Dynamics of euphotic zone depth in the Bohai Sea and Yellow Sea.

Euphotic zone depth (Z) plays an important role in studies of marine biogeochemical processes and ecosystems. Remote sensing techniques are ideal tools to investigate Z distributions because of their advanced observation ability with broad spatial coverage and frequent observation intervals. This study aims to develop a new approach that derives Z directly from remote sensing reflectance (R(λ)) values rather than by using other intermediate variables and then reveals the dynamic characteristics of Z in the Bohai Sea (BS) and Yellow Sea (YS). To do this, in situ data collected from various seasons were first used to assess the ability of several spectral indicators of R(λ) for deriving Z and the optimal spectral indicator was determined to build a Z retrieval model. This model was further applied to Geostationary Ocean Color Imager (GOCI) data to study the spatial and temporal variations in Z. The results showed that the new Z retrieval model performed well with R, RMSE and MAPE values of 0.843, 4.42 m and 17.9%, respectively. High Z levels were generally observed during summer for both coastal and offshore waters while the lowest Z values were observed during winter. Changing concentrations of total suspended matter, which are often modulated by sediment resuspension and transportation, are probably the main factor responsible for the spatial and temporal variability of Z. These findings provide crucial information for modeling primary production, carbon flux, and heat transfer, etc., in the BS and YS, as well as contribute a useful alternative approach that will be easily implemented to study Z from satellite data for other water environments.