Abundance and properties of dissolved organic matter in pore waters of a freshwater wetland

The abundances, molecular weights, molar absorptivities, and polydispersities of sediment pore water dissolved organic matter (DOM) and ferrous iron, Fe(II), were measured from three sites within a freshwater wetland (Old Woman Creek) adjacent to Lake Erie. DOM concentrations (expressed as organic carbon) increased with depth. Number-average molecular weight, weight-average molecular weight, and molar absorptivities (at 280 nm) also increased with depth up to 5 cm and remained relatively constant below this depth. These properties are indicative of the respective changes in the size of the DOM constituents and the number of aromatic moieties present. Moreover, a strong correlation was observed between Fe(II) and DOM, which suggests that both constituents coaccumulate in these pore fluids. It is hypothesized that reduction of iron oxides coated with organic matter releases both DOM and Fe(II). Increases in the molecular weights and molar absorptivities of the DOM constituents suggest that the "sorbed" component released by iron oxide dissolution is composed of larger and more aromatic molecules. The molecular-weight data also suggest that much of the refractory organic carbon in the interstitial waters comprises smaller-than-expected polymeric molecules. Thus, the release of the sorbed organic materials during the reductive dissolution of the oxide phases in anoxic sediments coupled with the selective preservation of refractory organic components may play an important role in both the DOM-mediated fate of pollutants and the cycling of carbon in wetlands.