Long-Term Changes in Colored Dissolved Organic Matter from Satellite Observations in the Bohai Sea and North Yellow Sea

Cong Xiao,Shengqiang Wang,Yu Huan,Deyong Sun,Jiabao Zhang,Zhongfeng Qiu

doi:10.3390/rs10050688

Abstract

Spatial and temporal variations in colored dissolved organic matter (CDOM) are of great importance to understanding the dynamics of the biogeochemical properties of water bodies. This study proposed a remote sensing approach for estimating CDOM concentrations (CCDOM) based on in situ observations from the Bohai Sea (BS) and the North Yellow Sea (NYS). Cross-validation demonstrated that the accuracy of the CDOM algorithm is R2 = 0.78, APD = 15.9%, RMSE = 0.92 (ppb). The CDOM algorithm was applied to estimate the 14-year (2003–2016) sea surface CCDOM in the BS and NYS areas using Moderate Resolution Imaging Spectroradiometer (MODIS) monthly products. The results showed a significant fluctuation in CDOM variations on a long-term scale. The highest values of CDOM were observed in the BS, the middle values were observed in the Bohai Strait, and the lowest values were observed in the NYS. Seasonal variations were observed with long-lasting high CDOM values from June to August in coastal waters, while relatively low values were observed in the NYS in the summer. In the spring and fall, a distinct increase appeared in the NYS. High CDOM values in the nearshore coastal waters were mostly related to terrestrial inputs, while CDOM in the offshore regions was mainly due to autochthonous production. Furthermore, ocean currents played an important role in the variations in CDOM in the BS and NYS areas, especially for variations in CDOM in the Bohai Strait.

Highlights

Colored dissolved organic matter (CDOM) is one of the optically active fractions in dissolved organic matter existing in natural waters [1] and participates in physical and biogeochemical processes in ocean ecosystems [2,3]
CCDOM values covered various environments, including the Bohai Sea (BS) and North Yellow Sea (NYS) areas, which were used for model development, and the values of CCDOM varied widely between 1.921 to 8.985, with an average of 4.430 and a standard deviation of 1.935
This study developed a regional empirical remote sensing approach to estimate CCDOM in the BS and NYS areas

Summary

Introduction

Colored dissolved organic matter (CDOM) is one of the optically active fractions in dissolved organic matter existing in natural waters [1] and participates in physical and biogeochemical processes in ocean ecosystems [2,3]. Numerous algorithms have been proposed to retrieve CDOM absorption based on satellite ocean color data, including empirical [10,11,12], semi-analytical [13,14,15], and matrix inversion as well as optimization methods. Empirical approaches require less fundamental knowledge of the mechanisms between water components and inherent optical properties but require adequate data to parameterize the models. These algorithms are sensitive to variations in water constituents [16]. Based on semi-analytical methodologies, matrix inversion methods require some certain knowledge about specific optical properties, such as the absorption slopes of CDOM [18]. While the above algorithms have been developed and successfully applied in specific regional natural water environments, their utility and prediction accuracy over a range of varying water conditions, for complex freshwater ecosystems or in high turbidity water, still need to be sufficiently tested

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