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

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. WC found that 34% of the DOC accumulating during degradation of salt marsh grass (Spartina alternif7oru) 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 1 l-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. Solubilization of particulate organic matter by bacteria has recently been recognized as an important feature of carbon cycling in marine ecosystems and a quantitatively important route by which carbon is transferred from the particulate to the dissolved phase (Moran and Hodson 1989; Smith et al. 1992). This process appears to be especially significant in coastal salt marshes of the southeastern U.S., where as much as 40% of the carbon in Spartina alterniflora detritus is converted to stable dissolved intermediates during bacterial decomposition (Moran and Hodson 1989). The conversion of vascular plant detritus to dissolved organic matter (DOM) is significant from an ecological standpoint because it provides plant-derived carbon to populations of salt marsh bacteria other than those attached to the particulate detritus, including free-living bacterioplankton (Moran and Hodson 1989). Furthermore, plant-derived DOM is susceptible to export from the coastal salt marshes to adjacent marine environments, whereas Acknowledgments Edward Sheppard provided technical assistance in the lab and field. WC appreciate the detailed reviews of this manuscript by John Ertel, Rick Keil, and John Hedges. This research was funded by grants NA80AA-DO0091 from the NOAA Office of Sea Grant and OCE 9 116450 from the National Science Foundation. plant material in particulate form generally is not (Chalmers et al. 1985; Williams et al. 1992). The ecological role of vascular plant-derived DOM, both within coastal marshes and in marine ecosystems to which it has been exported, will depend on its relative biological lability among other factors. We know from previous studies that rates of utilization of DOM derived from Spartina detritus are highly variable: some components are rapidly assimilated by bacteria over time scales of hours to days, whereas other components have much longer turnover times (Moran and Hodson 1989). The refractory components are of particular interest because they appear to be enriched in lignin byproducts and are potential precursors of humic substances (Moran and Hodson 1990b). The refractory components are also the components most likely to persist long enough to be available for export. Indeed, the export of humified, lignin-rich DOM to marine environments has been demonstrated by the identification of lignin biomarkers in the humic substances of several coastal and oceanic systems (Meyers-Schulte and Hedges 1986; Moran et al. 199 la,b). Although the rates of turnover and the ecological roles of chemically identifiable, low molecular weight components of marine DOM have been studied for nearly three decades (Wright and Hobbie 1966; Azam and Hodson 1977), little is known about the fate of refrac-