The importance of fractional light absorption by photosynthetic pigments for phytoplankton productivity in Lake Constance [Federal Republic of Germany]

The role of light interception by photosynthetic pigments on the productivity of phytoplankton was studied year-round in mesotrophic Lake Constance. The phytoplankton showed wide seasonal fluctuations in both biomass (Chl a) and photosynthesis. Linear regression was used to determine pigment-specific light attenuation and nonalgal light extinction. Fractional light absorption by photosynthetic pigments ranged from 4 to 70%. Pheopigments comprised $27.4 \pm 8.2%$ of total pigments. Since pheopigments do not transfer energy to the photosystems, their light absorption must be added to the background light attenuation by nonphotosynthetic material. If pheopigments are considered, estimates of fractional light absorption decrease accordingly. A plot of integral photosynthesis vs. maximum photosynthesis at the depth of the optimum light intensity gives a saturation curve, due to increasing fractional light absorbance by photosynthetic pigments at rising concentrations of biomass. Integral photosynthesis would reach a theoretical upper limit if all light were absorbed by active photosynthetic pigments. Since pheopigment concentrations usually rise with active Chl a, background light attenuation increases also, leading to diminished productivity per unit of lake surface area.