THE FATE OF TERRIGENOUS DOM IN MARINE SYSTEMS: WHAT CAN WE LEARN FROM DISSOLVED LIGNIN PHENOLS?

Terrigenous dissolved organic matter (DOM) discharged into marine systems is quantitatively significant (0.25 Pg yr^-1) while heavily degraded by processes in soils and rivers. However, all evidence indicates that terrigenous DOM is efficiently degraded in marine systems and comprises only a small fraction of oceanic DOM. This presentation highlights the use of lignin phenols as a tracer of terrigenous DOM to explore photochemical and microbial degradation of DOM in the Mississippi River plume. Field and experimental data were collected during a cruise in the northern Gulf of Mexico and Mississippi River plume to quantify rates and extent of dissolved lignin removal and accompanying compositional transformations. Lignin analyses were conducted on solid-phase extractions of DOM (<0.2 µm pore size) as well as DOM that was size-fractionated by ultrafiltration into high-molecular-weight (>1 kDalton) and low-molecular-weight (<1 kDalton) components. At salinities <25 psu, the primary controls on lignin concentrations were flocculation and microbial degradation. At salinities >25 psu, decreased turbidity lead to significantly greater light penetration and the dominant control of photooxidation on lignin compositions and concentrations. Photooxidation effects include a sharp decrease in the percentage of lignin in the HMW size fraction, changes in ratios of syringyl to vanillyl phenols, and increases in LMW acid:aldehyde ratios for both vanillyl and syringyl phenols. Microbial degradation rates of dissolved lignin were measured at 30% of photooxidation rates in surface waters during a 10-d incubation experiment with plume water, indicating the importance of both processes in the fate of terrigenous DOM in coastal waters. Data from in situ and experimental samples indicated that lignin composition was not significantly altered by either flocculation or microbial degradation, suggesting that composition is primarily determined by source and photochemical transformation.