## 4 Discussion
### Influencing Factors of U. prolifera Migration  
Generally, the changes in _U. prolifera_ distribution are closely related to environmental factors. The growth of _U. prolifera_ has a wide range of adaptation to seawater temperature, salinity, pH, and light intensity due to its undemanding requirements for marine environmental conditions [29]. Therefore, the main reasons for the migration were considered to be wind fields and ocean currents. [PERSON] et al. [16] used a coastal ocean model to identify the processes that drive green algae drift. The results showed that, without wind, the research object displayed a tendency to move northward, but it was unable to move out of Subei Shoal. According to the drift path shown in Figure 8, a large area of algae was distributed outside Subei Shoal. Hence, this study focused on the relationship between _U. prolifera_ migration and wind direction and speed. To achieve the aim mentioned above, weekly average sea-surface wind (SSW) in the Yellow Sea during the monitoring periods in 2016-2018 was obtained from remote-sensing systems, (RSS) and is shown in Figure 15. Taking 2018 as an example, the detailed analysis is as follows: clearly, from early to mid-June, the northwest SSW gradually strengthened. During this period, the center of gravity of _U. prolifera_ presented a constant northwesterly movement, with an increasing amplitude of variation. From June 17 to June 23, a circular wind in the clockwise direction appeared. In the occurrence area of _U. prolifera_, the wind direction changed from northwest to northeast, which led to a sudden change in the migration path of the center of gravity. Starting from June 21, the barycenter moved eastward, reaching its easternmost position of this year on June 29. However, after the beginning of July, the direction of the SSW reversed again, resulting in the center of gravity turning from east to west until mid-July. After that, the SSW remained constant from the northwest for a long time, and the wind speed was relatively stable. From the end of July to early August, the wind speed obviously weakened, and the direction deflected southward from the northwest. This explained the abnormal phenomenon of the southward barycenter movement. After August 4, _U. prolifera_ moved northward  
Figure 14: (**a**) Histogram of _U. prolifera_ distribution and coverage area in 2018. The abscissa is the date, and the ordinate is the area in square kilometers. The stable period of the coverage area in 2018 is highlighted and marked by magenta boxes in the graph. (**b**) Histogram of _U. prolifera_ distribution and coverage area in 2017. (**c**) Histogram of _U. prolifera_ distribution and coverage area in 2016.  
again as the SSW turned north, and the wind speed slightly increased. Based on the above analysis, the time nodes of SSW changed in direction and speed, and they were consistent, with changes in the _U. prolifera_ trajectory during the whole monitoring period in 2018. Figure 15: _Cont._  
Most of time during 2017, the drift trajectory of _U.prolifera_ basically coincided with the direction of the sea wind. It was worth noting that from May 18 to June 4, the movement of the trajectory to the west was tremendous. Especially from May 27 to June 4, in a short period of one week, the span of distance was extremely significant, regardless of the north-south or east-west directions. During this period, although the direction of the sea breeze was consistent, there was no sudden increase in wind speed. It was hard to imagine that such large-span movements were caused by the influence of the sea breeze. Therefore, based on the results of the _U. prolifera_ growth situation analysis in Section 4.1, it could be speculated that two sources of _U. prolifera_ existed in 2017, with great possibility of this being the case. The tight relationship between the trajectory and the SSW also occurred in 2016.