Distribution characteristics of green algal micro-propagules in the East China Sea in winter and their relationship with green tide macroalgae in the Yellow Sea

Green tide blooms have occurred in the Southern Yellow Sea (SYS) for the last 16 years. Current studies show that the main source of the green tides in the region is the Subei Shoal. However, some scholars maintained that the germplasm of Ulva green tides may originate from the adjacent sea area in 2021. In the present study, floating green macroalgae and their micro-propagules, and surface water were collected in the East China Sea (ECS) and the waters near the Changjiang River estuary in December 2020, and their spatial and temporal distribution characteristics in winter were analyzed, in order to investigate whether they contributed to green tide blooms in the SYS. No floating green macroalgae were found in the ECS in winter, and there were few micro-propagules in the surface water. Only one and two U. prolifera individuals were detected near Qushan Island, Zhoushan City, respectively, and they did not belong to the type 5S-II U. prolifera. A phylogenetic tree-based comparison and haplotype analysis were carried out between U. prolifera cultured from the micro-propagules and U. prolifera attached on Neoporphyra cultivation rafts, and that floating in the SYS (in 2020 and 2021). The results showed that the haplotypes of U. prolifera cultured from the micro-propagules were significantly different from the haplotypes of the latter two. The ECS does not have the “seed bank” for overwintering of the green tide macroalgae blooming in the SYS. This study may provide a reference for further research on the germplasm sources of green tides in the SYS.