Quality improvement of the bottom water and sediment at anoxic water in a brackish lake

除冬季外，日本中海咸淡湖疏浚区（水深15m）的低磷水长期处于缺氧状态。2007 - 2009年在疏浚区进行了高溶解氧水补给试验。在本研究中，我们采用了一种新技术（WEP系统：MATSUE DOKEN株式会社）将高溶解氧水引入疏浚区。结果表明，在WEP系统出口附近，低磷离子中溶解氧浓度升高至15~29mg/L，整个试验点在6个月（5月9日- 10月）的时间内，无论潮流如何，溶解氧浓度都保持在5~ 15mg/L。对疏浚区底水硫化氢（H_2S）和甲烷（CH_4）的月变化进行了观测。此外，还对同一区域沉积物中H_2S浓度和CH_4产率进行了观测。在WEP系统运行过程中，底部水体中没有检测到H_2S和CH_4的积累，并且在0 ~ 4cm深度沉积物中H_2S几乎消失，CH_4生成速率也较对照区明显降低。在淡水湖中，铁（II）通常以还原状态从底部沉积物中释放出来。然而，在半咸淡的中海湖中，它被硫酸盐还原产生的硫化氢所困，无法释放。在本研究中，硫化氢被氧化消失，形成了铁（II）可以从底部沉积物中释放出来的环境。因此，观察了底泥对磷磷释放的抑制作用。研究了高溶解氧水对基于硝化和反硝化的氮营养物自净功能的影响。底水溶解氧（DO）的增加诱导了活性硝化，随后又诱导了沉积物-水系统的反硝化活性。溶解氧浓度超过200%时，硝化作用受到抑制。在整个实验期间，疏浚区均可形成多毛类、拟螺旋体、pulchra等底栖动物的栖息地。然而，在此期间，双壳类动物并没有定居下来。沉积物捕集器（长40cm，内径10cm的管道）中捕获的生物表明，在3m以下的深度，沙蛤（Asari clam）、菲律宾蛤（Ruditapes philippinarum）和亚洲贻贝（Musculista senhousia）的幼虫可以到达底层，但可能是由于pH值低于8而无法招募。因此，本文提出的新技术对改善湖泊底水和沉积物的环境是非常有用的。少

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 … More 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