River Organic Matter Changes with the Seasons
Michael A. Greenwood
The vast amounts of dissolved organic matter that flow through the Amazon River and its network of tributaries play a significant environmental role in one of the world’s great ecosystems and beyond.
As the matter absorbs sunlight, it eventually degrades, providing a steady supply of nutrients for aquatic organisms. The photodegradation also contributes to the water’s transparency and to the output of carbon dioxide.
Figure 1. Water samples were drawn from the Negro River, a major tributary of the Amazon, to determine how exposure to UV irradiation affected organic matter within.
Researchers seeking to understand how exposure to UV radiation affects the optical properties of this matter found that it had a direct impact on the structure and behavior of the matter and that the level of photodegradation was seasonally dependent.
Investigators from three Brazilian institutions, Embrapa Agricultural Instrumentation and the University of São Paulo, both in São Carlos, and Paulista State University in Araraquara, collected samples of the organic, or humic, matter from the Negro River, a major tributary of the Amazon, in both the winter flooding season — June and July — and in the summer when the river is at its lowest level — December and January (Figure 1).
The samples were taken from about 25 cm below the surface, and their initial spectra were recorded with a Varian nuclear magnetic resonance spectrometer. Researchers led by Débora Marcondes Bastos Pereira Milori also performed fluorescence tests to acquire a satisfactory spectroscopic signal.
The samples were irradiated with a Hamamatsu mercury-xenon lamp with an emission range from 290 to 475 nm. A light intensity of 5.8 mW cm–2 was applied to the samples, equivalent to four times the solar light on a sunny day in the region.
The samples of organic matter were subjected to irradiation exposure times of 0, 60, 120, 180, 300, 660, 1500, 2100, 3000 and 5000 min. All irradiation was performed in the dark, under aerobic conditions, and with temperature and pH controls.
Figure 2. Researchers found a strong correlation between fluorescence photobleaching and the amount of irradiation exposure in samples of dissolved organic matter. The black line represents a sample that has not been irradiated. The red line shows the intensity of a sample irradiated for 5000 min. Courtesy of Ursula Fabiola Rodrìguez-Zúñiga.
As a result of irradiation, both summer and winter samples exhibited a marked loss of absorbance in the near-UV and in the visible region — 200 to 500 nm — after 48 h of exposure. For the winter sample, the absorbance loss was 79.05 percent and for the summer sample, 86.38 percent. The total organic carbon content also fell sharply in both samples as a result of exposure. Decrements of around 70 percent in both samples were observed.
In addition, the absorbance loss was the greatest in samples that had the most induced organic radical formation as a consequence of their phenolic content. The loss was most pronounced in both samples in the UV, from 200 to about 300 nm. The same experiment revealed that the organic matter exhibited different fluorescence intensities when excited with a 250-nm source. In the winter sample, the organic matter exposed to 300 min. of irradiation had the highest fluorescence intensity, peaking at 430 nm. In the summer sample, the matter exposed to 660 min. of irradiation had the strongest intensity, peaking at about the same wavelength.
Environmental Science and Technology, ASAP Edition, Feb. 7, 2008, doi: 10.1021/es702156n.
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