富栄養化貯水池

A large amount of nitrogen and phosphorus flows into agricultural reservoirs through domestic and agricultural drainage in flat, low-lying agricultural areas. The availability of excess nutrients causes a profuse growth of phytoplankton, thereby resulting in water bloom, and leading to various water quality problems. Water quality assessment is very important to prevent eutrophication of reservoir, and to elucidate the movement of phytoplankton and circulation of nutrients within the water body. Therefore, a water quality prediction model is the most effective approach for water quality assessment, and an ecosystem model is frequently utilized to analyze and predict the dynamics of water quality. In this study, a one-box ecosystem model considering 4 primary algal groups (green algae, blue- green algae, diatoms/dinoflagellates, and cryptophytes) was constructed for 3 eutrophic reservoirs in the Itoshima Peninsula, located in the western part of Fukuoka City. These reservoirs were utilized for agricultural water resource, and they differed in characteristics each other, such as water depth, storage capacity, and presence of vegetation (floating-leaved plants and emersed plants). Water quality data of chlorophyll-a content of each algal group, TOC, DOC, TN, TP and DO were weekly observed from July to November 2009 to verify the validity of the ecosystem model. As a result, the variations of water quality related to eutrophication could be evaluated and predicted by this model differed in the water characteristics. Further, the seasonal change in chlorophyll-a concentrations of each algal group in relation to the temporal variations of environmental factors of water temperature, solar radiation and nutrients concentrations, could be quantitatively estimated by this model. Seasonal characteristics that enable proliferation of each algal group under the condition of nutrients limitation could be determined by focusing on the biochemical variable of DIN and DIP calculated from the numerical simulation results.