## 4 Discussion  
### Spatiotemporal Features of U. prolifera  
The spatiotemporal features of _U. prolifera_ were compared from three viewpoints: the drift path, the overall situation, and the growth stages. First of all, this study found that the origination and the drift path of _U. prolifera_ in 2016 and 2018 were still consistent with previous years. A large number of studies have shown that the green tides originate along the Jiangsu coast [30; 31; 32; 33], which was first proposed by [PERSON] et al. [2]. According to the growth of _U. prolifera_ discovered for the first time on June 3 in 2018 (Figure 4a), the scale was still small, and growth was mainly distributed along the coastline of Jiangsu Province. However, because of the poor quality of MODIS images due to meteorological factors  
Figure 13: (**a**) Distribution area and perimeter ratio in 2018 are shown in the diagram as two broken lines. The transverse coordinates represent the time series. The left vertical coordinate is the distribution area, and the right vertical coordinate is the ratio of the perimeter to area. (**b**) Distribution area and perimeter ratio in 2017. (**c**) Distribution area and perimeter ratio in 2016.  
in this year, it was impossible to judge the exact starting position and time of _U. prolifera_. Fortunately, the initial time that _U. prolifera_ was found in 2016 was May 15, with a smaller distribution area and a similar position as in 2018 (Figure 5a). This discovery may help with speculation on the origination of _U. prolifera_ in 2018. In terms of source tracing, unlike other years, 2017 presented a double-source phenomenon. On April 30 (Figure 6a), the _U. prolifera_ was firstly discovered in the southeast corner of the Yellow Sea. On May 18 (Figure 6c), _U. prolifera_ was founded near Jiangsu Province. Since the location span was very large, the time interval was short, and since then the growth of entemporpha shown an obvious characteristic of rapid outbreak of one end and rapid disappearance of the other end, two different sources may be considered. Based on the analysis above, the hypothesis that Subei Shoal was the source of phytoplankton was confirmed again, though it was probably not the only source. Earlier investigations also indicated that the free-floating green tides in the Yellow Sea drifted to the southern coast of Shandong Province from June to August [10; 16]. From 2016 to 2018, the floating angle has maintained a trend of gradual northward drift, accompanied by one or more north-to-south detours and finally drifted and died out near the vicinity of Qingdao. Secondly, the overall situation of green tide events in 2016-2018 remained serious. [PERSON] et al. [34] stated that from 2008 to 2012, the areas in 2008 and 2009 were obviously larger than in other years, with a maximum coverage area of 3451.19 km\({}^{2}\) and 2767 km\({}^{2}\) and an affected area of 23,475 km\({}^{2}\) and 26,234 km\({}^{2}\) in 2008 and 2009 respectively. From 2010 to 2012, the area was greatly reduced and relatively stable. However, [PERSON] et al. [35] claimed that the maximum distribution area of green tide was increasing year by year from 2011 to 2016. During the three years 2016-2018 in this study, the maximum distribution and coverage area were 57,384 km\({}^{2}\) and 2906 km\({}^{2}\) in 2016. In the following two years, the distribution and coverage area relatively decreased, but still lead to a tremendous influenced marine area. Normally, the development of macroalgal blooms in the Yellow Sea goes through five major stages: appearance, development, outbreak, decline (management), and disappearance [36]. From an interannual perspective, the five stages occur at different time periods in different years. Generally speaking, the earliest time of emergence was between the end of April and the beginning of May [31], with a relatively small coverage area and taking the form of algae filaments floating on the sea surface.