A stochastic event with an expectation value of waiting time of roughly 1200 years occurred during August to October 2012 in Lake Shinjiko, a brackish lake. Abnormally strong salt stratification gave rise to oxygen deficiency at the lake bottom and the movement of a large amount of soluble phosphorus to the surface to flow out of the lake eventually. As a result, the former generation of blue. green algae has not been observed in fact since 2013. The reduction in phosphorus at the lake bottom is thought to have caused a regime shift from blue-green algae to diatoms. A series of phenomena is also important in considering the water pollution mechanism of Lake Shinjiko. For example, the effect of extension of the public sewerage system by Shimane prefecture may be hidden by the stochastic occurrence of soluble phosphorus and the reproductive cycle of the withered blue-green algae. If this assumption holds true, it means that the desirable regime shift is sustainable. It is necessary, however, to make clear the contribution of rivers smaller than the River Hiikawa in the phosphorus balance, and to promote immediately executable countermeasures to reduce phosphorus in order to make the current diatom regime more reliable.