Determination of the fluorescence quantum yield by oceanic phytoplankton in their natural habitat

Sun-stimulated chlorophyll a fluorescence has been measured in situ, within the upward and downward light fields, in oceanic waters with chlorophyll c oncentrations of 0.04-3 mg m(-3). We combined these signals with phytoplank ton absorption spectra to derive the fluorescence quantum yield, phi (numbe r of photons emitted by fluorescence/number of absorbed photons). phi was d erived separately from hyperspectral (upward and downward) irradiance measu rements (with a LI-COR Instruments spectroradiometer) and fi om nadir radia nce near 683 nn (with a Biospherical Instruments profiler). The contributio n of inelastic Raman scattering to the signal in the red band was assessed and subtracted. Raman-corrected phi values derived from the two instruments compared well. Vertical phi profiles were strongly structured, with maxima l (5-6%) values at depth, whereas phi was congruent to1% in near-surface wa ters (measurements made approximately at solar noon). These near-surface va lues are needed for interpretation of remotely sensed fluorescence signals. This optical study shows that the fluorescence yield of algae in their nat ural environment can be accurately derived in a nonintrusive way with avail able instrumentation and adequate protocols. (C) 2000 Optical Society of Am erica OCIS codes: 010.4450, 260.2510.