다중위성센서를 이용한 황해와 동중국해의 녹조, 갈조 탐지

Abstract

The macro algae blooms (MAB) has been increase along the shores of industrialized countries in the 1970s. In the Yellow Sea (YS), the green tide has been occurred every summer since 2008, and in recent years, the golden tide has also appeared on a large scale in the East China Sea (ECS), which is becoming a serious social problem. The damage caused by these are the cleanup of large amounts of seaweeds entering the coast, obstruction of movement of ships, influx in to aquaculture, and tourism damage. In this study, we compare the detection ability of floating algae by satellite sensors and propose a method of distinguishing between the green and golden tide using the reflective characteristics. We investigated the changes of floating algae in the YS and ECS through long-term monitoring using satellite images. We compared the detection area difference of floating algae according to various spatial resolution sensors. The areas extracted from low resolution sensor increased as the threshold value decreased. Also, as the threshold value increased, the difference in the areas decreased. The areas extracted from the high resolution sensor is widened from a certain threshold or more. The correlations (R2) with OLI, OLCI, GOCI and MODIS were 0.347, 0.144, 0.194 and 0.157, respectively, as compared with other sensors based on the location of the floating algae detection of MSI with the highest spatial resolution. It is considered that the floating algae is a factor that not only the spatial resolution but also the small space-time difference cans greatly reduce the correlation. We measured the reflectance of Ulva prolifera and Sargassum horneri, which are main species of the green and golden tide in the YS and ECS. U. prolifera showed a peak at 555 nm, while S. horneri showed a peak at 602 and 646 nm and a lower reflectivity at 632 nm. To distinguish between the green and golden tide, we proposed a method of using the difference of red and green reflectance (difference of Red-Green; DRG). The DRG in the golden tide was always positive, while those of the green tide was negative. The reflectance spectra of the green and golden tide were compared for various sensors (MODIS, GOCI, OLCI, ETM+, OLI, MSI). When the DRG was applied to the spectral difference between the floating algae and nearby water, the golden tide was always positive and the green tide was always negative. When red band was applied to the MODIS 645 nm band and the OLCI 620 nm band, the DRG of the green and the golden tide showed a larger difference. Based on MODIS and GOCI images, we detected green and golden tide from the YS and ECS from 2000 to 2018. From 2000 to 2003, golden tide occurred in the ECS and the floating algae were not detected from 2004 to 2006. Since 2007, the YS has had a green tide every year, and golden tide has appeared on the ECS since 2012 on a large scale. The area of the green tide was the largest at 20,603 km2 in 2016 and the golden tide was the largest at 7,827 km2 in 2017. The movement of floating algae is mainly influenced by wind and current, and the floating algae occurring in the China coast has moved to the Korean Peninsula, Jeju Island, and Japan. The Yellow Sea and East China Sea are emerging as the largest seas of floating algae in the world. The systematic and continuous monitoring is expected to help mitigate the damage from floating algae.