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-mu-g liter-1 and accounted for 0.002-0.13% 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 (> 31 parts per thousand) 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 0.14-1.0% of the DOC. A simple mixing model which assumes no biological or physical sinks of lignin-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 3-18% of mid- to outer-shelf DOC is of terrestrial origin.