Time- and Irradiance-Dependent Behavior of the Quantum Yield of Chlorophyll a Fluorescence

Fluorescence-based approaches for determining phytoplankton biomass have been under-utilized in comparison to applications based on blue/green wavelength reflectance ratios. In part, the under utilization is due to attenuation of the fluorescence signal by high water absorption in the red, in contrast to the greater transmission, and hence, depth penetration, of blue and green wavelengths. In near-shore mesotrophic and eutrophic environments with moderate to high concentrations of phytoplankton, the difference in absorption coefficients among all visible wavelengths diminishes, resulting in more spectrally flat reflectance spectra. At these high phytoplankton concentrations, the natural (or solar-stimulated) chlorophyll fluorescence signal increases. The net effect is that chlorophyll a fluorescence becomes a significant feature of the reflectance spectra in these waters.