## 4 Discussion
### Directionality Change and Sea Wind  
Based on the change in SDE parameters, the influencing factors on algae distribution were further explored. It was also assumed that the sea breeze was the main influencing factor. During the three monitoring years in this study, the changes in oblateness and direction angle were quite different. The patterns in 2018 was easy to find, with three sharp increases in SDE oblateness (Figure 11a), and the direction being rotated clockwise with the passage of time (Figure 11b). Furthermore, the time periods where the two parameters changed greatly were important. In 2017 and 2016, the direction angle of SDE swung back and forth with no certain regularity, and there was a time period for where the angle rotated more than 100\({}^{\circ}\) clockwise in both years. Moreover, as in 2018, the flatness mutation occurred in the death stage of _U. prolifera_, with the maximum value of the oblateness occurring at the time when _U. prolifera_ was close to extinction. The analysis started from 2018. The three periods of oblateness change were in June 3 to 12, July 19 to 23, and August 2 to 11. In the first two time periods, the direction angle also changed greatly. Because the time span of the three periods was not long, the weekly averaged SSW data could not be used as a good basis. Therefore, a time interval of three days was used to determine whether the SSW changed suddenly, with the starting and ending date being taken as the search range. The results are shown in Figure 16. In these three periods, the wind direction changed, and the wind speed increased greatly. Figure 16: Three-day average surface wind speed and direction of three periods of oblateness change in 2018, obtained from RSS. In each specific period, the upper sub-graph represents the sea-surface wind (SSW) at the starting time, and the lower vector graph shows the changes in wind speed and direction within this period. (**a**) The change of SSW in period one. (**b**) The change of SSW in period two. (**c**) The change of SSW in period three. Period one: From the end of May to the beginning of June, the SSW in the Yellow Sea was about 2 m/s, with inconsistent directions. However, the direction changed significantly heading north, and the speed increased two or three times. On June 3, the direction angle of the _U. prolifera_ distribution was 21\({}^{\circ}\) north by west, approaching the coastline of Jiangsu Province. By comparison, on June 12, the north-south trend of the distribution was prominent. The direction angle was rotated strongly northward, and it almost reached the north direction. During this time period, the dramatic change in the shape of the distribution was closely related to the change in the SSW direction and the increase in wind speed. Period two: From July 21 to 23, the wind speed suddenly increased to a maximum of 20 m/s, forming a huge windy area in the southern part of the Yellow Sea. On July 19 and 23, the distribution direction angle was 19.5\({}^{\circ}\) and 30.2\({}^{\circ}\) north by east, respectively. During this period, the wind direction was west and northwest, and the wind speed decreased with increasing latitude. The southern part of the distribution area was more strongly affected by SSW than the northern part, which may have accounted for the clockwise rotation of the direction angle. Period three: At the beginning of August, the SSW was weak, at about 2 m/s, and the direction was westerly in the area where _U. prolifera_ was distributed. From August 2 to 11, the wind speed increased significantly. The northwest wind directly pointed to the coastline of Shandong Province. In early August, _U. prolifera_ was distributed in the sea area near Shandong Province. When the wind strengthened and the direction changed, _U. prolifera_ was pushed further towards the shoreline.