Isotopic composition of nitrogenous compounds with emphasis on anthropogenic loading in river ecosystems

摘要

Nitrogenous compounds with high δ~(15)N values were recently found in human-dominated small rivers in the Lake Biwa area. A detailed survey was performed to determine the distribution and variation of δ~(15)N values in nitrogenous compounds in a representative small river (Hebisuna River) that flows into Lake Nishinoko, an inner bay of Lake Biwa. A high δ~(15)N value was detected in the lower reaches of the river and the inner bay, most likely due to denitrification. These results strongly suggest that denitrification in small river systems such as the Hebisuna watershed has contributed to ~(15)N enrichment in the Lake Biwa ecosystem during the past 40 years. We also observed a clear, stepwise, positive correlation between population density and δ~(15)N values for paniculate organic matter or sediments. These results demonstrate that δ~(15)N_(POM) and δ~(15)N_(sediment) are helpful indicators for assessing nitrogen loading from domestic sewage. Moreover, they will aid in the development of new concepts in the environmental capacity of river ecosystems and its relationship to redox conditions. Finally, our data suggest that a population density of 100-200 persons per km~2 is the upper limit for a watershed in which only simple conventional sewage treatment is in effect.