Three-Dimensional Hydrodynamic and Water Quality Modeling of a Brackish Lake

Water environment in brackish lake is very complex. To analyze the water qualities, it needs to approach from physical aspect and ecological aspect. Moreover, it needs to consider not only the influence of ecology on the water qualities but also the influence of water qualities on ecology itself. We developed ecological model to consider the biochemical processes of the bivalve, that calculates water temperature, salinity, chlorophyll-a, inorganic nitrogen (NH₄, NO₃) and phosphorous, organic nitrogen and phosphorous, COD and DO. Applying this model to Lake Ogawara, we analyzed the characteristics of the water qualities in the brackish lake. In order to clarify the influence of water qualities on ecosystem, we establish the model that predicts the variation of Biomass of Corbicula japonica, that is the dominant bivalve in Lake Ogawara. Main results in the present study are summarized as follow. It can be calculated the spatial and seasonal variation of water qualities with accuracy. It is found that spatial distribution of water qualities is dependent on the physical factor such as a flow scale and a lake size, and on the biochemical factor such as a time scale of production and decomposition. We can clarify the influence of water qualities on the bivalve's growth and population. It is found that the growth rate is faster in the southern area than northern area, and the population tends to decrease because of the lack of spawning.