Creeping Environment Problem of Rapid Growth of Water Hyacinth in the Shallow Reservoir of Ishiduchi with Influence of Global Warming at Warm Temperate Region in Japan

Masahiro MURAKAMI and Akihiro NISHIYAMA*

* School of Environment Science and Technology, Kochi University of Technology, 185 Miyanokuchi, Kami city, Kochi 782-8502, Japan

Abstract

Under the influence of global warming and climatic change in the south-west warm temperate region in Japan, rapid growth of water hyacinth of sub-tropical origin is a major source of thick muddy sludge sedimentation in the shallow lakes and/or reservoirs, which deteriorates water quality environment and eco-system as well as decreases the capacity of effective storage volume for flood control. This study aims to evaluate the influence of free-floating aquatic plant such as water hyacinth (Eichhornia crassipes) on the water quality and eco-system of the shallow semi-artificial flood retention reservoir in Kochi, Japan. A series of laboratory experiments have been carried out to examine the absorption-emission capacities of nitrogen (N:T-N) and phosphorous (P:PO4-P) in the growing-dying processes of water hyacinth in the aquarium at controlled water temperature in both summer and winter. The rate of absorption-emission of N-P amounts to 40-95% in a week, which is the major factor to control the nutrient (N-P) level of the reservoir. Vulnerability assessment is being carried out to restore the Ishiduchi reservoir taking into account of the application of bio-manipulation technology.

Key words: aquatic plants, water hyacinth, quality, purification, eco-system, bio-manipulation

要旨

　温暖な温帯気候に属している西南日本では、地球温暖化と気候変動の影響下で浅い湖沼や貯水池に亜熱帯原産の外来植物であるホテイアオイが大増殖し始めている。その結果水界の生態系や水質が悪化して池底にヘドロの堆積が進む過程で雨水調整池の治水調整容量が減少してきている。本論の目的は浮遊性水生植物(ホテイアオイ：Eichhornia crassipes)が高知県の浅い湖沼や貯水池の水質と生態系に与える影響を評価することにある。夏期と冬期を代表する水温にコントロールした室内実験水槽に石土池のホテイアオイと水を移し、成長過程（夏）と枯死過程（冬）における窒素 (T-N)と燐(PO4-P)の濃度変化、すなわちN-Pの吸収量と放出量を測定した結果、ホテイアオイの成長と枯死過程が池の栄養塩濃度を大きく支配していることが判明した。バイオ・マニピュレーション(Bio-manipulation)手法を適用した石土池の自然再生計画における脆弱性評価(Vulnerability assessment)に取り組んでいる。