Utilizing Downwelling Irradiance for Phytoplankton Determination

Abstract

The downwelling irradiance is rarely utilized to determine optical parameters of a water body since the ever-changing water surface induces strong signal fluctuations in the upper meters of typically 20 to 40 %. An analytic model has been developed that calculates the spectra of the direct and diffuse component separately. It can cope with the fluctuations by treating, in addition to the unknown optical parameters of the water body, also the intensities of the two irradiance components as fit parameters during data analysis. The potential of this model for the determination of phytoplankton concentration was analyzed using data from two German lakes. Three phytoplankton classes could be identified above thresholds between 0.4 and 0.8 μg/l, and total pigment concentration (sum of chlorophyll-a and phaeophytin-a) could be determined with an uncertainty of 0.7 μg/l. This new approach may be of particular interest for shallow waters, where it is difficult to apply the usual reflectance-based algorithms due to bottom influences.