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, 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.
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