Spatiotemporal characteristics and remote sensing estimation of colored dissolved organic matter around the Leizhou Peninsula

Colored dissolved organic matter (CDOM) is an important optically active substance in marine environment. Its biochemical conservatism makes it an important indicator to offshore pollution process. Monitoring the content, composition, and diffusion process of CDOM is a good approach to analyze the terrestrial input, optical properties, and ecological environment of offshore areas. The spatiotemporal characteristics of CDOM around the Leizhou Peninsula were analyzed based on field observation data collected in the autumn 2020 and spring 2021, and an empirical inversion model of the a g (355) (absorption coefficient at 355 nm) and spectral slope ( S g ) (g stands for gelbstoff/gilvin, which is called CDOM) based on Sentinel-3A ocean and land color instrument (OLCI) images was constructed. The results show (1) the order of average a g (355) value around the Leizhou Peninsula was east coast (0.503/m)>Qiongzhou Strait (0.502/m)>west coast (0.365/m); (2) the best band combinations of CDOM inversion in spring and autumn were (B4+B11)/B3 and (B7–B1)/B6 (B stands for the band of spectral images), and the final inversion results are close to the measured results, indicating that the model has good accuracy; (3) the S g value of the CDOM absorption spectrum was fitted to the hyperbolic-exponential model. The fitting accuracy of the model was higher than those of the exponential model and the hyperbolic model, and the best S g inversion model was constructed by selecting S g (275–295) and S g (250–295) in spring and autumn; (4) the spatial distributions of a g (355) and S g were inverted, and CDOM in the waters around the Leizhou Peninsula originated from terrestrial organic matter carried by coastal aquaculture zones and runoff in the northeast Zhanjiang and the northern Beibu Gulf.