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

Abstract

Nitrogenous compounds with high δ¹⁵N 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 δ¹⁵N values in nitrogenous compounds in a representative small river (Hebisuna River) that flows into Lake Nishinoko, an inner bay of Lake Biwa. A high δ¹⁵N 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 ¹⁵N enrichment in the Lake Biwa ecosystem during the past 40 years. We also observed a clear, stepwise, positive correlation between population density and δ¹⁵N values for particulate organic matter or sediments. These results demonstrate that δ¹⁵N _POM and δ¹⁵N _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.

Keywords:

• environment
• human population
• nitrogen-15
• river ecosystem
• watershed

Acknowledgements

The authors express special thanks to Prof. N. Yoshida of Tokyo Institute of Technology for his kind help in δ¹⁵N analysis of nitrous oxide.