An experimental study of the flocculation of colored dissolved organic matter from the Great Dismal Swamp during estuarine mixing

Bio-optical properties of colored dissolved organic matter (CDOM) is critical for change detection and ship wake detection in coastal waters. A systematic mixing experiment of Great Dismal Swamp (GDS) water and the North Atlantic seawater was conducted to study the flocculation of CDOM in estuary. CDOM concentration was determined by absorption spectrum in ultraviolet (UV) and visible wavelengths with a spectrophotometer. Temporal variation of absorbance was monitored initially using 50% seawater and 50% GDS water to determine the speed of flocculation and the time needed to complete the subsequent experiment. Absorbance of filtered mixtures of different fractions of GDS water and North Atlantic seawater showed a maximum at 202 nm. In the far ultraviolet spectrum, the absorbance decreased as wavelength decreased from 202 nm to 190 nm. In the middle and near ultraviolet and visible spectrum, the absorbance decreased as wavelength increased from 202 nm to 600 nm. The later decrease trend in CDOM absorbance is similar to those reported for other estuarine and coastal waters and follows an excellent log linear trend with wavelength. The slope of the log linear regression was relatively constant for seawater concentration of less than 50% and decreased from -0.0137 at 50% to -0.0171 at 90% seawater implying that the major part of CDOM removal or modification likely occur in high salinity regions of estuarine mixing. In contrary to a reported study conducted in Orinoco River estuary, these experiment indicates that CDOM is different in high salinity mixtures and is extensively modified in these waters.