Author(s): Takeshi Takemura; Jiro Maruyama; Masanori Nakai; Masamitsu Arita
Linked Author(s): Takeshi Takemura
Keywords: Nagara rivermouth barrage; River flow rate; Phytoplankton; Nutrient; Intensity of solar radiation; Water temperature; Retention time
Abstract: Nagara rivermouth barrage is located at a 5. 4km upstream from the rivermouth and its upstream reach is changed to a reservoir-like area. Oxygen-deficient water masses appear in the bottom layer and deterioration of native habitats reduces biodiversity. One of the reasons for the large environmental degradation has been attributed to great multiplication of phytoplankton after operation of Nagara rivermouth barrage. An analysis was carried out with the monitoring data to investigate the production process of phytoplankton in the upstream reach of the barrage. The aims of this study are the following two. One is clarification of the production process of phytoplankton, and the other is a quantitative evaluation of main environmental impact factors (water temperature, intensity of solar radiation and amounts of dissolved inorganic nutrients) connected with multiplication of phytoplankton. The results revealed that there were two kinds in the production process of phytoplankton. Pattern A is large self-multiplication of phytoplankton in the flow-down and Pattern B is movement of phytoplankton masses with high concentration. The appearance conditions of amounts of dissolved inorganic nutrients (for nitrogen and phosphorus) for Pattern A at St. Nagara were given by PO4-P (N) / T-P (N) ≥ 0. 622 and DIN (N) / T-N (N) ≥ 0. 845. In addition, the conditions of water temperature and accumulated intensity of solar radiation were more than 22 °C (at St. Ise) and 20 MJ/m2 (in flow-down), respectively. Thus, Pattern A appears under the conditions of sufficient amounts of dissolved inorganic nutrients (DIN, PO4-P), high water temperature and intensity of solar radiation, and a medium river flow rate (90-180 m3/s).