Global Occurrence of Macroalgal Blooms and Driving Factors of Ulva prolifera Green Tides in the Yellow Sea

Macroalgal blooms are increasing globally due to anthropogenic stressors and climate change. Globally, green macroalgae are the most frequently reported and widely distributed type of blooming algae, causing negative environmental and socio-economic impacts. Recurring annually in the Yellow Sea for 17 consecutive years since 2007, the massive green tides caused by Ulva prolifera have been recognized as the largest-scale green tide events in the world. Consequently, the prevention and mitigation of Yellow Sea green tides have become an important component of China’s coastal ecological security. This paper provides an overview of the main types, geographical distribution and ecological consequences of macroalgal blooms worldwide and in China, and systematically reviews the origin, formation mechanism and the key driving factors on the outbreak and maintenance of Yellow Sea green tides. Current research indicated that the Ulva prolifera green tide in the Yellow Sea originats from the Subei Shoal, and attached Ulva on the Porphyra aquaculture rafts is the only identified source of the early stage floating macroalgae. The development of Yellow Sea green tide is influenced by multiple environmental factors and hydrodynamic field combined with human interventions and climate change. Specifically, Porphyra yezoensis culture activities primarily affect the initial biomass of the green tide, while seawater temperature and nutrient level mainly influence the growth and demise of Ulva prolifera. Monsoon and the wind-driven surface currents determine the drifting path and the residence time of floating algae at different stages. Overall, climate change influences the meteorological and circulation conditions in the Yellow Sea region, altering background environmental dynamic fields such as water temperature, salinity, nutrient levels, and monsoon intensity, thereby affecting the interannual variability of the green tide, specifically reflected in outbreak timing, drift path, bloom scale, and duration.