Estimation of hypolimnetic discharge on improvement of water quality and control of phytoplankton blooms in yamaguchi reservoir

Sakai water purification plant (Sakai WPP) and Higashi-murayama water purification plant (Higashi-murayama WPP) take water from Yamaguchi reservoir (20 m-effective depth) by Intake tower No.1 and No. 2. Yamaguchi reservoir, refilled with water after seismic reinforcement to the dam body, started its operation again in 2002. In the spring of 2004 and 2005, massive blooms of Asterionella spp. and Fragilaria crotonensis (diatoms) occurred, which resulted in sharp decreases in filtration capacity of Sakai WPP, taking slow sand filtration. Results of water analysis for the reservoir between 2003 and 2004 indicated that nutrients eluted from sediments during stratification periods were utilized for the blooms in the following springs. Since intake gates at five different depths enable us to conduct selective intake, we conducted hypolimnestic discharge; while Sakai WPP continued taking water with low turbidity and phytoplankton production, the lowest gates of Intake tower No.1 were also slightly opened in the middle of 2005. In April 2006, the lowest gate of Intake tower No. 2 was fully opened instead of the upper ones. Consequently, the scale of spring diatom blooms reduced after 2006. Nevertheless, rise in turbidity increased T-P, which resulted in massive blooms of Synedra spp. (diatoms) in 2009 when the water level was lowered to 8 m. This illustrated that this operation of the selective intake was not effective for conservation of the reservoir at low water levels.