Effect of hydrological changes on algal blooming in the reservoir and the required modification for treatments

The severe incidences of Microcystis blooming in 1980s of the Sagami and Tsukui reservoirs, located in Sagami river, Japan were effectively controlled by the installation of 8 and 5 bubbling circulation systems, respectively. However, the blooming had started again in those reservoirs. It was found that those algal blooming occurred due to the formation of strong stratification following a temperature gradient between upper and lower water levels of those reservoirs. In recent years, algal blooming re-appeared due to the rise in temperature of the spring inflow. Another reservoir namely the Sensui lake, located at the tributary of Takahashi river in the western part of Japan experienced severe droughts in winter between 2002 and 2003 and between 2008 and 2009. After the drought, Microcystis declined while Anabaena increased at the deeper zone. It appeared that the improved light condition at the deeper sediment surface might provoke the germination of Anabaena. These results indicate that snow fall reduction due to the global warming increased the inflow temperature in the reservoir. Thus, there is a possibility that the present destratification systems are not effective enough to suppress the blooming. On the other hand, the exposure of the bottom sediment can prevent algal blooming by changing the species composition. The artificial drawdown of the water level and the exposure of the bottom sediment for twenty days in early spring in Watarase reservoir, Japan, delayed the Phormidium cell density peaks to late summer and the 2-MIB production rate also declined.