Variations in the spectral properties of freshwater and estuarine CDOM caused by partitioning onto river and estuarine sediments

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 (Kp) of Satilla River (Georgia) and Cape Fear River estuary (North Carolina) CDOM ranged from 19 to 233 L kg−1 when model freshwater and organic-rich estuarine sediments were added to solution (concentrations of 0.1, 1, or 10 g L−1), with the largest Kp 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 ∼100 mg L−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 > 10 g L−1) may exist as CPOM. Spectral slope coefficients (300-700 nm) of initial CDOM samples increased as much as 20% after mixing with 10 g L−1 sediment and 5% after mixing with 1 g L−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.