Sources and composition of high-molecular-weight dissolved organic carbon in a southern Louisiana tidal stream (Bayou Trepagnier)

The composition and cycling of high-molecular-weight dissolved organic carbon (HMW-DOC) were examined in a tidal stream (Bayou Trepagnier) with seasonally high DOC concentrations (1.0–5.6 mM). 13C nuclear magnetic resonance (13C NMR) was used to examine the bulk chemical composition of natural HMW-DOC from two field sites over 1.5 yr. The HMW-DOC at both sites was dominated by aliphatic (41%), carbohydrate (33%), and carboxyl (16%) carbon, with relatively low aromatic carbon abundance (10%). A comparison of 13C NMR signatures of bayou HMW-DOC and leachate HMW-DOC collected from leaf litter and soils revealed that plant litter leachate appears to be a more important source of HMW-DOM than soil. Dominant sources of HMW-DOC were likely allochthonous inputs of terrestrial plant litter with periodic inputs of soil organic matter during flooding events. The low aromaticity of bayou HMW-DOC may reflect the influence of low-oxygen conditions, which inhibit the decomposition of particulate aromatic macromolecules such as lignin and humic material to HMW-DOC. Ligninphenol biomarker concentrations (L6, in mg/ 100 mg OC) were much lower in HMW-DOC (1.2) than in plant (5.2) and soil (6.8) organic matter, indicating that a significant fraction of this highly aromatic material was not degraded to HMW-DOC. Finally, this study demonstrated that lignin and other compounds from terrestrially derived organic matter in sediments and adjacent soils are not a significant source of more soluble moieties that enter the HMWDOC pool of the bayou. In both freshwater and marine systems, a significant fraction of total dissolved organic carbon (DOC) is composed of high-molecular-weight DOC (HMW-DOC) or colloidal organic carbon. The composition of HMW-DOC is important to nutrient cycling and aquatic chemistry, because this fraction contains substantial amounts of potential bacterial substrates and influences physicochemical characteristics through light attenuation, pH buffering, and metal complexation (McKnight et al. 1994; Battin 1998). Despite the abundance of HMW-DOC in inland waters, most studies advancing our understanding of HMW-DOC cycling have been conducted in the open ocean or large estuaries. In those systems, HMW-DOC appears to be derived mainly from direct exudation by phytoplankton and is rich in polysaccharides, pigments, lipids, and nucleic acids (Bianchi et al. 1995; Biddanda and Benner 1997). In freshwater systems, humic substances derived from diagenesis of structural materials from soils and higher plants are also important contributors to HMW-DOC (Frimmel and Christman 1988; Gustafsson and Gschwend 1997). Relatively little is known about HMW-DOC cycling in coastal wetlands, although wetlands have high DOC concentrations and could be important HMW-DOC sources to es1 Corresponding author.