INFLUENCE OF URBANIZATION ON RECENT WATER QUALITY IN LAKE USHIKUNUMA

Abstract

 The watershed of Lake Ushikunuma, which is located in the southern part of Ibaraki Prefecture, has been urbanized in recent years. The aim of this study was to clarify the factors affecting changes in water quality in this lake. The water quality and phytoplankton community were surveyed and the changes were analyzed. The results showed that the chemical oxygen demand (COD), total nitrogen, and total phosphorus concentrations in the inflow rivers were all decreasing, due to a decrease in the area of the field and diffusion of the sewage system. COD and chlorophyll a concentrations at the center of Lake Ushikunuma were also decreasing. It was shown that the reduction in inflow load due to urbanization resulted in improved water quality in the lake. Moreover, the results indicated that phytoplankton had a significant influence on the COD changes in the lake because the ratio of particulate COD to total COD was greater than 50%, and because COD and chlorophyll a concentrations in the surface layer were significantly correlated. The phosphorus limited phytoplankton production during most of the year. These results suggest that the phosphorus load and phosphorus release flux from the bottom sediment should be monitored carefully to understand the water quality changes in Lake Ushikunuma.