湖沼生態系モデルによる富栄養化防止対策の評価

抄録

An ecological model of Lake Yunoko, a dimictic lake, was developed to obtain quantitative information on the nutrient cycle in Lake Yunoko, particularly the sediment-waterinteractions and to assess various programs for eutrophication control. A multi-component dynamic model for nutrients in the bottom sediment-water system was incorporated into the model, one dimensional water temperature-water quality-ecological model. Good agreement between the model calculations and observed nutrients, chlorophyll-a, and dissolved oxygen concentrations in the water and nutrients concentrations in the bottom sediment was noted. The agreement between the model calculations and the measured nutrient flux from bottom sediment during the summer stagnation period was good. The effectiveness of eutrophication control programson chlorophyll-a concentration in the surface water and dissolved oxygen concentration in the bottom water was assessed using the calibrated model. Sediment dredging decreased nutrient flux from bottom sediment. As a result, chlorophyll-a concentrations in the surface water at autumncirculation period decreased in comparison with those of the control simulation. However, sediment dredging had not an effect on the depression of nutrient release flux after 4 years as compared with the control simulation. The depletion of dissolved oxygen in the bottom water during the summer stagnation period was scarcely restored by dredging only. Artificial circulation increased dissolved oxgen in the bottom water more than 5.2 mg 1^-1 throughout the year, whereas chlorophyll-a concentration in the surface water scarcely decreased compared with the control simulation.In the case of 50 percent reduction of external phosphorus loading from the wastewater treatment facility, the annual average chlorophyll-a concentration in the surface water decreased to 79 percentof that of the control simulation for the 4 th year.