On the relation between vicissitude process of nutrition and the non-pointpollution load in a mountainous lake under restricted phosphorus nutrition

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This study focuses on the vicissitude process of nutritive balance in a mountainous lake based on the relation between amount of non-point pollutant load in the basin and nutritive condition of the lake. The study conducted in mountainous Lake Onogawa which have typical characteristics in condition with mesotrophic and stratification.At first, pollution load in the catchment-area of the lake Onogawa was estimated. Natural load of the basin and the inner load in hypoliminon of the lake estimated larger than the artificial pollution in the basin. Inner load of hypoli-minon in summer season, especially, ammonia and nitrate that brought by the density current of a rain was notable. So, it was recognized that the non-point pollution in this lake is mainly consisted of the nitrate.And next, the vertical distribution of ammonia in hypoliminon of the lake was approximated by the one-dimensional differential equation of diffusion.Finally, the heat balance of the lake in summer season was analyzed. So, the negative heat flux were brought by the river and the flow from the upper lake was given positive heat flux, it was recognized that the restoration of the stratification was accelerated by this positive heat flux.