Dissolved humic substances of vascular plant origin in a coastal marine environment

Abstract

Bacterial decomposition of vascular plant detritus in coastal wetlands results in the conversion of particulate organic matter to dissolved form and causes the release of humic substances into the bulk dissolved organic carbon (DOC) pool. We found that 34% of the DOC accumulating during degradation of salt marsh grass (Spartina alterniflora) from coastal wetlands of the southeastern U.S. fits the definition of humic substances and that lignin is the primary source of the dissolved humic substances (66% of the total). Although marine bacterioplankton used the lignin‐rich humic substances more slowly and less efficiently than other components of the DOC pool, a significant fraction (24%) of these substances was mineralized within 7 weeks. Concentrations of vascular plant biomarkers (lignin phenols) indicate that 11–75% of the dissolved humic substances on the southeastern U.S. continental shelf is from vascular plant‐dominated environments. Calculations indicate that about half this material is contributed by the coastal salt marshes and half by river export. Vascular plant influence was lower in the bulk DOC pool (2–38%), indicating that terrestrially derived material is harbored preferentially in the humic substances subcomponent of marine DOC.