Abstract
The water quality of Lake Hashie and the Kanzawa and Nishikatsura rivers, which flow into the lake, was measured to identify the main cause of eutrophication in the lake. A land use survey was also carried out of the watershed of Lake Hashie. The total nitrogen (TN) and total phosphorus (TP) concentrations in the lake and rivers were relatively high, in the ranges of 1-15 and 0.1-1.5mg·l-1, respectively. The TN and TP concentrations of the lake were increased by the influx of water from the rivers. From the results of the land use survey, nitrogen and phosphorus loads from stockbreeding and farmland in the upper basins may have caused the pollution of the rivers. Therefore, to prevent the eutrophication of Lake Hashie, the proper disposal or recycling of livestock waste and the optimum use of chemical fertilizer in the farmland are recommended in the upper basins of the Kanzawa and Nishikatsura rivers.
