Calculation of solar‐induced fluorescence in surface and subsurface waters

Solar‐induced chlorophyll a fluorescence can be observed in up welling irradiance spectra, but two major corrections are necessary before the signal may be used to detect phytoplankton pigment or primary production. A two‐component correction model was developed for use in both surface and subsurface waters. The first component separates up welling irradiance resulting from fluorescence from total up welling irradiance using a simple linear approximation between two wavelengths on either side of the chlorophyll a emission spectrum. Fluorescence accounted for 10 to 40% of total upwelled irradiance at the surface and up to 80% in subsurface waters. The second component corrects for attenuation of irradiance between the phytoplankton source and the detector using attenuation terms that incorporate the angular distributions of solar excitation and isotropic fluorescence emission irradiances. Results from our model compare favorably with models rigorously derived for surface application, and ‐ because it is not based on the single‐scattering assumption, ‐ our model also can be used in subsurface waters. The diverse water masses sampled (fjord, coastal, and open ocean) belonged to a single optical type, according to a classification using K d (490) and a phy (490) (rather than chlorophyll a ); these results indicate that our model can be used in diverse environments to extract a corrected fluorescence signal from upwelling irradiance spectra.