Bohai Sea, with the highest suspended sediment concentration in the world, has experienced a sharp decrease in sea surface suspended sediment concentration (SSSC) in the latest two decades. However, limited by the shortage of long‐term field observations, the spatiotemporal characteristics and mechanisms of the long‐term variation in the SSSC are still unclear. Based on remote sensing data, this study reconstructed the spatiotemporal SSSC distribution from 2003 to 2016, and found significant spatial difference in decadal SSSC variation in the Bohai Sea. The largest decadal SSSC decrease was observed in the southern Bohai Sea with an average annual decrease of 1.3 mg/L (decrease of 4.4% per year), while significant SSSC increase was observed in the central Bohai Sea with largest annual increase of 0.4 mg/L (increase of 5.0% per year). Significant seasonal differences in the SSSC decreasing were also revealed, and the SSSC decrease in winter and spring (0.46 mg/L/a) was 27.8% higher than that in summer and autumn (0.36 mg/L/a). Decadal wind speed data measured at 14 meteorological stations around the Bohai Sea indicated that wind speed reduction played the most important role in the decadal decrease in SSSCs, and significant SSSC decrease can only be found in sea areas with a significantly reduced wind speed. In addition, although the runoff and sediment discharge of the Yellow River also decreased during 2003–2016, they were not related to the spatiotemporal variation of SSSCs and their direct impact was limited to the range of 15 km from the estuary.

This research used the profile data measured extensively in the Yellow Sea and Bohai Sea (YSBS) to explain the temporal and spatial distribution characteristics of optical properties and systematically analyzed the influencing mechanisms of the seasonal variations of optical properties in the YSBS in conjunction with synchronously measured hydrological and biogeochemical data in vertical profiles. The main conclusions obtained are as follows: the vertical variations in the optical properties in the YSBS are mainly influenced by the stratification effect, vertical mixing function, and phytoplankton growth process; and the variations of optical properties are dominated by the change of particle characteristics. The backscattering ratio can be used to discriminate particle types as a proxy of particulate bulk refractive index. In the YSBS, for waters with a backscattering ratio of less than 0.012, the variations of optical properties are dominated by the phytoplankton particles. For waters with a backscattering ratio greater than 0.012, the variations of optical properties are dominated by inorganic sediment particles. In addition, for the YSBS, the variations in optical properties of upper surface layer waters can be elucidated well by the vertical variations.

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