Characteristics of Absorption Spectra of Chromophoric Dissolved Organic Matter in the Pearl River Estuary in Spring

Xia Lei,Jiayi Pan,Adam Devlin

doi:10.3390/rs11131533

Xia Lei, Jiayi Pan + Show 1 more

Open Access

https://doi.org/10.3390/rs11131533

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Abstract

In this study, absorption variation of chromophoric dissolved organic matter (CDOM) was investigated based on spectroscopic measurements of the water surface and bottom during a cruise survey on 2–12 May 2014 in the Pearl River Estuary (PRE). Multiple spectral signatures were utilized, including the absorption ratios E2/E3 (a(250)/a(365)) and E2/E4 (a(254)/a(436))) as well as the spectral slopes over multiple wavelength ranges. The horizontal variations of a(300), E2/E3, spectral slope (S) of Ultraviolet C (SUVC, 250–280 nm), Ultraviolet B (SUVB, 280–315 nm), and S275–295 (275–295 nm) were highly correlated, revealing that CDOM of terrigenous origin in the upper estuary contained chromophores of larger molecular size and weight, while the marine CDOM in the lower estuary comprised organic compounds of smaller molecular size and weight; the molecular size of surface CDOM was generally larger than that at the bottom. Results of Gaussian decomposition methods showed that CDOM in the middle estuary of terrigenous origin produced more Gaussian components per spectrum than those of marine origin in the lower estuary and the adjacent Hong Kong waters. The surface CDOM composition was more diverse than at the bottom, inferred by the finding that the average number of Gaussian components yielded per surface sample (5.44) was more than that of the bottom sample (4.8). A majority of components was centered below 350 nm, indicating that organic compounds with relatively simple structures are ubiquitous in the estuary. Components centered above 350 nm only showed high peaks at the head of the estuary, suggesting that terrigenous CDOM with chromophores in complex structures rapidly lose visible light absorptivity during its transport in the PRE. The relatively low and homogenous peak heights of the components in Hong Kong waters imply higher light stability and composition consistency of the marine CDOM compared with the terrigenous CDOM.

Highlights

Dissolved organic matter (DOM) is one of the earth’s largest exchangeable reservoirs of organic material and an important component of the marine biogeochemical system
The S275–295 and S350–400 were quantitatively close to S value over the UVB band (SUVB) and S over the UVA band (SUVA), respectively, due to the similar spectral range employed for S calculation (Table 1)
These strong correlations illustrate the consistency of absorption ratios and spectral slopes in describing the spectral shapes of CDOM absorption if they are calculated in the similar spectral range

Summary

Introduction

Dissolved organic matter (DOM) is one of the earth’s largest exchangeable reservoirs of organic material and an important component of the marine biogeochemical system. Different spectral ranges produce different spectral slopes because the CDOM absorption spectra do not exactly follow a continuous exponential decay with an increase in wavelength. This makes it difficult to compare S values across studies [6,25]. In a previous investigation [29], the mixing behavior of CDOM in the Pearl River Estuary (PRE) was examined, revealing that CDOM with different sources (terrigenous or marine) and depths (surface or bottom) have distinct mixing characteristics (conservative or nonconservative). Absorption ratios and spectral slopes over diverse wavelength ranges were used as signatures to indicate CDOM molecular information. The two ratios were expected to provide a general depiction of the horizontal and vertical variation of CDOM molecular size and weight in the estuary

Gaussian Decomposition Approach

Spectral Slopes and Absorption Ratios

Gaussian Decomposition

Conclusions