**4. Discussion**

For dissolved matter in water, the variation of CDOM is greatly influenced by hydrodynamic conditions, such as horizontal transport and vertical mixing. The PRE circulation is dominated by the gravitational exchange current with the surface freshwater flowing seaward in the surface, mainly on the west side, and the compensated seawater in the lower layer flowing landward along the east coast [31, 32].

As illustrated by in-situ observations and satellite interpretation, CDOM shows the strongest absorption and lowest spectral slope at the head of the estuary (Station A1, A2 and A3) (**Figures 4** and **5**). Water in this region comes from the discharge of the Humen and Jiaomen Gates (**Figure 2**), carrying abundant dissolved organic materials. These CDOM components with complex composition are then transported seaward along the west coast, causing the western estuary to have stronger CDOM absorption and lower spectral slope than the eastern estuary, where the intrusive seawater brings large amount of marine CDOM with lower absorption and higher spectral slope.

It has been reported that terrestrial CDOM is much more sensitive to photochemical reactions than marine CDOM [33]. DOM originating from upper streams during high-flow events is abundant in aromatic compounds, and therefore is highly susceptible to solar irradiation, which is in agreement with laboratory

experiments which reveal a high degree of riverine DOM photoreaction [34]. Thus, photodegradation can be a possible reason for the removal of terrigenous CDOM in the surface water of the PRE, therefore the decrease of CDOM absorption. Furthermore, photobleaching can convert high molecular weight CDOM species to lower molecular weight species, result in a great decrease in ag(295) than in ag(275), and consequently cause the spectral slope increase [30, 35, 36].

With respect to the ocean color applications, the spatial distribution pattern of CDOM absorption in upper layer is successfully captured by the algorithm, and the time-series CDOM variation which collaboratively changes with river discharge, is also reflected from satellite multi-senor imagery. Furthermore, the variation of CDOM spectral slope (Sg) within the PRE is depicted with very high spatial resolution, by retrieving Sg from satellite imagery pixel by pixel using Eq. (5). This can lead to much more delicate CDOM absorption products (**Figures 4** and **5**), because previous algorithms to derive CDOM absorption spectra generally use a uniform Sg for the entire water area of interest to calculate ag [37], which is evidently unreasonable for estuarine and coastal waters, where the CDOM of different sources may have different levels of Sg.
