Study on the underwater light field and water quality parameters along the coast around the Guangdong-Hong Kong-Macao Greater Bay Area

The underwater light field and water quality parameters serve as important indicators for evaluating ecosystem structures and the capacity of the marine carbon cycle. Investigating the underwater light field and water quality is essential for the sustainable management of the marine ecological environment. The coast around the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), an important blue economic region in China, is characterized by a complex hydrodynamic system. Based on in situ optical properties and water quality parameters, the underwater light field along the coast around the GBA was investigated using the downward diffuse attenuation coefficient Kd, euphotic depth Zeu, and Secchi disk depth ZSD through radiative transfer numerical simulation software. Kd with or without the bottom irradiance reflectance was studied, and the relationships between the underwater light field and water quality parameters (especially for chlorophyll-a concentration (Chla) and turbidity) were analyzed. The results indicate that 1) Bottom irradiance reflectance has a negligible impact on Kd values in this study area; 2) Kd exhibits significant positive correlations with both Chla and turbidity. A superior linear regression is found between Kd(570) and turbidity (R = 0.894), while a strong exponential regression is observed between Kd (430) and Chla (R = 0.718); 3) The spatial distributions of the underwater light field and water quality parameters are influenced by the Pearl River plume (PRP) and upwelling in eastern Guangdong. Specifically, the eastern coast exhibited lower Chla and turbidity together with higher water transparency than the western coast, while the eastern offshore region showed higher Chla and turbidity along with lower water transparency compared to the western offshore region; 4) Comparative analysis revealed a decrease in ZSD average from 2020 (11.1 m) to 2023 (6.8 m), potentially attributed to intensified PRP effects and increased regional precipitation during 2023. This study provides essential data support for evaluating the ecological health along the coast around the GBA.