In both sub-basins, CDOM profiles displayed a large seasonality, which can be summarized by a strong surface photobleaching in summer time, a high mixing in winter, and a seasonal subsurface enhancement. In summer, owing to exposure to intense sunlight, surface CDOM lost its absorptivity and fluorescence efficiency remarkably. Consequently, the minimum of ay(412) (less than 0.005 m 1) always appeared at surface in summer. Another reason leading to the seasonality of surface CDOM was the strong mixing effect in winter, which not only generated a very deep homogenous CDOM layer that even may exceed 350 m in the eastern sub-basin, but also brought some deep CDOM into the upper layer. Besides, from summer to winter,