CHARACTERIZATION OF DISSOLVED ORGANIC CARBON IN AQUATIC SYSTEMS OF NORTHWESTERN SOUTH CAROLINA

Dissolved organic carbon (DOC) compounds play key biogeochemical roles in aquatic systems by binding and transporting transition metals, serving as energy sources for microbes, and influencing light attenuation. DOC represents a broad class of structurally complex macromolecules, which makes it difficult to characterize. The purpose of our study was to better understand the character and sources of DOC in freshwater environments in the Piedmont of northwestern South Carolina. Over 40 water samples were collected from wetlands, lakes, rivers, and headwater streams. DOC concentrations obtained from UV-persulfate digestion were combined with UV spectrum analysis (SUVA₂₅₄) to estimate percent aromaticity for the DOC in each sample. High aromaticity reflects terrestrially-derived humic acids, and low aromaticity reflects microbially-derived humic acids. Highly aromatic DOC also is more likely to react with metal oxides because the functional groups associated with aromatic compounds such as carboxylic acid ligands have been found to sorb to oxide surfaces. A spectrofluorometer also was used to calculate a fluorescence index (FI) of fulvic acid in each sample, with high FI indicating a microbial source and low FI indicating a terrestrial source. DOC concentrations ranged from 0.66 to 5.6 mg/L, lowest in headwater streams and highest in wetlands. Headwater stream DOC had high percent aromaticity (70-85%), four times greater than has been reported from large rivers. Aromaticities of river, wetland, and lake samples were lower, ranging from 22 to 44%. The FI of the samples ranged from 1.14 to 1.53, highest in wetlands and lakes and lowest in rivers and streams. The weak relationships (r² = 0.22) between DOC concentration and both aromaticity and FI suggests that concentration and composition are determined by separate processes. The high FI and low aromaticity of lake and wetland DOC suggests microbial sources dominate, whereas the low FI and high aromaticity of headwater streams suggests terrestrial sources dominate. The results also suggest that thesource of DOC changes downstream from terrestrially derived in headwater streams to increasingly microbially derived in larger rivers. Alternatively, the decline in aromaticity downstream may suggest progressive oxidation of organic carbon macromolecules.