Spatiotemporal distribution and influencing factors of Ulva prolifera and Sargassum and their coexistence in the South Yellow Sea, China

Marine ecological disasters occurred frequently in recent years and raised widespread concerns about the ecological health of the ocean. We analyzed the spatiotemporal distributions of Ulva prolifera and Sargassum from April to July each year between 2016 and 2020 in the South Yellow Sea using multisource (GF-1 and HJ-1A/1B) remote sensing images, combined with the MODIS sea surface temperature (SST) data, photosynthetically active radiation (PAR) data, and Quick SCAT sea surface wind (SSW) data, to explore the potential influencing factors. The results show that (1) U. prolifera and Sargassum appeared mainly from May to July and April to June, respectively; (2) U. prolifera showed an impact in larger spatial scope than that of Sargassum. U. prolifera originated in the shoal area of northern Jiangsu and finally disappeared in the sea near Haiyang-Rongcheng area. The spatial scope of the impact of Sargassum tended to expand. Sargassum was first detected in the ocean northeast of the Changjiang (Yangtze) River estuary and disappeared near 35 degrees N; and (3) correlation analysis showed that the SST influenced the growth rate of U. prolifera and Sargassum. PAR had varied effects on U. prolifera and Sargassum at different times. A moderate light conditions could accelerate the growth and reproduction of U. prolifera and Sargassum. High irradiance levels of ultraviolet radiation may cause photoinhibition and damage on U. prolifera and Sargassum. The southeast monsoon and surface currents promoted the drift of U. prolifera and Sargassum from the southeast to the northwest and north. Therefore, the spatial and temporal similarities and differences between U. prolifera and Sargassum were influenced by a combination of factors during their growth processes.