Urbanization Impacts Dissolved Organic Matter Concentration and Quality in a Southeastern United States Watershed

Blackwater rivers are named due to their exceptionally high concentrations of chromophoric dissolved organic matter (CDOM). They are the predominant lotic ecosystem in the United States Southeastern Coastal Plain, a region experiencing some of the nation’s highest rates of development. This study assessed variability in DOM concentration and composition across forested to urbanized blackwater systems in coastal South Carolina, U.S. Dissolved organic carbon and nutrient concentrations as well as absorbance and fluorescence optical properties reveal that urban sites have lower concentrations, elemental ratios, and less complex DOM. In contrast, forested blackwater sites have concentrations an order of magnitude higher, elevated elemental ratios, and molecular size dominated by refractory terrestrial-like DOM. Urban blackwater rivers were observed to have DOM concentrations and composition more similar to brown water systems than rural blackwater systems. These findings suggest that the urbanization of blackwater ecosystems results in lower concentrations and the export of simpler, more labile DOM, potentially lowering dissolved oxygen concentrations, increasing atmospheric carbon emissions and other negative impacts. To protect blackwater systems, baseline DOM concentrations and composition must be established to decipher impacts on water quality due to naturally occurring versus anthropogenic activities and to properly assign classifications to these diverse systems across the U.S.