Validation and application of a three-dimensional coupled water quality and sediment model of the Pearl River Estuary

We developed a 3D water quality⁃sediment model, which depicts the dynamic changes of nutrients in water column and sediments of the Pearl River Estuary (PRE), based on a coupled 1D⁃3D hydrodynamic model. The proposed model was validated using observations collected during summer in years 1999 and 2006 and then it was applied to simulate the distribution of water quality and sediment nutrient fluxes in the PRE and to quantify the contribution of sediment fluxes to nutrient inputs. Validation results show that our model simulates the spatio⁃temporal distributions of nutrients and dissolved oxygen concentrations successfully both in water column and sediments, with relative model / data errors less than 38%. In addition, the simulated sediment nutrient fluxes match the observed ones reported in published literatures, suggesting that our model reasonably reproduces the key sediment biochemical processes and flux characteristics. Our model results indicate that NH4 ⁃N and PO34 ⁃P fluxes are mainly directed from sediments to water column during summer in the PRE, which means that sediments are the source of nitrogen and phosphorous to water column. With respect to NO2 ⁃N+NO3 ⁃N (NO23), it is transported from water column to sediments, which act as the sink of NO23. The sediment nutrient fluxes mainly decrease from the inner estuary towards offshore; the fluxes are estimated 0.24~8.88, -10.06~ -0.14, and -0.37~0.41 mmol·m-2 ·d-1 at Lingdingyang for NH4 ⁃ N, NO23, and PO34 ⁃P, respectively, and estimated 0.01 ~ 4.14, 1.45 ~ 0.68, 0.12 ~ 0.09 mmol·m-2·d-1 at the coastal sea, respectively. In 环 境 科 学 学 报 36 卷 general, the sediment fluxes have notable contributions to nutrient inputs in the PRE and the sediment fluxes of NH4 ⁃N and PO34 ⁃P during summer can reach as high as 12% and 22% of terrestrial TN and TP inputs, respectively.