Two drifting paths of Sargassum bloom in the Yellow Sea and East China Sea during 2019–2020

The macroalgal blooms of floating brown algae Sargassum horneri are increasing in the Yellow Sea and East China Sea during the past few years. However, the annual pattern of Sargassum bloom is not well characterized. To study the developing pattern and explore the impacts from hydro-meteorologic environment, high resolution satellite imageries were used to monitor the distribution, coverage and drifting of the pelagic Sargassum rafts in the Yellow Sea and East China Sea from September 2019 to August 2020. Sargassum blooms were detected from October 2019 to June 2020 and presented two successive drifting paths that both initiated from around 37°N. The first path spanned smaller spatial scale and shorter period, starting with a bloom of 3 km2 distribution area near the eastern tip of Shandong Peninsula in late October 2019 and drifted southwards, hit the Pyropia aquaculture area in early January 2020, then vanished in the northwest of East China Sea (ca. 32°N) around end of January. The second path began with a large distribution area of 23 000 km2 east of 123°E in late January 2020, firstly moved southwards in the central Yellow Sea and northern East China Sea (north of 29°N) till late April, then turned northwards with monsoon wind and vanished from late June to August. The mean sea surface temperature of 8°C to 20°C in the Sargassum bloom areas corresponded to in situ observed temperature range for vegetative growth and floating of S. horneri. There was no observed floating Sargassum blooms during July through September in the Yellow Sea and East China Sea. The results indicate that floating S. horneri is unable to complete life cycle in the Yellow Sea and East China Sea, and provide insights to the future management of Sargassum blooms. Further studies are needed to validate the pattern and source of annual Sargassum bloom in the Yellow Sea and East China Sea.