Sunlight-Induced Changes in Photosensitizing Properties of Dissolved Organic Carbon in Canadian Prairie Pothole Wetland Ponds

Abstract

Dissolved organic carbon (DOC) is an important component of the global carbon cycle, acting in part as a potential source of carbon for microorganisms, support of flocculation of heavy metals and nutrients to sediments, as well as a protective barrier between ultraviolet radiation and aquatic organisms. In my thesis, chapter 1 is a literature review with general background information about the photodegradation process of DOC in Prairie Pothole Region (PPR). In chapter 2, I used an incubation experiment with light and depth as treatments to examine photodegradation impacts on the optical characterization of DOC in prairie wetland ponds at the St. Denis National Wildlife Area in central Saskatchewan, Canada. Surface water was collected from two ponds with seasonal average DOC concentrations of 71.1 mg L-1 (More-DOC Pond) and 32.7 mg L-1 (Less- DOC Pond) and filtered through 0.45μm GF/Fs into Teflon bottles that were transparent to UV light. Samples were then divided into dark and light treatments and incubated at three different depths (surface, 0.25m, and 0.5m) in situ and collected every 2 weeks for 4 months. Excitation emission matrix scans from 300nm to 800nm and absorbance at 254nm were collected on each sample. Absorbance at 254nm in More-DOC Pond and Less-DOC Pond waters decreased by 34% and 50%, respectively. We identified two fluorescence components in both ponds which corresponded to peak C and peak M in literature. Although bulk DOC concentrations did not show significant differences over time, fluorescence intensity of peak C decreased by 39% and 47%, and of peak M increased by 19% and 29%, in the More-DOC Pond and Less-DOC Pond, respectively. Increases in other fluorescence indicators such as fluorescence index (35% and 22%) and FRESH (29% and 30%) were observed in the More-DOC Pond and Less-DOC Pond, respectively. Overall, terrestrial DOC from our sampling ponds became less aromatic, more aliphatic, lower molecular weight, and lost absorptivity and fluorescence intensity after 12 weeks of incubation. Lastly, in chapter 3, I outlined the future research using absorbance and fluorescence to study DOC in multiple ponds at SDNWA seasonally as well the connection of DOC to mercury and methyl mercury in PPR. Results of this study will provide a better understanding at the optical characterizations, sources, and act as a foundation to further study on the role of DOC in methylation of mercury