| Budget Amount *help |
¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 2006: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2005: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2004: ¥800,000 (Direct Cost: ¥800,000)
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| Research Abstract |
Dissolved organic matter (DOM) dominates over organic matter pool in lakes. Until 1980s, DOM was thought to be waste and useless substances in the energy flow in the lakes. However, the importance of DOM was re-evaluated. Azam et al. (1983) named "microbial loop" in which bacteria used DOM to build up their body, protozoa ate bacteria, and then zooplankton ate protozoa. Thus energy flow of microbial loop stems from DOM.
On the other hand, hypolimnion of a productive lake becomes anoxic in summer, because of bacterial decomposition of organic matter from productive photosynthetic layer. Fe(III) and Mn(IV) in the lake sediment are reduced to Fe(II) and Mn(II), respectively and dissolved into hypolimnetic water. On the autumnal water circulation, these dissolved metals are oxidized to Fe(III) and Mn(IV) and the solubility of these compounds are small, resulting in the precipitation. Fe(III) precipitates coprecipitate many inorganic and organic compounds. Some DOM in natural lake water shou
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ld be coprecipitated by Fe(III) precipitates. However, no information is available on this phenomenon.
The present study revealed that when hypolimnetic anoxic water from Lake Hibara agitated by a magnetic stirrer for 24 hr, dissolved oxygen (DO) increased from 0 to 7.6 mgl^<-1>, Fe(II) decreased from 98% of the total Fe to 5% and simultaneously massive yellow precipitate formed, and 32-48% DOC disappeared. When the water sample left unstirred for 48 hr, DO increased to 2.2 mgl-1, Fe(II) decreased to 31%, and 20% of DOC disappeared. The amount of disappeared DOC is too large to explain bacterial decomposition. The yellow precipitates should be Fe(III) precipitates and these precipitates must coprecipitated disappeared DOC.
The mixing experiments of hypolimnetic anoxic water with aerobic surface water to simulate autumnal water circulation of a lake were conducted in this year. Duplicate incubations were carried out. The results likely suggested the coprecipitation. However, precision of the incubations were not enough to predict the difference between controls and treated samples. Methodological improvement is desired in the nest step. Less
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