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 concentrations of 0.04-3 mg m(-3). We combined these signals with phytoplankton absorption spectra to derive the fluorescence quantum yield, phi (number of photons emitted by fluorescence/number of absorbed photons). phi 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 phi values derived from the two instruments compared well. Vertical phi profiles were strongly structured, with maximal (5-6%) values at depth, whereas phi was approximately =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.