## 1 Introduction  
Searseed tides, including green tides and golden tides, have been a nuisance in recent decades, with their occurrence increasing worldwide; the resulting consequences include deteriorating tourism environments, hindering marine aquaculture, and disrupting fishing work [1, 2]. If not cleaned up in time, seaweed biomass will rot into a pritd mass, and its oxygen-deficient interior can generate toxic hydrogen sulfide (H\({}_{2}\)S) [3], which can cause adverse impacts on coastal ecosystems. Two genera, _Ultra_ and _Sargassum_, are primarily responsible for the cosmopolitan seaweed tide bloom. The former genus, _Ultra_, has a major causative role in green tides, and the latter genus, _Sargassum_, the source of the Sargasso Sea's name, has a major causative role in golden tides. In terms of the vegetative body, the thallus thickness of _Ultra_ can reach up to the level of one or two cells, and the shapes are diverse and can be sheet-like, tubular, or fern-shaped [4]. The thallus of _Sargassum_ is leathery and tough, and _Sargassum_ is made up of leafy appendages, branches, and round, berry-like structures, which are called pneumatocysts, that are filled with gas and help the plant's body to float on the sea surface. In the 1900s, the green tide issue emerged in Europe, and it occurred extensively in temperate and tropical coastal waters all over the world during the 1970s-2000s [3; 5; 6]. The golden tides induced by _Sargassum_ mostly took place in the Sargasso Sea, Gulf of Mexico, and Caribbean Sea [1; 7], while recently they have intruded into the west coastline of the northern and tropical Atlantic Ocean, and even further to the coasts of western Africa and northern Brazil in the southern Atlantic Ocean [1; 8; 9; 10; 11; 12]. Despite the different vulnerable locations of these two ecological disasters due to the different characteristics of _Ulva_ and _Sargassum_, it is surprising to find the concurrence of both seaweed tides in the Yellow Sea of China [13; 14]. Before 2007, several gathering events of green macroalgae had occurred in China; however, these were small in scale. The large-scale green tides detected in 2007 appeared near the northern coastline of Jiangsu Province, but they did not draw much attention [15; 16]. The next year, in 2008, an enormous green tide inundated the coastal waters of Qingdao, threatening the Olympic sailing regatta [17]. Since then, large-scale green tides have recurred annually from spring to summer in the Yellow Sea, with a maximum coverage area of 2100 km\({}^{2}\)[2; 3; 15; 17; 18; 19; 20], and it has been recognized as the largest green tide in the world in terms of the distribution, the coverage, and the biomass of the green macroalgae [17; 18]. It was found by remote sensing that the green tide was tracked back to the Subei Shoal in the southwestern Yellow Sea [15; 17; 21; 22; 23; 24; 25; 26], and further field surveys indicated that the _Ulva prolifera_ attached to the aquaculture rafts of _Pyropia_ were scraped and discarded in situ, which was the primary source of the green tides in the Yellow Sea [18; 20; 27; 28; 29]. Some other green tides were reported in Qinhuangdao of Hebei Province, Yantai and Haiyang of Shandong Province, Xiamen of Fujian Province, Shantou and Zhanjiang of Guangdong Province, Haikou of Hainan Province, and Beihai of Guangxi Province, demonstrating that it is a nationwide ecological issue in China [2]. In particular, a local green tide happened in Jinmeng Bay, Qinhuangdao, in 2015, and was recurrent yearly thereafter every April to September [30; 31]; the latest one, in August 2021, was studied in detail in this research. When green tides occur, field surveys are the most direct measure of acquiring access to first-hand data in order to determine the cause of the disaster; however, they are expensive and time-consuming, and have a measuring point limitation unable to obtain the overall spatiotemporal variations of green tides in a study area [19].