Dissolved Organic Matter Biogeochemistry Along a Transect of the Okavango Delta, Botswana

Biogeochemical processing of dissolved organic matter (DOM) in aquatic environments can alter its chemical quality and its bioavailability to the microbial loop. In this study, we evaluated the relative importance to DOM character of allochthonous and autochthonous DOM inputs and photo-degradation in a large, pristine wetland, the Okavango Delta of Botswana. We performed an intensive spatial sampling of surface water and analyzed for chemical and physical parameters (pH, conductivity, dissolved oxygen saturation, temperature, and channel depth), dissolved organic matter (DOM), and particulate organic matter (POM). We used UV–vis absorbance, fluorescence spectroscopy, and parallel factor analysis of excitation emission matrix data (EEM-PARAFAC) to characterize DOM. Our findings from principal component analysis (PCA) show downstream changes in DOM chemistry to be dominated by photo-degradation, suggesting that DOM in the Okavango Delta is transformed photo-chemically in shallower downstream reaches after being mobilized from the permanent swamp and seasonal floodplains. Additionally, we found that the PARAFAC model developed for the Everglades, a large, anthropogenically-altered wetland in North America, was well suited to tracking DOM dynamics in the Okavango Delta and may be useful for characterizing DOM in other sub-tropical, seasonally flooded wetlands.