| Co-Investigator(Kenkyū-buntansha) |
HIGASHI Masahiko LECTURER RYUKOKU UNIVERSTY, 理工学部, 講師 (40183917)
TATSUKAWA Kenichi INSTRUCTOR TOKYO UNIVERSITY, 海洋研究所, 助手 (20013584)
IWATA Katsuya PROFESSOR WAKAYAMA UNIVERSITY, 教育学部, 教授 (10031816)
KONAGAYA Tsuneo PROFESSOR MIE UNIVERSITY, 生物資源学部, 教授 (30024821)
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| Budget Amount *help |
¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 1988: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
Donghu (East Lake), 12 km^2 and 4.5 m deep, in Wuhan City, China was chosen as a research object. We intended to clarify the space-time distribution of nutrients such as nitrogen and phosphorus, and to evaluate roles of every biota in the lake system on recyclng of the nutrients. Japanese researchers took over a portion of the project which included evaluation of phytophagous fishes as an important activater of the material recycling.
Stocking silver carp, a phytoplankton feeder, and bighead carp, a zooplankton feeder, into the eutrophic lake in an extremely high density caused a drastic change of the lake system. Microcystis, a colony forming bluegreen alga, which had been dominated in summer and had been the main food of the both kinds of carps before became scarce while green algae smaller than 10 m became dominant. Actually the algal biomass became larger than before and the photosynthetic activity also became higher than before. Simultaneously, the growth of silver carp became poor while bighead carp grew faster than before.
Based on these findings we assume,
(1) The bluegreen algae had no opportunity of growing because of a higher grazing pressure by the fishes.
(2) The tiny green algae had an opportunity of growing because the competitive pressure of the bluegreen algae didn't act.
(3) The green algae gave a better food condition to zooplankton. Thus, the zoophagous fish which had mainly ingested Microcystis before could consume a higher quality food. While the consumption rate of silver carp was lowered because the green algae are too small even for the phytophagous fish.
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