Characterization of Optical Attenuation by Colored Dissolved Organic Matter (CDOM) in the Red Sea

Optical properties of colored dissolved organic matter (CDOM) control the downward irradiance in the ultraviolet and visible range of the electromagnetic radiation. CDOM is the strong absorber for shorter wavelengths (ultraviolet light) with a steep spectral slope in the open ocean. Despite the importance of CDOM in understanding physical and biogeochemical processes in the marine environment, in situ measurements of optical properties in the Red Sea are sparse. This study assesses the variations in optical properties of CDOM in the Red Sea using data collected during in 2014 and 2015. Absorbance of filtered seawater samples was measured between 280 to 800 nm at 1nm intervals. Spectral remote sensing (R_rs), optical, and biophysical properties were obtained from HyperPro data.

This study provides the first characterizion of in situ CDOM absorption and its vertical distribution in the Red Sea. The spectral slope of CDOM absorption within the range of 300-650 nm was determined by least square curve fitting applied to 23 Red Sea samples. S_cdom varied between 0.011 to 0.030 nm^-1 with an average slope of 0.018 nm^-1 (std. dev. 0.005 nm^-1). However, there was a strong inverse correlation between S and a_cdom(443) (Pearson correlation coefficient, R² = 0.94).

We applied three standard global inversion algorithms (Linear Matrix [LM], Garver-Siegel-Maritorena [GSM], and Quasi Analytical Algorithm [QAA]) to recent data collected in the Red Sea, providing comparison of R_rs inverted values of the CDOM absorption coefficients (a_cdom) with in situ CDOM absorption coefficients (a_cdom). Comparison at five key selected wavelengths (412, 443, 490, 510, and 555 nm) demonstrated that in situ a_cdom values were higher than predicted values from the three inversion algorithms underestimating in situ measurements, consistent with the conclusion of Brewin et al. (2015) that overestimation of chlorophyll in the Red Sea could be due to excessive CDOM. A fixed slope value or its varying values for the global inversion algorithms may bias the retrieval of CDOM information for the Red Sea owing to the absence of region-specific algorithms. Further studies are required to improve our understanding of the sources and sinks contributing to high CDOM concentrations in the Red Sea.