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Abstract: The optical properties and geochemical cycling of chromophoric dissolved organic matter (CDOM) are altered by its sorption to freshwater and estuarine sediments. Measured partition coefficients (K p) of Satilla River (Georgia) and Cape Fear River estuary (North Carolina) CDOM ranged from 19 to 233Lkg−1 when model freshwater and organic-rich estuarine sediments were added to solution (concentrations of 0.1, 1, or 10gL−1), with the largest K p values measured in solutions with the lowest sediment concentrations. Sorption of Satilla River CDOM was augmented upon raising the ionic strength of solution by mixing with natural seawater, likely due to the ‘salting out effect’ of Ca+2 and Mg+2 ions. For turbid estuarine systems with particle loads of ∼100mgL−1, we estimate that ∼1–2% of the CDOM pool sorbs to settling particles, facilitating the transfer to a potentially large colored particulate organic matter (CPOM) reservoir within sediments. Our results also indicate that >30% of the colored organic matter pool within sediment pore waters (sediment concentration>10gL−1) may exist as CPOM. Spectral slope coefficients (300–700nm) of initial CDOM samples increased as much as 20% after mixing with 10gL−1 sediment and 5% after mixing with 1gL−1 sediment indicating that sorption to particles has the potential to significantly alter the optical properties of CDOM in the water column of turbid shallow environments or in areas of high benthic exchange. [Copyright &y& Elsevier]

River input of allochthonous organic matter dominates the strong complexation capacity of dissolved copper in the organic-rich Cape Fear River (CFR) estuary, North Carolina. This slightly stratified estuary is characterized by conservatively mixed dissolved organic carbon (DOC=200–1200 μM C), high river flow, and low biological productivity. Copper speciation data measured using competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV with 8-hydroxyquinoline (8-HQ)) for seven Cape Fear estuarine transects revealed that strong (mean detection window αCu8HQ=105.2) Cu-complexing ligands range in concentration from 7 to >200 nM (at fixed KCuL′=1013.5), are conservatively mixed below the turbidity maximum zone (S∼5), and exist in substantial excess of dissolved Cu levels (3–25 nM). Strong ligand and DOC concentrations exhibited strong linear correlations among transect samples at DOC concentrations <1000 μM C (r2=0.93, p≪0.01) and among all size fractions of ultrafiltered estuarine samples (r2=0.94, p≪0.01). A 300 μM C solution of Cape Fear River humic substances isolated using C18 extraction exhibited a strong Cu ligand concentration of 143 nM (at fixed KCuL′= 1013.5). Comparison with speciation data from estuarine transects indicates C18 isolated humics could account for 100% of the typical strong Cu-complexation capacity in the Cape Fear estuary. [Copyright &y& Elsevier]

Benthic fluxes were evaluated as a source of strong Cu-complexing ligands (K′CuL>1011) to the water column of the organic-rich Cape Fear River estuary along the North Carolina coast. A core incubation technique was used to measure sediment-water exchange of strong Cu-complexing ligands, as well as total dissolved Cu and dissolved organic carbon, at two sites within the estuary. Substantial ligand fluxes were observed only during summer experiments (850±610 nmol m-2 d-1 in June 2001 at lower estuarine site and 870±580 nmol m-2 d-1 in June 2002 at upper estuarine site). Strong ligand fluxes were correlated with DOC fluxes only for the June 2002 experiment. The ratio of strong Cu ligand:DOC fluxes among all cores from the June 2002 incubation (3×10-5 mol g-1 C) was nearly equivalent to the ratio measured in the water column in an earlier study of the Cape Fear estuary (2×10-5 mol g-1 C). Fluxes of TDCu were not statistically significant during any experiment and were not related to the behavior of strong Cu ligands. Although fluxes of strong Cu ligands were large during the summer months, short estuarine residence times (<1 week) and a large ambient water column pool of ligands (30–200 nM) limit the importance of sediment-water exchange as a major source to Cape Fear estuarine waters. However, the fluxing ligands are experimentally indistinguishable from those in the water column, suggesting that benthic fluxes may provide an important source to estuaries with long residence times or lower ambient ligand levels. [Copyright &y& Elsevier]