Sun-stimulated chlorophyll <i>a</i> fluorescence has been measured <i>in situ</i>, within the upward and downward light fields, in oceanic waters with chlorophyll concentrations of 0.04–3 mg m<sup>−3</sup>. We combined these signals with phytoplankton absorption spectra to derive the fluorescence quantum yield, φ (number of photons emitted by fluorescence/number of absorbed photons). φ was derived separately from hyperspectral (upward and downward) irradiance measurements (with a LI-COR Instruments spectroradiometer) and from nadir radiance near 683 nm (with a Biospherical Instruments profiler). The contribution of inelastic Raman scattering to the signal in the red band was assessed and subtracted. Raman-corrected φ values derived from the two instruments compared well. Vertical φ profiles were strongly structured, with maximal (5–6%) values at depth, whereas φ was ≅1% in near-surface waters (measurements made approximately at solar noon). These near-surface values are needed for interpretation of remotely sensed fluorescence signals. This optical study shows that the fluorescence yield of algae in their natural environment can be accurately derived in a nonintrusive way with available instrumentation and adequate protocols.
© 2000 Optical Society of America
Stéphane Maritorena, André Morel, and Bernard Gentili, "Determination of the Fluorescence Quantum Yield by Oceanic Phytoplankton in Their Natural Habitat," Appl. Opt. 39, 6725-6737 (2000)