Sediment Control on Lake Eutrophication Using an Ecological Model

Abstract : Eutrophication of closed water bodies is still one of the major environmental concerns in Japan. The percentage of lakes and reservoirs whose water cannot satisfy the Environmental Quality Criteria is the lowest when compared with those of rivers or sea areas (Environment Agency of Japan 1985). For the conservation and restoration of closed fresh waters, the Environment Agency enacted a new law, Law Concerning Special Measures for Conservation of Lake Water Quality on 13 May 1983. Local governments are required to make all possible efforts for eutrophication control, such as removal of polluted bottom sediments, artificial aeration/circulation, and reduction of nutrient loads. Ecological models are widely used for assessing the effectiveness of various means for eutrophication control (Nyholm 1978, Orlob 1983, Matsuoka 1984). However, most conventional models presuppose that release rates of nutrients from bottom sediments are constant or are given by a forcing function (Larsen, Mercier, and Malueg 1973; Imboden and Gachter 1978; Gulliver and Stefan 1982). Sediment submodels, therefore, are rarely incorporated into the ecological models (Jorgensen 1976). Strictly speaking, it is almost impossible for these models to predict the effects of sediment dredging and/or reduction of external loadings on the profiles of nutrient concentrations along the depth of sediments and, consequently, interaction (i.e. release and sedimentation) between bottom sediment and overlying water. The purpose of this study is to develop an ecological model of a lake, Lake Yunoko, combined with a dynamic model of bottom sediments and to assess various policies for eutrophication control with special emphasis on management of the bottom sediment.