[Hyperspectral Applications] Monitoring of Cyanobacteria in Taihu Lake Based on Xiguang-1 05 Satellite (Tianxianpeihao)

Background Introduction

Satellite remote sensing technology plays a crucial role in inland water quality monitoring, enabling the retrieval of key water quality parameters over a large area, relatively quickly, and at low cost. However, inland water quality monitoring also places high demands on satellite remote sensing technology: small water bodies require high spatial resolution; rapidly changing water quality necessitates high temporal resolution; and complex and variable optical characteristics of water bodies require high spectral resolution. Therefore, satellite data that simultaneously meets high spatial, high temporal, and high spectral resolution has significant advantages in inland water monitoring.

Phycocyanin (PC) is a common co-pigment in cyanobacteria. Its absorption peak is concentrated in the range of about 620–650 nm, so there will be local absorption depression (or relatively low reflectance) in this band.

When there is an abundant cyanobacterial community on the water surface, Rrs(l) will be relatively smaller in this absorption band, and there is usually little or no absorption on both sides of the band. Based on this principle of "local depression", the absorption depth can be quantified by the "peak height/baseline" method, thus serving as an indicator of the presence and concentration of PC.

Methods and Principles

The surface reflectance data of sentinel-2 L2C and Xiguang-1 05 satellite (Tianxianpei) that passed over the area around June 18, 2025 were selected for calculation. The cloud cover was less than 10%, and the weather was clear before and after the satellites passed over the area.

Table 1. Parameters of Xiguang-1 05 Satellite (Tianxianpei)

Table 2 Sentinel-2 band parameters

Phycocyanin Index (PCI)

The optimal hyperspectral band is 620–650 nm. A linear interpolation is performed between 600 nm and 650 nm as a baseline. The difference between this baseline value at 620 nm and the actual Rrs(620) is then used as the PCI. When the difference is negative (absorption), the absolute value or the opposite sign is usually taken to express the "absorption intensity".

Results Analysis

This comparison uses data from Xiguang-1 05 satellite (also known as Tianxianpei) and Sentinel-2. The comparison results are as follows:

Figure 1. Sentinel-2 RGB composite image 20250616

Figure 2. RGB synthesis of Xiguang No. 1 05 Star (Tianxianpeihao) 20250618

Figure 3 Sentinel-2 PCI Distribution (20250616)

Figure 4. PCI distribution map of Xiguang-1 05 satellite (Tianxianpei) 20250618

The comparison of the above distribution maps shows that the spatial distribution trends of Xiguang-1 05 satellite (Tianxianpei) and Sentinel-2 PCI are quite consistent and similar overall. The western Taihu Lake region experiences severe cyanobacterial blooms, likely due to the concentration of rivers flowing into the lake, bringing large amounts of nutrients that promote algal growth, leading to cyanobacterial blooms and increased chlorophyll a concentration. The central and eastern Taihu Lake regions are less affected, resulting in better water quality than the western region. Aquatic plants grow in the eastern part of the lake, and chlorophyll a concentration is lower. The central lake area is a large-scale bloom distribution zone; once a bloom occurs in the summer, it covers a large area. Blooms are rare in the southwest lake area, and almost no blooms occur in the eastern Taihu Lake, indicating a better environment in the eastern Taihu Lake. Some differences exist in the central lake area, mainly due to variations in satellite transit times; the summer cyanobacterial blooms in Taihu Lake show significant temporal differences.

Overall, the PCI index of Xiguang-1 05 satellite (Tianxianpeihao) maintains a high degree of consistency and usability with the Sentinel-2 data results.