The Basic model for aquatic ecosystem assessment using carbon and nitrogen stable isotopes

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In the Yoshino River, The Total-N concentration were from 0.19 to 0.55mg/l in the top to middle reaches, which most in the catchment is a forest, in the lower reach, where population and farmland increased, the concentration rose from 0.58 to 1.19mg/l. δ^<15>N value of Total-N were 1.0% in the head water, 3.6-5.9%0 in the middle reach, and 4.2-8.9%0 in the lower reach. There was positive correlation (r=0.91, p<0.001) between the δ^<15>N_<Total-N> and Total-N concentration. The increase in the δ^<15>N_<Total-N> and Total-N concentration with the increase in the farmland and population in the catchment indicated that the effect of the nitrogen loading by agricultural and life drainage was large as a downstream.The relation between the δ^<15>N_<Total-N> and Total-N concentration in 10 main river flowed into Lake Biwa was similar to the relation got in the Yoshino Rivet However, about 8% was the maximal value, the δ^<15>N_<Total-N> decreased in the river in the paddy field region. In such river, the ammonia concentration was high. It was indicated that the δ^<15>N_<Total-N> decreased in the anaerobic environment because nitrification is not sufficiently carried out. The fluctuation of such δ^<15>N shows δ^<15>N is the good parameter to reflects the effect of nitrogen loading by the human activity. It is possible to make use of The δ^<15>N_<Total-N> for the diagnosis of the aquatic environment.From the investigation of the Yoshino River, the relation of δ^<15>N_<Total-N> (%o)=3.7×Total-N concentration(mg/1)-0.30 was obtained. and it becomes a standard in considering the soundness of the river in future. In the future, such relational expression will become a standard of the assessment of the waters ecosystem. In the future, such relational expression will become a standard to diagnose the health of the aquatic ecosystem.