In summer, the abundance of Dinophyceae decreased from 51.0% to 25.9%, while Diatomea fluctuated from 1.0% to 1.4%. Moreover, the abundances of Noctilucales, Chlorophyta and Chrysophyceae were much higher in summer than that in other seasons. Notably, as an important food source for Yesso scallop, the abundance of diatoms was as low as 0.7% during the scallop's growth period, which might result in the scarcity of energy source. In addition, temperatures, content of dissolved oxygen as well as silicate concentration were found to be associated with the annual pattern of phytoplankton variation. It was suspected that the change of phytoplankton community structure and the decreasing amount of bait algae in the aquaculture waters might result in adverse impacts on the health of Yesso scallops.