Quantum Yield of in Situ Fluorescence of Pyhtoplankton in Kiel Bay Under Daylight, Comparison with Primary Production

Abstract

We investigated the upwelling radiance signal between 650 and 750 nm in turbid coastal waters. The upwelling radiance is composed of elastically backscattered light and chlorophyll fluorescence. We calculated fluorescence upwelling radiance from the vertical distribution of chlorophyll, a standard specific absorption spectrum, the vertical spectral downwelling irradiance distribution and the diffuse downwelling attenuation coefficient. The subtraction of the calculated fluorescence radiance from the measured radiance yielded the elastic backscatter spectrum. In turbid coastal seawater at the surface the elastic backscatter spectrum is expected to be depressed near the chlorophyll absorption band at 675nm. The position of this depression at 675 nm was used as a sensitive criterion for the proper calculation of the fluorescence radiance. The quantum yield of fluorescence calculated this way correlated well with quantum yield of primary production measured by ¹⁴C uptake in situ. Critical evaluation of the calculated quantum yields suggests the following average values: 0.02–0.03 (Quanta_em/Quanta_abs) for fluorescence and 0.03 (Mole C / Einstein_abs) for photosynthesis.