Influence of water discharged from a reservoir on reclaimed land into Isahaya Bay (Kyushu, Japan) on the regeneration of NH4+ in the water column

  • Hiroyuki TakasuEmail author
  • Tomohiro Komorita
  • Takaya Okano
  • Miki Kuwahara
  • Keita Hoshimoto
Original Article


In 1997, the inner part of Isahaya Bay on the west coast of Ariake Bay, western Japan, was separated from the sea by dikes, leading to the formation of a freshwater reservoir. Since the Isahaya Reclamation Project began in the 1990s, red tides and hypoxia have become more common in the bay. Eutrophic high-turbidity water from the reservoir is frequently discharged into the bay, so NH4+ regenerated from organic matter in the discharge may be a source of nutrients for the red tides. In this work, we mixed reservoir water and seawater from the bay in experiments aimed at quantifying the NH4+ regeneration caused by the direct decomposition of labile organic matter in the high-turbidity drainage water. By comparing the results of field monitoring and mixing experiments, we estimated that the NH4+ regeneration rate under the influence of the drainage water was 192–501 μmol m−2 h−1, which was 31.5–46.8% higher than the natural NH4+ regeneration rate. This suggests that the additional NH4+ regeneration induced by the drainage water is non-negligible and that the organic matter in the drainage water may be an important source of NH4+ for red tides in the bay. This is the first study to provide data on the non-negligible amount of NH4+ regenerated upon the direct decomposition of labile organic matter in high-turbidity water drained from a reservoir on reclaimed land into the bay.


NH4+ regeneration Organic matter decomposition Reservoir of reclaimed land Drainage Isahaya Bay 



This study was supported in part by JSPS KAKENHI grant nos. 16K18737 and 18K14511 to HT, 18K11625 to TK, and 18H03360, and by the Ministry of Education, Culture, Sports, Science and Technology as well as Saga University as a part of the Cooperative Monitoring Program of the Ariake Sea (COMPAS) project. We thank S. Ijima, T. Inoue, and M. Kusaka for their assistance during the field sampling. We are grateful to the anonymous reviewers as their comments helped us to greatly improve the manuscript.


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Copyright information

© The Oceanographic Society of Japan and Springer Nature Singapore Pte Ltd, part of Springer Nature 2019

Authors and Affiliations

  • Hiroyuki Takasu
    • 1
    Email author
  • Tomohiro Komorita
    • 2
  • Takaya Okano
    • 3
  • Miki Kuwahara
    • 2
  • Keita Hoshimoto
    • 2
  1. 1.Graduate School of Fisheries Science and Environmental StudiesNagasaki UniversityNagasakiJapan
  2. 2.Faculty of Environmental and Symbiotic SciencePrefectural University of KumamotoKumamotoJapan
  3. 3.Faculty of Environmental ScienceNagasaki UniversityNagasakiJapan

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