1Okayama Prefectural Technology Center for Agriculture, Forestry and Fisheries, Research Institute for Fishries Science, Setouchi, Okayama 701-4303, 2Tomata Dam and Reservior Management Office, Kagamino, Okayama 708-0433, 3Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 620-0861, Japan
To clarify the change in water quality caused by dam impoundment, we researched variance of total nitrogen (TN), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), total phosphorus (TP), total dissolved phosphorus (TDP), dissolved inorganic phosphorus (DIP), dissolved silicate (DSi), water temperature and amount of phytoplankton in Lake Okutsu (Tomata dam). A thermocline developed in summer. Planktonic algae dominated by dinoflagellate bloomed in the surface throughout the seasons. The concentration of each kind of nutrient in the outflow water was lower than that in the inflow water. The lower nutrient surface water consumed by planktonic algae was drained to downstream. Concentrations of TN, TP, DSi in the outflow water became lower than those in the inflow water by 27.8%, 21.2%, and 18.9%, respectively.