An Analysis on the Distribution of Floating Seaweed in the East China Sea and Southern Yellow Sea in 2015–the Case of Sargassum observed by the Geostationary Ocean Color Imager

In early 2015, a large amount of brown seaweed, known as Sargassum horneri macroalgae, was piled up along the shore of Jeju Island and the southwest islands of the Korean Peninsula. This event was associated with a huge bloom of floating Sargassum in the East China Sea (ECS) and southern Yellow Sea (SYS). Ship surveys or aerial surveys can only cover a limited space and are time consuming and expensive. This study aims to capture temporal variation in the geographical distribution of floating S. horneri using satellite imagery obtained from the Geostationary Ocean Color Imager (GOCI). The GOCI acquires eight images a day with a 500-m spatial resolution, a high signal-to-noise ratio and a constant viewing angle, providing imagery suitable for monitoring a large-scale floating algae event and its temporal evolution. Semi-monthly aggregated images were generated to determine fractional coverage area per pixel or density of the floating algae from January to June, 2015. The results are consistent with previous field-survey-based studies, but also reveal a number of new findings. Unexpectedly, S. horneri patches were detected as early as January over a broad area of the ECS continental shelf. The floating algae were detected not only near the outer continental shelf area along the Kuroshio front and Eastern Kuroshio Branch current as previously reported in literature but also along the western inner continental shelf. The floating seaweed patches along the eastern outer shelf proliferated during the second half of March, then moved north, entering the Korea-Tsushima Straits in the west of Kyushu to the north. The algae patches in the western shelf moved north in April and May and separated, one entering the Jeju Strait and the Korea-Tsushima Straits, and the other entering the SYS. Overall, S. horneri density peaked in late April, then decreased in May and June before disappearing from the ECS in July.