Distribution of Carbon and Nitrogen Isotope Ratios in the Yodo River Watershed

Abstract

Nitrogen and carbon isotope ratios of surface sediments and benthic animals, and nitrogen isotope ratio of nitrate were measured to establish the isotopic biogeochemical structure in the Yodo River watershed. The δ15N of sediments in Lake Biwa, located in the middle reach of the Yodo River watershed, were the highest values in the watershed. The δ15N values of nitrate were also significantly higher in comparison with those of lakes in other watersheds. This fact suggested that denitrification was occurring on a large scale in Lake Biwa. Based on the isotope mass-balance, ca. 50% of nitrogen introduced into the North Basin of Lake Biwa seemed to be released as nitrogen gas through the denitrification process. δ13C values of benthic animals (aquatic insects) in the upper reach of the Yodo River watershed were 6 to 13‰ higher than those of sediments. This fact suggested that the aquatic insects in the upper reach foraged organic matter produced in the river ecosystem. The higher δ13C values of sediments and benthic animals in the South Basin of Lake Biwa and the Yodo River indicated that the middle to lower reaches in the Yodo River watershed were progressively influenced/transformed by human activities.