Restoration and conservation of “naiko” lagoon ecosystem around Lake Biwa (1): A study on the improvement of water quality and residence time of “Hira-ko” and “Yanagihira-ko” lagoons

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  • IKEYA Tohru
    Research Institute for Humanity and Nature Center for Ecological Research，Kyoto University
  • ISHIDA Takuya
    Research Institute for Humanity and Nature Graduate School of Advanced Science and Engineering，Hiroshima University
  • YI Rong
    Department of Ecosystem Studies，School of Environmental Science，The University of Shiga Prefecture School of Geography and Environmental Engineering，Gannan Normal University
  • BAN Syuhei
    Department of Ecosystem Studies，School of Environmental Science，The University of Shiga Prefecture
  • OKUBO Takuya
    Department of Biological Resources Management，School of Environmental Science，The University of Shiga Prefecture
  • OKUDA Noboru
    Research Institute for Humanity and Nature Research Center for Inland Seas，Kobe University

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Other Title
  • 琵琶湖周辺内湖の再生と保全（第1報）: 平湖・柳平湖の水質改善と滞留時間調整の検討
  • ビワコ シュウヘン ナイコ ノ サイセイ ト ホゼン(ダイ1ポウ)ヘイコ ・ ヤナギ ヘイコ ノ スイシツ カイゼン ト タイリュウ ジカン チョウセイ ノ ケントウ

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<p>We examined improvements of water quality and residence time after a river rehabilitation project in the “Hira-ko” and “Yanagihira-ko” lagoons, and the effective conservation of water quality in the semi-enclosed lagoons around the coastal area of the Lake Biwa. When we started the research, the residents had been unable to feel the improvement of the water quality even after countermeasures by the project. According to the project report data, the lagoon dredging could reduce phosphorus efflux rate from sediments, and then such as phosphorus, total (TP) and soluble reactive phosphorus (SRP) from August to November, and cell densities of phytoplankton species occurring from August to September, have decreased after 2010, when an intake from “Ohta-gawa” river was installed. Further analyses revealed that several species, such as Anabaena (Dolichospermum) macrospora, which is one of typical blooming and musty odor producing cyanobacteria, decreased until 2015. The dominant blooming cyanobacteria shifted to several other species, such as Anabaena (Dolichospermum) affinis, and relative abundance of diatoms increased after 2016. We estimated the overall inflow rate to discuss the appropriate residence time of the “Naiko” lagoons. The estimation of inflow by the river rehabilitation project was significantly underestimate the inflow rate in irrigation period (May to August) because the most of inflow measurements undertook in non-irrigation period (September to April), so the residence time in irrigation period, which was 15-17 days, was overestimated in the report, although inflow rate from “Ohta-gawa” river was around one-half of the initial project plan. The introduction of the additional inflow, which has been conducted by the residents' organizations of Shina-cho for rural community in order to compensate the shortage in the intake from “Ohta-gawa” river, could decrease residence time as much as 7 days in monthly average during operation months, and even 26 days in October 2018 when the evaporation exceeded precipitation and inflow rate was minimum.</p>

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