Measurement of solar‐stimulated fluorescence in natural waters

Abstract

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.