| Variation in the shape of solar absorption (Fraunhofer) lines are used to study the inelastic scattering in natural waters. In addition, oxygen absorption lines near 689nm are used to study the solar stimulated chlorophyll fluorescence. The prototype Oceanic Fraunhofer Line Discriminator (OFLD) has been further developed and improved by using a well protected fiber optic - wire conductor cable and underwater electronic housing. A Monte-Carlo code and a simple code have been modified to simulate the Raman scattering, DOM fluorescence and chlorophyll fluorescence. A series of in situ measurements have been conducted in clear ocean waters in the Florida Straits, in the turbid waters of Florida Bay, and in the vicinity of a coral reef in the Dry Tortugas. By comparing the reduced data with the model simulation results, the Raman scattering coefficient, {dollar}bsb{lcub}rm r{rcub}{dollar} with an excitation wavelength at 488nm, has been verified to be 2.6 {dollar}times{dollar} 10{dollar}sp{lcub}-4{rcub}{dollar}m{dollar}sp{lcub}-1{rcub}{dollar} (Marshall and Smith, 1990), as opposed to 14.4 {dollar}times{dollar} 10{dollar}sp{lcub}-4{rcub}{dollar}m{dollar}sp{lcub}-1{rcub}{dollar} (Slusher and Derr, 1975). The wavelength dependence of {dollar}bsb{lcub}rm r{rcub}{dollar} cannot be accurately determined from the data set as the reported values ({dollar}lambdasb{lcub}rm m{rcub}sp{lcub}-4{rcub}{dollar} to {dollar}lambdasb{lcub}rm m{rcub}sp{lcub}-5{rcub}{dollar}) have an insignificant effect in the natural underwater light field. Generally, in clear water, the percentage of inelastic scattered light in the total light field at {dollar}lambda {dollar} 510nm. At low concentrations ({dollar}asb{lcub}rm y{rcub}(lambda{dollar} = 380nm) less than 0.1m{dollar}sp{lcub}-1{rcub}{dollar}), DOM fluorescence plays a small role in the inelastic light field. However, chlorophyll fluorescence is much stronger than Raman scattering at 685nm. In shallow waters where a sea bottom affects the ambient light field, inelastic light is negligible for the whole visible band. Since Raman scattering is now well characterized, the new OFLD can be used to measure the solar stimulated in situ fluorescence. As a result, the fluorescence signals of various bottom surfaces, from coral to macrophytes, have been measured and have been found to vary with time possibly due to nonphotochemical quenching and photoinhibition. |
