Remote sensing of spatial and temporal variations of euphotic zone depth in the Bohai Sea and Yellow Sea during recent 20 years （2002—2020）

Euphotic zone depth (Zeu) is defined as the depth at which photosynthetic available radiation is 1% of its surface value. This zone is in the upper water column, where marine phytoplankton can effectively photosynthesize, which is essential in air–sea interaction through transfer of either gases or heat, especially with reference to greenhouse gases, such as carbon dioxide. Accordingly, the euphotic zone has an important influence on research into marine primary productivity, phytoplankton biomass, and global carbon cycle. Meanwhile, the spatial and temporal variations of Zeu are closely related to the variability of water color elements. Consequently, Zeu is regarded as an indicator of water clarity, which may even have a certain indicative significance for ecosystems. Thus, marine researchers have prioritized Zeu monitoring. In this study, a remote sensing model was proposed to estimate Zeu from the moderate resolution imaging spectroradiometer (MODIS) satellite data based on in situ data collected from several cruises in the Bohai Sea and Yellow Sea. The designed model uses the logarithm of slope of the remote sensing reflectance (Rrs) between 443 nm and 6S67 nm as an input. In situ data validations indicated that the algorithm shows good performance, with 0.86 R2 (coefficient of determination), 4.14 root-mean square error, and 17.2% mean absolute percentage error. The model based on Rrs efficiently performs compared with the current common models. The long term MODIS satellite data (2002—2020) were further used to investigate the spatial and temporal distributions of Zeu in the Bohai Sea and Yellow Sea. Results indicate that: (1) Zeu is low in the coastal regions but high in offshore waters. Meanwhile, clear temporal variability in Zeu was observed, showing that Zeu is typically high in summer but low in winter for most regions. (2) The tongue-shaped structure with a low value in the North of Yangtze River Estuary extends to the northeast in summer and turns to the southeast in early autumn. (3) Zeu monotonously varied in the Bohai Sea, Northern Yellow Sea, and Subei Shoal from 2002 to 2020. In the Bohai Sea and Subei Shoal, Zeu showed a downward trend, while it displayed an upward trend in the Northern Yellow Sea. Meanwhile, Zeu indicated a fluctuating trend in the Southern Yellow Sea, South of Jeju Island, and North of Yangtze River Estuary. Furthermore, the potential driving factors responsible for these spatiotemporal variations were examined based on multi-source satellite data. The results indicate that the spatial and temporal variations of Zeu in the Bohai Sea, Southern Yellow Sea, Northern Yellow Sea, and Subei Shoal are influenced by a variety of driving factors. Zeu is positively driven by the sea surface temperature and photosynthetic active radiation but negatively driven by wind speed and total suspended matter concentration. Specifically, the total suspended matter concentration has a significant effect on the Zeu variations. Meanwhile, Zeu in the North of Yangtze River Estuary is strongly related with the amount of runoff (correlation coefficient R=-0.55). © 2022 National Remote Sensing Bulletin. All rights reserved.