Molecular indicators of the sources and transformations of dissolved organic matter in the Mississippi river plume

The dynamics of dissolved organic matter (DOM) in the Mississippi river plume were investigated during four cruises to the region in 1990–1993. During each cruise, large-volume water samples were collected from a broad salinity gradient (0–36 psu) for the determination of dissolved organic carbon (DOC) concentrations and the isolation of DOM by tangential-flow ultrafiltration. The fraction of DOC recovered by ultrafiltration ranged from 49% in the river to 22% in the Gulf of Mexico. The ultrafiltered DOM (UDOM) was further characterized for the concentrations and compositions of combined neutral sugars and lignin-derived phenols. Concentrations of DOC ranged from 359 μM in the river during the summer to 65 μM in surface waters of the Gulf of Mexico during the winter. Mixing curves indicated minor losses of DOC at low salinities and a major source of DOC at mid salinities. Combined neutral sugars had elevated concentrations at mid salinities where plankton biomass and rates of autotrophic and heterotrophic processes were maximal, indicating that elevated concentrations of sugars and DOC were freshly-derived from plankton. In contrast, concentrations of lignin phenols in UDOM were depleted at low salinities, indicating losses due to flocculation of terrestrially-derived humic substances. The selective removal of syringyl phenols from dissolved lignin was observed in UDOM from higher salinity (>25 psu) waters, providing the first molecular evidence of the removal of terrigenous DOM from the coastal ocean due to photooxidation. Together, these molecular biomarkers indicated much greater dynamic behavior of DOM than was apparent from bulk DOC analyses, and they demonstrated that biological as well as physico-chemical processes were important in controlling the concentrations and compositions of DOM in coastal waters.