Modeling Sediment Impact on Nutrient Cycles and Phytoplankton Dynamics in the Three Gorges Reservoir

Sediment can have a significant effect on phytoplankton dynamics in some cases, which has already been observed in field studies. However, limited efforts have been made to numerically model the impact of sediment on nutrients and phytoplankton dynamics. In this study, a model based on the Environmental Fluid Dynamics Code (EFDC) model has been developed, and three aspects of sediment impact have been integrated into this model. Firstly, suspended sediment could reduce light penetration in the water column and thus restrict the growth rate of phytoplankton. This effect is represented by a linear relationship between light extinction coefficient and suspended sediment concentration. Secondly, the adsorption and desorption processes lead to the exchange of nutrients between water column and sediment, and further affect the nutrient availability for algal growth. Two separate equations for the dissolved and particulate phases of nutrients along with an empirical partition coefficient are introduced to characterize these processes. Thirdly, an improved approach has been adopted to calculate the nutrient fluxes at the sediment-water interface caused by diffusion and sediment resuspension. The model was applied to simulate the effects of sediment on nutrient cycles and phytoplankton dynamics in the Three Gorges Reservoir, and a hypothetical scenario with no external source of nutrients was also considered to evaluate the role of internal loadings. Results showed that sediment transport had only minor effects on phytoplankton dynamics but it could affect nutrient cycles and retention in the Three Gorges Reservoir. Moreover, internal loadings could make a difference in the long term. This model can provide a useful tool for assessment and management of water quality when sediment impact cannot be ignored.