Optical properties of chromophoric dissolved organic matter along a transect in the Barataria Bay, Louisiana

Optical properties of chromophoric dissolved organic matter or CDOM were studied in the Barataria basin from field samples collected on 26 September 2007 from stations located in the lower and upper Barataria bay influenced by marine waters to the upper basin influenced by freshwater sources. CDOM absorption at 412 nm acdom(412) increased seven-fold from 0.74 m⁻¹ near the Barataria Pass to about 5.2 m⁻¹ at locations influenced by freshwater sources in the upper basin. Although CDOM absorption generally increased with decreasing salinity (in salinity range 23.26 to 1.03), nonlinearity at the mid-salinity range suggested additional CDOM sources in the bay. At the freshwater end (salinity between 0.12–0.38) CDOM absorption varied between 3.4 to 5.2 m⁻¹ suggesting the various streams and small lakes to have varied concentrations of CDOM possibly contributing to the large CDOM variability. CDOM spectral slope coefficient S (nm⁻¹) provides a measure of the decrease of absorption with increasing wavelength and has been used as an indicator of water types. S varied from a low of 0.0154 nm⁻¹ at a station near the Barataria Pass to a high of 0.0176 nm⁻¹ at one of the freshwater sources upstream in the bay with S increasing gradually with decreasing salinity suggesting a strong freshwater influence on CDOM spectral slope. However, it was observed that below the 1.03 salinity level values of S were between 0.0164–0.0176 nm⁻¹ and variable indicating that the different freshwater sources to the Barataria bay to potentially have different CDOM spectral characteristics.