| Co-Investigator(Kenkyū-buntansha) |
OKUMURA Minoru 島根大学, 総合理工学部, 教授 (30032650)
MITAMURA Osamu 滋賀県立大学, 環境科学部, 教授 (50030458)
SENGA Yukiko 立正大学, 地球環境科学部, 助教 (30434210)
YAJIMA Hiroshi 鳥取大学, 工学部, 准教授 (10283970)
INOUE Tetsunori 独立行政法人港湾空港技術研究所, 海洋-水工部, 主席研究官 (70311850)
NAKAMURA Yoshiyuki 港湾空港技術研究所, 海洋-水工部, 領域長 (90172460)
AIZAKI Morihiro 島根大学, 生物資源科学部, 教授 (20109911)
YAMAGUCHI Keiko 島根大学, 生物資源科学部, 准教授 (80322220)
HIGANO Junya 水産総合研究センター, 生産システム部, 二枚貝増養殖チーム長 (80372019)
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| Research Abstract |
Hypolimnion water in the dredged area (Water depth : 15m) of brackish Lake Nakaumi, Japan, has been exposed to anoxic condition for a long term except for winter season. The experiment supplying the high dissolved oxygen water was performed at the dredged area during 2007 to 2009. In this study, we used a new technology (WEP system : MATSUE DOKEN Co., Ltd.) for introducing the high dissolved oxygen water into the dredged area. As a result, the dissolved oxygen concentration in hypolimnion was increased up to 15～29mg/L nearby the outlet of WEP system, and was kept at 5-15mg/L in the whole experimental site, for a 6 month (09May-Oct) in spite of the tidal current.
Monthly variations of hydrogen sulfide (H_2S) and methane (CH_4) were observed at the bottom water in the dredged area. In addition, H_2S concentration and CH_4 producing rate in sediments were also observed at the same area. No accumulations of H_2S and CH_4 were detected in the bottom water during the operation of the WEP syst
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em. Moreover, H_2S was almost disappeared at 0-4cm depth in sediment, and also CH_4 producing rate decreased markedly compared to that at the control area.
In the case of a freshwater lake, generally, the iron (II) is released from the bottom sediment in a reduction state. In brackish Lake Nakaumi, however, it is trapped by the hydrogen sulfide derived from sulfate reducing and cannot be released. In the present study, hydrogen sulfide was oxidized and disappeared, and the environment that iron (II) can be released from the bottom sediment was formed. Thus, the effect of release inhibition from bottom sediment for phosphate phosphorus was observed.
The effect of high dissolved oxygen water on self-purifying function for nitrogen nutrient based on nitrification and denitrification was investigated. An active nitrification was induced by the increase of dissolved oxygen (DO) in the bottom water, followed by induction of denitrifying activity in the sediment-water system. It was also suggested that nitrification was inhibited by high concentration of dissolved oxygen exceeding 200%.
Benthos such as polychaetes, Paraprionospio patiens, Prionospio pulchra etc. could form their habitat at the dredged area throughout the experimental periods. However, bivalves were not settled during the periods. The captured organisms in the sediment traps (40cm long and 10cm inner diameter pipes) showed that the larvae of Asari clam, Ruditapes philippinarum and Asian mussel, Musculista senhousia, could reach the bottom layer but could not recruit probably due to low pH less than 8 in deeper than three 3m.
Thus, it was suggested that the present new technology is very useful for an environmental improvement of the bottom water and sediment in lakes. Less
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