Geo-Environment Science

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.