## 1 Introduction  
_Margalefdinium polykrikoides_ blooms occur in Korean coastal waters in a repetitive manner. This type of bloom has gradually become larger, wider, and more frequent since first appearing in 1995 [1, 2]. _M. polykrikoides_ blooms with a cell abundance greater than 1000 cells mL\({}^{-1}\) can have a negative impact on fish or shellfish [3]. Large-scale _M. polykrikoides_ blooms first formoffshore and are then generally transported to coastal waters, where they gradually accumulate [4]. Because there are many aquaculture cages found along the south coast of Korea, The occurrence of blooms with a high cell abundance in this area would cause considerable economic damage. Since the outbreak of red tide blooms in 2015, _M. polykrikoides_ blooms have not occurred on a large scale (maximum cell abundance >20,000 cell mL\({}^{-1}\) and duration >40 days) [4] between 2016 and 2020. Nevertheless, _M. polykrikoides_ blooms with a cell abundance of over 1000 cells mL\({}^{-1}\) have occurred on the southern coast of Korea every year from 2016 to 2020. To mitigate the damage caused by red tide blooms, it is important to identify their spatial distribution. Daily red tide reports provided by the National Institute of Fisheries Science (NIFS) include many types of data, including the nature of the causative organisms, cell abundance, and affected and warning areas [5]. NIFS operates a red tide warning system to provide red tide information through field sampling at discrete locations, focusing on _M. polykrikoides_. This system is composed of four levels: red tide emergence notice (>10 cells mL\({}^{-1}\)), red tide notice (>100 cells mL\({}^{-1}\)), red tide alert (>1000 cells mL\({}^{-1}\)), and warning lift. If the red tide observation areas are very wide, field sampling based on discrete locations is limited in terms of manpower, cost, and time. Hence, remote sensing approaches for red tide monitoring can be an effective tool and can overcome the spatial and temporal limitations of field sampling. Satellite-based red tide detection has been widely studied using ocean color sensors, such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), MODerate resolution Imaging Spectroradiometer (MODIS), Medium Resolution Imaging Spectrometer (MERIS), and Geostationary Ocean Color Imager (GOCI) [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Red tide blooms are detected by satellite-based products, such as chlorophyll concentration [6; 7; 8; 9] and normalized fluorescence line height [10; 11; 12; 13]. [PERSON] et al. [6] and [PERSON] et al. [7] demonstrated that a chlorophyll anomaly determined by pixel-wise means of a chlorophyll concentration map for 60 days was suitable for detecting _Karenia brevis_ blooms. This method was used as an early warning system to forecast _K. brevis_ blooms in the eastern Gulf of Mexico from the National Oceanic and Atmospheric Administration. [PERSON] et al. [13] used the fluorescence properties in the red part of the spectrum for red tide detection. [PERSON] et al. [14] detected red tide blooms using the increase in suspended particulate matter (SPM) caused by the occurrence of red tide blooms. In addition, optical properties, such as the absorption coefficient (a), backscattering coefficient (b\({}_{\text{bp}}\)), and remote-sensing reflectance (R\({}_{\text{rs}}\)) of red tide waters have also been applied to detect red tide blooms [15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. [PERSON] et al. [15] found that _K. brevis_ blooms have the optical properties of low backscattering. [PERSON] et al. [16] used the spectral slope of the normalized water-leaving radiance (n\({}_{\text{Lw}}\)) at approximately 490 nm. The use of this wavelength enables the distinction of _K. brevis_ blooms from other algal blooms. [PERSON] et al. [23] and [PERSON] et al. [24] reported that the peak of R\({}_{\text{rs}}\) shifted to longer wavelengths (570-590 nm) when red tide blooms occur.