Seasonal Change and Trap of Dissolved Silicate in Hyugami Reservoir

Abstract

 We estimated seasonal changes in the dissolved silicate (DSi) in Hyugami reservoir in Fukuoka prefecture by field surveys, and also calculated trap values. DSi concentrations in surface water decreased in periods of diatom blooms from spring through autumn, while the concentrations in bottom water increased from one to three months after the blooms. In winter, the breakdown of thermal stratification led to homogenized DSi values throughout the whole water. These seasonal changes altered concentrations in the outflow water as well. From spring through autumn, concentrations in outflow tended to be lower compared to those of the inflow. Conversely, the concentration of outflow in winter tended to be higher than that of the inflow. This regression from bottom water in winter reduced DSi traps through the year. Material balance analysis revealed that the DSi trap values in Hyugami reservoir in FY2010 and FY2011 were -2.0% and 10.9%, respectively. We estimated that the DSi trap for the full-year was 5.2%.