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Huynh, Cuong T ¹ ; Mladenov, Natalie ²

¹ University of babyÖ±²¥app at Boulder
² University of babyÖ±²¥app at Boulder

The aquatic and terrestrial plants of the Okavango Delta are important contributors to the pool of dissolved organic matter (DOM) and have an important role in biogeochemical cycling in this large tropical wetland system (Figure 1; Mladenov et al., 2005). As the DOM flows across the length of the Delta (approximately 400 km), we hypothesize that sunlight essentially degrades the dissolved organic compounds derived from vegetation allowing it to be more readily available for uptake by microbes and other plant life. In order to evaluate the effects of ultra-violet (UV) light on the spectroscopic (fluorescence and UV-vis absorbance) properties of plant-derived DOM from this wetland, we exposed fulvic acids, isolated from the Okavango Delta, to UV light in the laboratory using a solar simulator. The two fulvic acids examined for this research study were collected from 1) a leaching Cyperus papyrus stand in the upstream Permanent Swamp of Okavango Delta (CPLFA) and 2) a channel in the Seasonal Swamp (SSFA), a seasonally-flooded hydrotone downstream that contains DOM derived from a greater variety of vegetation.

The effects of UV light on DOM are believed to result in a breakdown of the aromatic compounds in DOM, reflected in lower absorbance values. Also, previous research has shown that the fluorescence index (FI), a ratio of fluorescence intensities measured at an excitation of 370 nm (McKnight et al., 2001), generally decreases over time of irradiation by UV light (Mladenov, unpublished). During the four hours of irradiation in the experiments of this study, we did not see a decrease in the UV absorbance in either the SSFA or CPLFA samples (Table 1). However, a decrease in the FI was observed (Table 1). Figure 2 shows that the dominant peaks in three-dimensional excitations and emissions matrices (EEMs) did diminish with time of irradiation. We also used parallel factor analysis (PARAFAC; Cory and McKnight, 2005) to fit our EEMs to a model (Cory and McKnight, 2005) of thirteen components responsible for fluorescence, consisting of oxidized and reduced quinones, proteins, and unknown compounds. Model results showed that all fluorescence components decreased evenly over time and that the sum of all components was reduced by almost half after four hours of irradiation. These findings suggest that compounds responsible for fluorescence are intensely degraded by UV light, while the aromatic nature of the DOM may be less affected than previously believed. Additional measurements of dissolved organic carbon concentrations (DOC) are needed to examine UV light effects on both DOC content and specific UV absorbance. We conclude that these changes happen relatively quickly after exposure of DOM to UV light, which may have important implications for biogeochemical cycling in the Okavango Delta.

Cory, R.M. and D.M. McKnight, 2005. Fluorescence spectroscopy reveals ubiquitous presence of oxidized and reduced quinones in dissolved organic matter. Environmental Science and Technology. 39: 8142-8149.

McKnight, D. M., E. W. Boyer, P. K. Westerhoff, P. T. Doran, T. Kulbe, and D. T. Andersen, 2001. Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity. Limnology and Oceanography. 46: 38-48.

Mladenov, N., D.M. McKnight, P. Wolski, L. Ramberg, 2005. Effects of annual flooding on dissolved organic carbon dynamics within a pristine wetland, the Okavango Delta of Botswana. Wetlands. 25 (3): 622-638.