Analysis of dissolved organic matter character and dynamics using optical techniques in prairie wetland ponds

Abstract

Carbon plays an important role in a variety of Earth’s biogeochemical processes, through its various reservoirs and the interconnections between them. The hydrosphere is an important reservoir for carbon, and inland waters have often been regarded as unimportant in carbon biogeochemical cycling. However, important transformations and transport mechanisms occur within inland waters. Wetlands are an example of this, where carbon is in flux with the atmosphere and hydrosphere. Dissolved organic carbon (DOC), the carbon fraction and majority of dissolved organic matter (DOM), is found in the earth’s aquatic environments, is an important connection in the carbon cycle between terrestrial and aquatic systems and plays a variety of biogeochemical roles. Within aquatic environments, DOM plays an intermediary role in trace metal cycling such as that of toxic mercury. DOM is highly heterogenous and can be characterized according to its structure, origin, and molecular weight. Optical techniques have emerged as a powerful tool to analyze DOM according to its origin and chemical character, and in several techniques such as derived fluorescence indices and parallel factor analysis (PARAFAC) models based on emission-excitation matrices have become common. Within Saskatchewan, Canada and located in the prairie pothole region (PPR), the St Denis National Wildlife Area is home to a series of wetlands where a more toxic form of mercury, called methylmercury (MeHg) is found in highly differing amounts within close proximity. DOM is known to influence MeHg concentrations and the process of mercury methylation. This thesis hypothesized that the differing characteristics of DOM may explain differences in high methylmercury concentrations. Through the use of optical techniques, including derived fluorescence indices and PARAFAC modelling, this thesis establishes that high MeHg concentration wetland ponds in SDNWA have more humic and terrestrially derived DOM, and low MeHg concentration wetland ponds have more aromatic DOM. This suggests that the highly aromatic DOM, combined with the high sulfur conditions of the SDNWA enable a greater amount of photodemethylation of MeHg in low MeHg concentration wetland ponds. Keywords: Dissolved organic matter, Methylmercury, Emission-excitation matrix, Parallel factor analysis, Prairie Pothole Region