Geological characteristics influencing dissolved inorganic phosphorus (DIP) concentrations in forested watershed rivers in eastern Hokkaido: A case study of the Lake Nukabira Dam Watershed in the Daisetsuzan National Park

Abstract

　The objective of this study was to clarify the effect of geology on dissolved inorganic phosphorus (DIP) concentrations in river water during normal water flow conditions. The study was carried out in the Lake Nukabira Dam watershed, located in eastern Hokkaido, which is part of a national park where forests cover most of the land use. We measured phosphorus concentrations by component at 14 river sites in the watershed and statistically analyzed the relationship between DIP concentrations and environmental variables, including surface geology, soil, vegetation, and topography in the catchment area. DIP concentrations in river water showed the strongest positive correlation with the area fraction of Quaternary lacustrine deposits classified as igneous rocks. DIP concentrations were significantly high in rivers located in the northern part of the watershed, where Quaternary lacustrine deposits were distributed in the catchment area. In some of these rivers, the concentration exceeded the environmental standard (0.01 mg L^-1 for total phosphorus) of Lake Nukabira Dam. DIP concentration in river water showed a significant positive correlation with silicate-form silicon (SiO₂-Si) concentration, suggesting the influence of rock weathering. The supply of DIP via groundwater may contribute to the high DIP concentration of the river water in the distribution area of Quaternary lacustrine deposits.