Quantum yield of photosynthesis and spectral absorption of phytoplankton during the stratified periods at the northern basin of Lake Biwa

Abstract

To understand what factors govern the quantum yield for phytoplanktonic photosynthesis, field research was conducted in the northern basin of Lake Biwa in September of 1994 and 1995, when the water column was still thermally stratified. Vertical distributions of underwater spectral light energy, its absorption by phytoplankton, as well as photosynthesis and primary productivity were observed, together with temperature, chlorophyll a fluorescence, beam transmittance, and nutrient concentrations. The optical absorption coefficient, averaged over the wavelengths of photosynthesically active radiation (PAR), co-varied with sum of chlorophyll a and phaeopigment concentrations as has been reported in previous studies. The quantum yield of phytoplanktonic photosynthesis, φ, increased exponentially with decreasing radiation. This was clear in the high radiation region where scalar PAR exceeded 10 E m-2 day-1. Also, φ increased with decreasing specific absorption by phytoplankton, and especially with increasing dissolved inorganic nitrogen concentration under weak light. These results suggest that phytoplankton could efficiently utilize the absorbed light energy for photosynthesis under weak radiation in deep water.