The green tide in Yingkou, China in summer 2021 was caused by a subtropical alga—Ulva meridionalis (Ulvophyceae, Chlorophyta).

The large-scale green tide caused by Ulva has occurred successively in the Yellow Sea since 2007, and new events of green tide also continued to appear in nearby sea areas, indicating an undergoing rapid development of occurrence patterns for harmful macroalgal blooms (HMBs) along coastal China. In August 2021, a green tide occurred for the first time in Bayuquan sea area of Yingkou city, Liaoning Province in the Bohai Sea. In this study, morphological and molecular approaches were used to identify the causative species as U. meridionalis, an alien subtropical alga previously found to dominate green tides in the South China Sea. According to the hydrological data of Bayuquan in summer 2021, combined with morphological and developmental observations for this alga, we hypothesized that the disturbance caused by the typhoon In-Fa might have detached the local U. meridionalis from substrates, especially for those thalli with poorly developed holdfasts, and the ensuing wave-free period with unusually high temperature, which fell in the reported optimum growth temperature for U. meridionalis, might have provided the favorable conditions for the final bloom of the floating seaweeds. This is the first report on the bloom of subtropical U. meridionalis in the north temperate sea zone, indicating that the ecological risk of causing green tides in the future by this rapidly spreading species deserves high attention. [ABSTRACT FROM AUTHOR]