**3. Results**

**Figures 4** and **5** present time-series products of ocean color-retrieved CDOM absorption (ag(290)) and spectral slopes (Sg(250–400)) from 2012 to 2018, which are derived from the satellite images listed in **Table 3** using Eqs. (3)–(5). All the products can capture the general distribution pattern of CDOM in the PRE,

*Variations of the Absorption of Chromophoric Dissolved Organic Matter in the Pearl River… DOI: http://dx.doi.org/10.5772/intechopen.90765*

**Figure 4.**

*Time-series of CDOM absorption (ag(290)) in the PRE in spring in 2012–2018 derived from VIIRS, OLCI, HICO, and OLI.*

showing the gradual decrease of absorption and the increase of spectral slope along the northwest to southeast direction.

In addition to the clear and consistent trend illustrated in **Figures 4** and **5**, the quantitative variations of CDOM absorption and spectral slope with time are also remarkable. The temporal fluctuation is especially evident in the upper and western PRE, where the water properties are greatly influenced by freshwater discharge, especially during the flooding season.

**Figure 5.**

*Time-series of spectral slope of CDOM absorption (Sg(250–400)) in the PRE in spring in 2012–2018 derived from VIIRS, OLCI, HICO and OLI.*

Source and removal are the two major aspects that control and balance the CDOM absorption budget in nature waters. In open oceans, phytoplankton production and upwelling of deep water can bring new CDOM into upper layer to elevate CDOM absorption in surface water, while the photobleaching and high stratification can strongly decrease the absorptivity of surface CDOM and increase the spectral slope. In estuaries and coastal waters, the most distinctive condition different from the open oceans is the terrestrial input, which is a significant source of organic *Variations of the Absorption of Chromophoric Dissolved Organic Matter in the Pearl River… DOI: http://dx.doi.org/10.5772/intechopen.90765*

#### **Figure 6.**

*Correlation between the monthly averaged river discharge of the sum of values measured by two gauging stations (Boluo and Shijiao) and the monthly averaged ag(290) maxima from the CDOM Ocean color products.*

matter in surface water, and leads to the highest variation of CDOM concentration in the region [1]. Therefore, in estuaries and coastal waters, river flow is always an important factor influencing the temporal variation of CDOM absorption during the wet season.

A positive correlation between the monthly averaged river discharge and the monthly averaged ag(290) maxima from the CDOM ocean color products is shown in the PRE (**Figure 6**), suggesting the high CDOM absorption in the PRE is always associated with high river flow. This pattern is especially typical in spring, when the Pearl River enters the flood season, and the river discharge can dramatically increase after a heavy rainfall. As a contrast to the high river flow, the existing CDOM in surface water of the PRE can be rapidly photodegraded or microbial consumed under the low river discharge with limited inputs of new CDOM, resulting in low level of CDOM absorption and large value of spectral slopes. In this point of view, the time-series of CDOM absorption can be well correlated with river discharge, and therefore is a good indicator of estuarine hydrodynamic conditions.
