Data from: Dissolved organic carbon and chromophoric dissolved organic matter in blackwater and non-blackwater streams across South Carolina, USA

The term “blackwater” is commonly used to describe low-gradient rivers and streams with characteristically clear, but darkly stained water due to naturally high concentrations of Chromophoric Dissolved Organic Matter (CDOM). Blackwaters are found in many ecoregions throughout the world but are especially prevalent in the coastal plains of the southeastern USA. A management challenge in protecting blackwater water quality is that natural conditions often lead to contravention of water quality standards based on regulatory criteria adopted for freshwaters more broadly. Addressing this challenge requires the objective differentiation of blackwater from non-blackwater systems. This study explored use of dissolved organic carbon (DOC) and CDOM for developing a quantitative blackwater definition. Across South Carolina, USA, 32 streams (17 presumed blackwater and 15 presumed non-blackwater) were sampled bimonthly during 2024 for direct measures of DOC concentration and CDOM optical properties determined by UV-Visible absorption coefficient (Abs) measurements. On average, DOC and Abs were significantly higher at blackwater sites. Across all sites, there was a single highly predictive (r2 = 0.977) relationship between DOC and Abs at 254 nm. Results suggest that the 90th percentile of the distribution in Abs at 254 nm measured in non-blackwater sites could be used as a defining threshold to predict blackwater conditions in the majority of cases. While questions remain concerning specific drivers of DOC and Abs variability within sites, results encourage using simple absorbance measurements in development of a classification systems for characterizing blackwater rivers and streams in the Southeast U.S. and potentially elsewhere.