Analysis of the optical properties of the Orinoco River plume by absorption and fluorescence spectroscopy

The discharge of the Orinoco River significantly affects the optical properties of the water in the Caribbean Sea by increasing primary productivity and introducing large amounts of colored dissolved organic matter (CDOM) to the region. The optical characteristics of the CDOM in the Orinoco River plume were investigated during two cruises to the eastern Caribbean using absorption and fluorescence spectroscopy. We found that high concentrations of dissolved organic carbon (DOC) and abundance of CDOM in the eastern Caribbean were related to the Orinoco River plume. We did not observe changes in optical properties of CDOM in waters with salinities lower than 30. Self-shading from sunlight and the preponderance of riverine material at these low salinities can explain the observed behavior. Changes in the optical properties of the CDOM were observed in stations with salinities higher than 30 and were related to mixing between riverine and marine end-members, and to photodegradation. Light penetration and, consequently, the position of the chlorophyll maxima in areas close to the Gulf of Paria were controlled by the concentration of CDOM. Of the two main fluorophores found in this study, only the humic fluorophore appeared to be related to the absorption of light by CDOM in the visible spectrum. A secondary fluorophore, with excitation and emission in the UV, appeared to be resistant to photodegradation and did not show any relationship with absorbance in the visible range.