Abstract
The influence of land use on river water quality was examined by conducting water quality surveys at 18 rivers in the Abashiri River basin flowing through agricultural land in the autumn, 2005. Previous studies have shown a correlation between NO3-N concentration in river water and the upland field and grassland field ratio, and they have found that NO3-N concentration is influenced by the amount of excess nitrogen from cultivated land. In the studied area, a high correlation was observed between NO3-N concentration and upland field and grassland field ratio. In the Abashiri River basin, NO3-N pollution in river water is influenced by the amount of excess nitrogen from cultivated land.
The influence of water quality in tributaries of the Abashiri River on eutrophication in Lake Abashiri was studied, by comparing the N/P ratio in those tributaries to the N/P ratio of the Abashiri River and to that of Lake Abashiri. A high correlation between N/P ratio and upland field and grassland field ratio was observed. In basins of tributaries where the upland field and grassland field ratio exceeds 30%, the N/P ratio is higher than that in the Abashiri River and that in Lake Abashiri. From this result, the eutrophication of Lake Abashiri is attributed to the load in river water from tributaries whose the upland field and grassland field ratio exceeds 30%.
