## 1 Introduction  
_Phaeocystis globose_[1] (_P. glob_) is a photosynthetic autotrophic planktonic alga belonging to the phylum Chrysophyceae and the order Flagellates. It exists in two forms of life: unicellular and colonial gelatinous cysts. During mass reproduction, it transforms from unicellular to more dominant cysts and produces harmful substances, such as haemolytic toxins, capable of killing fish and shellfish [2]. This leads to damage in the environment of marine water bodies, potentially resulting in the severe degradation of marine ecosystems in both structure and function [3]. Since 2015, _P. glob_ red tides have frequently occurred in Qinzhou Bay, Guangxi, China (Table 1). A red tide not only affects marine fisheries and the ecological environment, but also leads to the blockage of cooling water intake systems in nuclear power plants [4]. As a result, many scholars have investigated the morphology [5; 6], growth history [7; 8], and genome sequence [9] of _P. glo_. They found that, compared to other algae, _P. glo_ is more adaptable to low temperatures and can survive in low-light environments. Consequently, _P. glo_ tends to aggregate in deep water and remains suspended in seawater after forming cysts. It only floats to the sea surface when its density is high. This phenomenon leads to the detection of _P. glo_ in surface seawater, indicating that the _P. glo_ red tide has already reached a high blooming stage, causing potentially irreversible harm to marine organisms and the ecological environment. Therefore, the development of effective early warning methods for _P. glo_ red tides is of paramount importance. In a study of [PERSON] et al. [10], it was pointed out that there exists a significant negative correlation between the concentration changes in _Synechococcus_ and _Prochlorococcus_ and _P. glo_. This finding offers a potential early warning indicator for _P. glo_ red tides. _Synechococcus_ and _Prochlorococcus_ belong to the same group of cyanobacteria, which are algae with a high phytoplankton abundance in the ocean [11; 12], whereas cyanobacteria are prokaryotic organisms with intracellular phycobilisomes. The phycobilisomes contain phycocyanin (PC), distinguishing them from other algae [13]. PC concentrations are usually utilized as markers for monitoring the concentration of cyanobacteria. Because PC has a strong absorption property at 620 nm, most water quality sensors use 620 nm fluorescence photometry to monitor the PC concentration [14]. The traditional manual monitoring methods are not only time-consuming and labor-intensive, but the monitored points are also difficult to represent the changes in PC concentration in the entire sea surface. Remote sensing monitoring can make up for the shortcomings of manual monitoring. It does so by analyzing the correlation between PC concentration and spectral characteristics of the water body. This involves the use of empirical, semi-empirical, and analytical methods to establish a mapping relationship between PC concentration and spectral features of water bodies. For example, [PERSON] et al. [15] developed a semi-analytical, nested band ratio-based algorithm and verified the feasibility of the method for PC retrieval in turbid water bodies dominated by cyanobacteria. [PERSON] et al. [16] proposed the band-subtraction algorithm for cyanobacterial bloom in Lake Taihu, China, where the remote sensing reflectivity at 620 nm was deduced from the baseline height between 560 nm and 665 nm, and then the obtained value was used to estimate the PC concentration. In addition to using the band ratio as independent variables for the input of the model, researchers have also proposed an enhanced three-band empirical model based on a three-band algorithm [17; 18] framework to detect high PC concentrations (PC \(\in\) [68.13, 3032.47] \(\mathrm{\SIUnitSymbolMicro g}\)/L) in inland water bodies.