The pattern of annual Sargassum bloom in the Yellow and East China Seas during 2019−2020

The macroalgal blooms of floating brown algae Sargassum horneri are increasing in the Yellow and East China Seas 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 and East China Seas 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 Peninsular 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 and northern East China Seas (north of 29°N) till late April, then turned northwards with monsoon wind and vanished in mid-June. 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. The surface temperature higher than 22°C in July-September prohibits Sargassum blooming or completing life cycle. The results 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 and East China Seas.