Diversity and distribution characteristics of phytoplankton in typical mariculture bays in northern China

The Rongcheng coastal regions, located outside the easternmost region of Shandong Peninsula, are the most typical high-density mariculture regions in northern China. To gain a detailed understanding of the diversity of phytoplankton in the Rongcheng coastal regions at the molecular level, we analyzed the phytoplankton community structure and compared the horizontal and vertical distribution in Rongcheng Bay, Lidao Bay and Sanggou Bay using metabarcoding analysis based on the 18S rDNA V4 sequences. A total of 186 phytoplankton species in 173 genera and nine phyla were detected in water samples collected from eight sampling stations in these three bays in summer, which was much higher than the historical survey results over the past 40 years. The amplicon sequence variants (ASVs) richness in Dinoflagellata showed no regular spatial distribution variation in the investigation area of the three bays, but the relative abundance in Dinoflagellata significantly increased on the outside of bays where no aquaculture facilities were set up. Both ASVs richness and relative abundance in Chlorophyta significantly decreased as the offshore distance increased in Rongcheng Bay and Lidao Bay, not in Sanggou Bay. In the vertical direction, Chlorophyta showed higher relative abundance in the surface layer in the bays. 47 potential bloom-forming algal species, which have been recorded as causal species and have led to the occurrence of harmful algal blooms, were detected in these three bays, and 26 species belonged to Dinoflagellata, posing a potential threat to mariculture industry. This study has helped us systematically understand the diversity and community structure of phytoplankton in typical mariculture bays in northern China, as well as potential harmful algal blooms, laying the foundation for long-term monitoring of planktonic ecosystems and prediction of harmful algal blooms in the future.