A possibility that heavy rainfalls with strong winds altered the seasonal color change in the water of lake Matsuo-Goshikinuma, in northern Honshu, Japan

In July of 1991, the water in Lake Matsuo-Goshikinuma turned from blue to yellowish red. This change in the color of the water was different from the usual seasonal change, namely blue in the spring, whitish blue in the summer, and yellowish red in the autumn and the winter. In order to explain this irregular change, the relationships among the color, temperatures of the water, dissolved oxygen concentrations, rainfalls, and wind for five years were studied. It became apparent that in June and July of 1991, there were repeated heavy rainfalls over 50mm/day accompanied by strong winds. On the other hand, the stagnation of the oxygen-poor (less than 1mg O sub(2)/l) water of the lake was interrupted by oxygen-rich (over 2mg O sub(2)/l) water in the upper layer of the water. These findings made it possible to conclude that heavy rainfalls with strong winds led to the increase in the dissolved oxygen concentrations of the water which resulted not only in an oxidation of hydrogen sulfide but also in an oxidation of ferrous ions. The resulting iron hydroxide caused the yellowish-red color of the water in July of 1991.