In situ determination of the quantum yield of phytoplankton chlorophyll a fluorescence: A simple algorithm, observations, and a model

Vertical profiles of the in situ quantum yield of fluorescence of chlorophyll a, ff, were derived with an algorithm from spectral underwater radiometer measurements. Select inherent optical properties were obtained from an initial radiance reflectance inversion that was optimized by comparing retrieved estimates of phytoplankton absorption with independent measurements. The comparison of chlorophyll concentrations produced by the algorithm to measured values allowed validation of the inversion. Fluorescence quantum yield values were calculated from the retrieved phytoplankton absorption and the upwelling radiance corrected for elastic and inelastic scattering. Raman scattered light was found to be a significant component of the upwelling light field at wavelengths of Chl a fluorescence. Values of ff determined using the algorithm, and therefore derived solely from the radiometer measurements, were not significantly different from those estimated using independent measurements of absorption by phytoplankton (r2 5 0.86). The profiles of ff were characterized by an initial increase with depth to a subsurface maximum followed by a subsequent decrease. The irradiances of the subsurface maxima and ff at high irradiances appeared to be well conserved. An irradiance-based model including photochemical and nonphotochemical quenching was developed to explain variations in the quantum yield. Variations in phytoplankton biomass have been studied using sun stimulated fluorescence, often termed natural or passive fluorescence, which can be detected as red peaks in both in-water (Gordon 1979; Kishino et al. 1984; Topliss 1985; Kiefer et al. 1989; Fischer and Kronfeld 1990; Stegmann et al. 1992; Roesler and Perry 1995; Babin et al. 1996; Culver and Perry 1997; Hu and Voss 1998; Maritorena et al. 2000) and water-leaving radiance spectra (Neville and Gower 1977; Gower 1980; Doerffer 1981). Global scale measurements are possible using satellite sensors capable of resolving these fluorescence peaks (e.g., Esaias et al. 1998; Gower et al. 1999) and may be of particular advantage in case II waters, where suspended particulate matter and colored dissolved organic matter (CDOM) can interfere with chlorophyll algorithms based on blue-green ratios (Gower et al. 1999). The derivation of primary production from sunstimulated fluorescence has also been proposed (e.g., Topliss and Platt 1986; Kiefer et al. 1989; Chamberlin et al. 1990; Chamberlin and Marra 1992; Kiefer and Reynolds 1992; Stegmann et al. 1992) and will be incorporated into satellite sensor mission data products (Esaias et al. 1998). Scattering of light, both elastic and inelastic, is responsible for redirecting incident photons into the upwelling light stream (Mobley 1994). Elastic scattering reflects ambient solar radiation, whereas inelastic scattering involves a shift in wavelength and includes chlorophyll fluorescence. At environmental temperatures, the majority of chlorophyll a fluo-