寡少な水中光環境下にある富栄養化水域の水質の動態特性

Abstract

In a eutrophic reservoir under a scarce underwater light environment due to a large amount of fulvic acid, the dynamics of chlorophyll-a and dissolved oxygen (DO) were examined by water quality monitoring and numerical simulations with a vertical one-dimensional ecosystem model. In addition, the effect of the improvement of transparency on their dynamics was evaluated by a scenario analysis. In the summer, the objective water body was under an extremely scarce underwater light environment since blue-green algae were generated in large quantities at the water surface. At a depth of more than 1m, chlorophyll-a concentration did not increase owing to the limitation of light intensity, and the water body entered the anaerobic state. In the autumn, the chlorophyll-a did not increase because of the nutrient limitation, although the influence of the light limitation decreased in a depth of less than 4m by the attenuation of blue-green algae. The vertical diffusion transport of DO, however, increased by the formation and progress of the mixed layer at a depth of less than 5m, resulting in an improvement of the anoxic state. The results of the scenario analysis indicated that the progressing of the current transparency (0.5〜1.4m) to 3m led to the decrease of phytoplankton due to the nutrient limitation and the improvement of the anoxic state in the whole vertical area, although phytoplankton temporarily increased from the decrease of the light limitation.