Nitrogen Mass Change and Cycle in Lake Biwa

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  • NAGARE Hideaki
    Research Center for Environmental Quality Control, Kyoto University
  • FUJII Shigeo
    Research Center for Environmental Quality Control, Kyoto University
  • SOMIYA Isao
    Faculty of Science and Technology, Ryukoku University

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  • 琵琶湖における窒素の水中内存在量と循環過程
  • ビワコ ニ オケル チッソ ノ スイチュウ ナイ ソンザイリョウ ト ジュンカン カテイ

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Abstract

The mass change of nitrogen in Lake Biwa was shown, and its cycle was discussed and compared with the input/output loading. Nitrogen exists at 10.5X103 ton in average, and its minimum and maximum mass are 7.6X103 ton, 13.4X103 ton, respectively. Its average mass is equivalent to 133% of the annual input loading, resulting in 1.6 months of residence time, which is shorter than 5.5 years of hydraulic retention time. This indicates that nitrogen in the lake is removed by settling onto the lake bottom or denitrification, and the removed mass is estimated to be 71% of the input loading. Phytoplanktons mainly utilize ammonium nitrogen in epilimnion whose mass is only 1% of the total mass in the lake. rather than other inorganic nitrogen amounting to more than ten-fold of ammonium nitrogen. During the stratification period, almost all decomposed nitrogen in the organic matter is oxidized into nitrate, and half of them is likely to be removed by denitrification.

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