Colors of Macroalgae: Distinguishing Ulva Prolifera and Sargassum Horneri Using Sentinel-2 L2A Images

Macroalgae blooms have occurred in the East China Sea (ECS) and Yellow Sea (YS) frequently in recent years. Two of the most common macroalgae are mainly Ulva prolifera and Sargassum horneri. In this article, a novelty method named commission internationale de l'éclairage (CIE) color space of macroalgae (CIE-M) was developed to distinguish Ulva prolifera and Sargassum horneri using Sentinel-2 L2A data. First, since Ulva prolifera and Sargassum horneri have biologically different pigment compositions, the color difference between them was plotted in the CIE color space using data extracted from the Sentinel-2 L2A images. It was found that most of the pixels from Ulva prolifera showed higher CIE_x and CIE_y than those of the Sargassum horneri. For practical application, thresholds of 0.345 and 0.35 were chosen for simple binary classification between Ulva prolifera and Sargassum horneri on satellite images. Second, the developed CIE-M method was compared with the other three previous methods, green algae index, slope of red–green, and Sargassum and Ulva prolifera index. The results showed that our newly developed method has satisfactory extraction results not only in clear region, but also in cloudy, and turbid region. Finally, the monthly spatial variation of Ulva prolifera and Sargassum horneri during 2019–2023 was analyzed separately, using the Sentinel-2 L2A images. It was found that Ulva prolifera mainly occurred in the YS from late spring to summer seasons, while Sargassum horneri blooms mainly occurred during winter to spring in the ECS and YS, with maximum area in February.