The world’s largest macroalgal blooms caused by Ulva prolifera have occurred in the Yellow Sea for 11 consecutive years. The area covered by blooms has been approximately 500 km2 in previous years, while in 2017, the maximum area decreased significantly to 312 km2. In this study, we concluded that species competition between Ulva and Sargassum (fast rise of the golden tides), extreme high sea surface temperature and harvest for floating Ulva macroalgae were the three critical factors influencing the sharp reduction in covered area for blooms in 2017. In addition, analysis of annual variations of Pyropia aquaculture area in the Southern Yellow Sea over the past two decades revealed that a great expansion in “Sansha” regions was mainly responsible for the initial blooms in 2007, and that this expansion supported the great biomass of the blooms in following years. Based on these findings, we suggest comprehensive utilization of the macroalgal blooms is a feasible way to control them.

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Since 2007, large-scale blooms have periodically broken out in the Yellow Sea, leading to serious economic and ecological problems in the surrounding coastal environments. Previous studies reported that the blooms originated in the Rudong oceanic area of the southern Yellow Sea and drifted almost 400 km into the Qingdao oceanic area of the northern Yellow Sea. In this study, chlorophyll concentrations, chlorophyll fluorescence, growth rates, and reproductive characteristics of floating Ulva prolifera collected along the Yellow Sea coast were assessed. The results revealed that these four physiological parameters in the Rudong area at the bloom source were considerably higher (2.02 mg/g, 0.68, 28.49% d −1 , and 4 × 10 7 , respectively) than in the Qingdao area (0.34 mg/g, 0.31, 5.23% d −1 , and 0.67 × 10 7 , respectively). However, the abundance of microscopic propagules of Ulva in the Qingdao area (3980 ind/l) was significantly higher than that in the Rudong area (667 ind/l). These findings help increase our understanding of the formation of the world's largest Ulva macroalgal blooms.

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