Distribution of Terrestrially Derived Dissolved Organic Matter on the Southeastern United States Continental Shelf

Dissolved lignin-derived compounds in seawater indicate the presence of organic matter originating from vascular plants and therefore from terrestrial (upland and coastal marsh) ecosystems. We used a hydrophobic resin to concentrate lignin-rich humic substances and to determine concentrations of lignin oxidation products (vanillyl lignin phenols) for waters of the continental shelf of the southeastern U.S. Lignin phenol concentrations ranged from 0.05 to 4.2 pg liter-’ and accounted for 0.002-O. 13% of the total dissolved organic carbon (DOC) pool in continental shelf waters. Dissolved lignin concentrations were generally highest near the shore and in those areas receiving greatest river and marsh discharge. Concentrations varied on both short-term (weekly) and seasonal time scales, however, indicating that the contribution of terrestrially derived dissolved organic matter to the C budget of the shelf is quite variable. Salinity (>~~YcQ) was significantly correlated (negatively) with lignin phenol concentrations during three of four cruises, suggesting largely conservative mixing of lignin-derived material on the shelf. In selected rivers and salt marshes contributing terrestrially derived organic matter to the continental shelf, lignin phenol C accounted for 0.14-l .O% of the DOC. A simple mixing model which assumes no biological or physical sinks oflignin-derived material during transport from terrestrial sources to the shelf predicts that an average of 6-36% of nearshore DOC derives from terrestrial ecosystems, depending on whether the terrestrial end-member (lignin source) is assumed to be a river or a salt marsh. while 5-26% of inner shelf DOC and 3-l 8% of midto outer-shelf DOC is of terrestrial origin. The importance of riverineand saltmarsh-derived organic matter to the carbon and energy budgets of coastal waters has been a focus of marine ecological research Acknowledgments We thank Laura Pittman and Georgia Zeigler for laboratory and held technical assistance. We appreciate reviews and discussions of this work provided by John Ertel and Chuck Hopkinson and advice in development of lignin phenol analysis techniques from Ronald Benner. This research was funded by grant NASOAA-DO009 1 from the NOAA ORice of Sea Grant, grant GCE 8718019 from the National Science Foundation, and grant DE-FG09-86ER60451 from the Department of Energy. for the past 30 yr. Questions regarding the magnitude and ecological significance of organic matter subsidies to the near-shore are particularly relevant for the southeastern coast of the U.S., where extensive salt-marsh and river systems empty into coastal waters, setting up turbid plumes visible many kilometers offshore (Blanton 1980). The phenomenon of “outwelling” from such marshes was examined initially by Teal (1962) and Odum and de la Cruz (1967) and more recently by Haines (1977) Peterson and Howarth (1987) and others. On the basis of energy budgets, C flux measurements, and stable isotope ratios, these studies implicated marsh detritus as an important, al-