**1. Introduction**

The chromophoric (or colored) dissolved organic matter (CDOM) is the lightabsorbing part of the dissolved organic matter (DOM) in natural waters, which may cause a yellow or brown water color at high concentrations [1]. Chemically, CDOM refers to an ensemble of structures including carboxylic acids and carboxyl-rich alicyclic molecules, substituted phenols, ketones, aldehydes, quinones, carbohydrates, saturated and unsaturated hydrocarbons, and nitrogenous material [2].

The major sources of CDOM in the marine environment are terrestrial-derived, oceanic-produced, or sediment-introduced [1]. The terrestrial CDOM is originally synthesized by land and water plants, subsequently processed and modified in limnic systems and eventually exported to coastal waters. The oceanic CDOM is originally fixed by marine plants or phytoplankton, produced by heterotrophic and autotrophic organisms, or formed by photooxidation of colorless DOM [3–5]. The sediment CDOM is often observed in coastal waters and shelf seas where sediment resuspension, hypoxia events or hydrothermal events occur [6–8].

The processes involved in the removal of CDOM in nature waters include photodegradation and microbial activities. The photochemical reaction triggered by CDOM absorption of high-energy (low wavelength) light can have great impacts on biogeochemical processes and water ecology [9–11]. Heterotrophic microbes either incorporate or respire organic matter and modify CDOM into labile DOM, which are rapidly degraded in the process [12].

Estuaries and coastal waters are very productive systems, where high loading of terrestrial CDOM and high local production are mixed, processed and exported to shelf seas [1]. Therefore, a better understanding of CDOM variation in estuaries and coastal waters can help estimating the oceanic carbon budget and evaluating the anthropogenic impacts on marine environments and global climate change. Furthermore, in estuarine and coastal waters, CDOM absorption usually co-varies with salinity. Variation of CDOM absorption can be used as a tracer of water mass mixing in near shore waters [13]. Investigating and explaining CDOM variations are therefore crucial for understanding various processes in the aquatic environment.

The Pearl River Estuary (PRE) is located on the southern coast of Guangdong, China (22–22.75°N, 113.5–114°N) (**Figure 1**), adjacent to the Northern South China Sea (NSCS). Its major part, the Lingdingyang Estuary, has a trumpet-like shape, with four gates (Humen, Jiaomen, Hongqili, and Hengmen) on the west side of the upper estuary, discharging freshwater of 6.83 × 1010 m3 into the NSCS every year [14]. The eastern side, however, has two deep channels, along which the coastal sea water can flood into [15]. The various water masses with complex properties make it difficult to interpret the CDOM variations in the estuary.

Furthermore, as China's second largest river in terms of water discharge, the Pearl River has an annual mean discharge of 10,000 m3 s<sup>−</sup><sup>1</sup> [15–17]. A significant seasonal variation exists in the Pearl River discharge. Therefore, the optical properties of the estuarine water can be strongly influenced by river discharge, especially during the wet season [15], which leads to distinctive characteristics when interpreting and quantifying CDOM variations in the PRE in different seasons.

Studies on CDOM absorption in the PRE last for decades. Previous studies of CDOM optical properties have covered the entire estuary in different seasons of the year, but detailed picture of CDOM variation in the estuary remains poorly understood, the dynamics of CDOM optical properties under control of multiple

**Figure 1.** *Location of Lingdingyang Estuary, the major part of the Pearl River Estuary.*

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

hydrodynamic and biogeochemical processes are not yet to be revealed satisfactorily, and the ocean color algorithms for CDOM optical property retrieval with high reliability are still needed to be developed, as well as the CDOM ocean color products with high spatial and temporal resolutions. Therefore, a comprehensive investigation of CDOM optical properties synthesizing advantages of in-situ observations and ocean color interpretations is still in necessity.

The aim of this research is to enhance the understanding of the spatial and temporal variations of CDOM optical properties in the PRE through analyzing in-situ measurements and interpreting satellite ocean color observations. The horizontal and vertical variations of CDOM absorption in the PRE are depicted based on dense and detailed in-situ observations and the dominant driven forces affecting the variations are discriminated. Spatial and temporal variations of CDOM optical properties in the PRE are analyzed based on the CDOM products derived from multi-source ocean color data with complemented spatial and temporal resolutions.
