Abstract
Lake Sanaru is a eutrophic lake and is occupying the 1st place in the worst ranking in Japan on the COD base. The water quality has not been improved at all, although the water quality improvement projects have been carried out heretofore. The water circulation in the lake is influenced by the tide and the brackish water is maintaining the biodiversity. The purposes of the study are to build an ecological model and to simulate the ecosystem of Lake Sanaru and finally to provide a useful tool to evaluate the cost-effectiveness of a technology improving the water quality. An ecological multi-compartment model combined with the hydrodynamics has been developed. In the hydrodynamic model, the set of equations for the fluid mass and momentum conservation in two horizontal directions are solved by the finite element method using the Galerkin method of weighted residuals. Good agreement has been achieved between hydrodynamic model predictions and field measurements. The result of hydrodynamic simulation is utilized to estimate the inflow and the outflow between ecological compartments and water levels in each ones. The ecological multi-compartment model has two compartments, the upper and the bottom ones. Each one consists of several sub-compartments describing behaviors of zooplankton, phytoplankton, bacteria, detritus, dissolved nutrients and so on. In the upper compartment, the effect of the physically forced by solar radiation, temperature and mixing layer depth is taking into account, while, for the bottom one, the effect of the diffusion of nutrients such as phosphate and nitrogen from the sediment to liquid is considered. The ecological model gave some promising initial results as a management tool to predict the expected reductions for the undesired consequences of eutrophication problem. The goal of this study is building the model which is reliable sufficiently to predict the water quality in the future, nevertheless as simple as possible.
