Concerns have recently been expressed that development, such as dam construction, in the Mekong River Basin will affect the flow of the river and the nutrient flow to the Tonle Sap (TS) Lake in Cambodia. This paper presents a dynamic phosphorus (P) model that was built to investigate the response of the lake’s phosphorus cycle to changes in Ploads under different development scenarios for the Mekong Basin including a high development (HD) scenario as well as current conditions. Understanding the P cycle is important as it is considered the limiting nutrient for organic productivity of the lake. The dynamic characteristics of the lake and the deficiency of data require a careful design of the P model structure. We developed a simple phosphorus budget model demonstrate here its application to the TS Lake. The P model was based on System Dynamics (SD) methodology and the model was constructed in Vensim, a software for building dynamic models. Results showed that the P peak concentration of the lake under the HD scenario would be lower than that under the baseline scenario by
about 30%. However, the basic cyclic behavior of P of the Lake under the two scenarios did not change significantly. Both actual data and the model output indicates two peaks per year in the P concentrations in the lake, likely a reflection of different sources during different stages of the annual flood/withdrawal cycle.