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A bio-optical model of chromatic dissolved organic matter in Lake Taihu, China
A linear correlation between absorbance at 350nm and concentration and fluorescence of chromatic dissolved organic matter in lake waters promises more accurate measurements.
20 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0611
Bio-optical models of chromatic dissolved organic matter (CDOM) describe the relative contributions of various factors to light attenuation in a given body of water. These models have been studied mostly in inland and coastal waters. We are extending work in this area by developing a bio-optical model for CDOM in Lake Taihu, a large, shallow lake.
CDOM absorbs light in both the ultraviolet and visible ranges,1 and different relationships between absorbance and fluorescence intensity have been demonstrated in the literature. Hoge et al.2 have shown a robust linear relationship between CDOM absorption and fluorescence in coastal and open ocean regions. Nieke et al.3 showed that this relationship also pertains to the St. Lawrence estuary, an environment similar to that of the open ocean. In the Baltic sea, Ferrari and Dowell4 concluded that the relationship between CDOM absorption and fluorescence was linear if self-absorption corrections were applied for the very high absorptions observed there (5m-1). Breves et al.5 found only a weak covariance of dissolved organic matter absorption and fluorescence data for the upper 100m of the Arabian Sea at the onset of the southwest monsoon.
We have carefully studied the effect of CDOM on attenuation of blue and ultraviolet in Lake Chao and Lake Longgan.6 In the present study, we provide information on light absorbance, fluorescence, and distribution of CDOM in Lake Taihu. Our objective was to examine the relationship between light absorption and fluorescence due to CDOM in Lake Taihu, and to investigate a regional bio-optical model of CDOM absorbance and fluorescence.
CDOM can emit fluorescence when excited by ultraviolet radiation (see Figure 1). We observed a peak at 430–450nm. The absorbance at 350nm—a(350)—is related to the fluorescence intensity. Specifically, fluorescence increased with increasing a(350), with relatively weak covariance between them (see Figure 2). The covariance can be described by the equation
For this equation, r2=0.498, SD=0.2659, n=215, and P>0.0001. For the waters of Lake Taihu, we calculated the slope value, S, comparing the absorbance at 350nm, a(λ0), and the absorbance at 440nm, a(λ). The resulting value of S is 0.0046nm-1 on average (SD=0.0015, n=284), with a range from 0.0004 to 0.0081nm-1. For comparison, the S value in the shallow waters of Lake Taihu was far below the value reported in the literature. The S value varies greatly with samples from different sources.
Figure 1. The shape of emission fluorescence excited by radiation of 341nm. The first and second peaks correspond to Raman fluorescence, and the third peak represents dissolved organic carbon fluorescence (DOC).
Figure 2. Comparison between a(350) and fluorescence intensity, showing the linear relationship between them. The samples with high absorbance and low fluorescence intensity were collected from the bottom.
In Lake Taihu, the high concentration of CDOM results in greater attenuation at the ultraviolet and blue end of the spectrum. The absorbance of CDOM, which decreased exponentially with increasing wavelength, had a linear relationship with concentration and fluorescence. Perhaps the high concentration and complicated composition of dissolved organic matter in a large, shallow lake such as Taihu are responsible for the nonlinear relationship between CDOM concentration and fluorescence that we also observed for some samples. The slope value, S, is far less than that reported in the literature. This may be due to the strong fluorescence emitted by high concentrations of CDOM in the area. It is also possible that a small volume of phytoplankton remained in the water sample. The good linear relationship between a(350) and CDOM concentration indicates that a(350) can be used directly to measure the concentration with greater accuracy than current methods.
Key Laboratory of Tropical Marine Environmental Dynamics (LED), South China Sea Institute of Oceanography, Chinese Academy of Sciences
Guang Zhou, China
Yang Dingtian is an associate professor at the South China Sea Institute of Oceanography, Chinese Academy of Sciences. He received his PhD from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. He has published more than 40 journal articles. In addition, he has served on several SPIE program committees, including Algorithms for Multispectral and Hyperspectral Imagery, Imaging Spectrometry, and Chemical and Biological Standoff Detection.