Seasonal change of the water color of Lake Goshikinuma was numerically explained from the absorption spectrum of the lake water.
In summer, the absorption spectrum did not have clear maximum value and the transparency of the lake water was high. Sunlight thus could reach the deep part of lake, and the sunlight is scattered by the water particle. Accordingly, the water color became to be blue. In contrast, the lake water itself was colorless in summer, because the lake water in summer had much Fe2+ which does not have absorption spectrum in visible range.
In autumn, the transparency was low and the lake water had the maximum absorption spectrum from 420 nm to 470 nm. In addition, from autumn to winter, the lake water circulated from the surface to the bottom, because the temperature of spring water at the bottom was higher than that of the lake water, and the lake water had much Fe3+ which has the maximum absorption spectrum at 407 nm and 550 nm. Consequently, the water color was olive in autumn. The water-color change from summer to autumn was judged to be caused by the oxidation of the iron ion from Fe2+ to Fe3+.