Measurement of Solar-stimulated Fluorescence in Natural Waters

The oceanic Fraunhofer line discriminator (OFLD), designed to measure the solar -stimulated inelastic scattering in the ocean, has been deployed in various types of water in Florida Bay and the Dry Tortugas to measure Fraunhofer lines and oxygen-absorption lines near 689 nm in the solar spectrum. The line-filling principle and previous work enable us to partition the measured light into elastic, Raman scattering, and fluorescence components. We show that in optically deep, oligotrophic water, where chlorophyll a (Chl a) concentration is as low as 0.1 mg m ⁻³ , fluorescence near 689 nm was still measurable by the OFLD. In moderately eutrophic shallow waters, where Chl a concentration ranges from 0.2 to 0.8 mg m ⁻³ , the fluorescence from either Chl a or dissolved organic matter in the water column was found to be a negligible component of the total light field due to the additional light reflected from the bottom. We also include measurements of the solar -stimulated fluorescence for benthic surfaces, such as brain coral, and have found these to be saturated under normal solar illumination.