Evaluation of Water Quality Environment in Eutrophic Reservoirs using an Ecosystem Model

An ecosystem model was developed to evaluate water environment in three eutrophic reservoirs located in the Obaru River basin of the Itoshima area in Fukuoka Prefecture, Japan from the viewpoints of dynamics of two nutrients: dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). This model was based on a completely mixed system and included nine water quality indices of phytoplankton, zooplankton, particulate organic matter, dissolved organic matter, phosphate-phosphorus, ammonia-nitrogen, nitrite-nitrogen, nitratenitrogen and dissolved oxygen (DO). First, the model parameters were determined for each reservoir by the optimization using a simple genetic algorithm in order that water quality observations were reproduced by the ecosystem model. The calculation results were in good agreement with the observations of chlorophyll-a, total and dissolved organic carbon, total nitrogen, total phosphorus and DO, and the validity of this model could be confirmed. Next, the dynamics of nutrients were examined by calculating the daily fluctuation of DIN and DIP, and their components of biochemical processes such as photosynthesis, respiration, mineralizarion, elution and denitrification. The results in common with the targeted reservoirs indicated that the fluctuation of DIN and DIP were similar, and the components of photosynthesis and respiration were superior to others. Also, it was found that the seasonal change in the dynamics and their components was different for three reservoirs, and that they reflected the water environmental features of water bodies such as the bottom sediment, a mass generation of blue-green algae and a vegetation of floating-leaved plants.本研究では,九州大学伊都キャンパスを源流とする大原川流域内の5号調整池,平川池,大久保池を対象に,有機汚濁および富栄養化関連項目に関する水質観測と,生態系モデルを用いた数値計算により,栄養塩の動態特性の観点から各水域の水環境評価を行った. まず,Chl.a,DO,TN,TPの水質観測結果を,水稲栽培に関する農業用水質基準ならびにOECDによる湖沼類型の分類基準の観点から考察した.つぎに, 植物プランクトン,動物プランクトン,溶存態有機物, 懸濁態有機物,リン酸態リン,アンモニア態窒素,亜 硝酸態窒素,硝酸態窒素,溶存酸素の計9つの状態変数で構成されるワンボックス型低次生態系モデルを構築し,観測結果の再現計算を行った.水域に応じたモデルパラメータの最適値をSGAにより決定したところ,特にChl.aとDOについては良好な再現結果が得られ,モデルの妥当性が確認された.また,SGAにより探索されたパラメータの中には,溶存態有機物の無機化速度定数や底泥による酸素消費速度など,水域の特徴を充分に反映する値がみられた. さらに,生態系モデルによる計算結果から,栄養塩にあたる無機態窒素と無機態リンの1日あたりの変動量,ならびに生物化学的循環過程による変動成分量を求め,栄養塩の動態特性を検討した.その結果,無機態窒素と無機態リンの変動量の季節的変化は,各水域で類似した傾向を示した.また,これらの変動量の季節的変化は各水域によって異なり,それぞれの特徴を反映した結果が得られた.すなわち,栄養塩の動態特性は,5号調整池では底質環境の影響を,また平川池と大久保池では藍藻類の大量発生の影響を強く受けることが示された