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

Abstract

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. Lignin‐phenol biomarker concentrations (del6, 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 HMW‐DOC pool of the bayou.