2025 Volume 48 Pages 39-50
When managers of water areas plan water quality management strategies in enclosed water areas, they refer to the calculation results of predicted impacts on water quality by using ecosystem models. Several phytoplankton growth and nutrient uptake equations in ecosystem models have been developed in previous studies; however, the differences in the calculation results of models for phytoplankton and nutrients in oligotrophic environments have not been clarified. In this study, we constructed a one-box ecosystem model with a closed condition and compared the differences in calculation results of models for phytoplankton biomass and nutrients obtained using different phytoplankton growth and uptake equations for an oligotrophic environment. The phytoplankton biomass was higher when using the model with no fixed phytoplankton cellular composition than when using the model with a fixed phytoplankton cellular composition. This is because the cellular composition of the phytoplankton decreased in response to a decrease in the nutrient concentration of the surrounding water in the model with no fixed phytoplankton cellular composition, leading to a decrease in the amount of nutrients required for growth. The nutrient concentrations of the surrounding water were lower when using the model that had no fixed phytoplankton cellular composition and considered cellular nutrients explicitly than when using other models. Therefore, this model was seen as more likely to represent oligotrophic environments.
