Abstract: An integrated model system that combines POM,a viscous sediment model and a water quality ecological model was applied to Changjiang River Estuary and its adjacent areas. The computational domain covered an area of 29°00'~33° 30' N, 120°00'~ 125°10' E with a mesh grid which has a finest horizontal resolution of 998 m and average of 2 789 m. In vertical, the grid was divided into 11 sigma layers. The model system was initialized and driven by averaged hydrometeorological conditions of the dry season from November to April. The runoff of the Changjiang Riveris given by the perennial average from December to April,with an average discharge of 15 200 m³/s and that for the Qiantang River was set as 925 m³/s. At the open boundary, eight tidal components were calculated, that is Q₁,O,P₁, K₁,N₂,M₂,S₂ and K₂ and the amplitudes and phase lags were interpolated from the results of OTPS. The sediment mass concentration from the upper stream of the Changjiang and Qiantang Riversis 0. 547 kg/m³ and 0. 2 kg/m³,respectively. The biochemical elements for the upper streams of the two rivers were DIN 100 imol/L,DIP 0.7 imol/L, detrital organic matter of N 1. 0 imol/L,detrital organic matter of P 0. 08 imol/L, phytoplankton N 1. 2 imol/L, zooplankton N 0. 4 imol/L, and dissolved oxygen 8. 62 mg/L. On the off-sea open boundary, all the scalar fields use the approach given by Thatcher-Harleman boundary conditions at the hypothesis that the scalar field values outside the domain all remain the initial set values, and the time lag is set as 2 h.
According to the observed data in March 2003,high DIN concentration occurred near the Changjiang Estuary and Hangzhou bay and the value near the surface is 65 imol/L while that of the bottom layer is a little bit higher with the highest value of 80 imol/L near the bottom. For the modeling result, the highest DIN concentration zone matches with the observation data only the value is greater than 50 imol/L at the surface. According to the observed data in March 2003,the highest surface DIP concentration is approximately 0. 8 imol/L and that at the bottom approaches 1 imol/L. In the modeling results, the high concentration zones of DIP are not only in the Changjiang River Estuary and Hangzhou bay,but also in other areas off the Zhejiang Province and DIP concentration near the surface is 0. 75 imol/L, a little bit lower than that near the bottom, which matches well with the observed data. Chlorophyll a is usually used to represent the phytoplankton. The observed data of chlorophyll a in March 2003 indicates that there's little difference between the mass concentration near surface and bottom while the highest chlorophyll a mass concentration zone is located to the north of Changjiang River Estuary and comparatively high mass concentration zone at the open sea near the Zhoushan Islands. According to the observed data in November 2002,the zone of highest chlorophyll a mass concentration near surface is located near 30°~31°N,123°E and the zone of highest chlorophyll a mass concentration near the surface is distinctly different from that near the bottom. The modeling result of chlorophyll a doesn't match with the observed data, which may be caused by the transparency of the seawater. Because chlorophyll a was measured with fluorescence method, it's affected badly by the suspended sediment. Comparisons above show that the modeling results match well with the distributions of temperature, salinity, suspended sediment, inorganic nitrogen and inorganic phosphorus both horizontally and vertically, which indicates that the model system has simulated the key processes of hydrodynamics, siltation and water quality ecology in the computational domain.
Finally,three numerical experiments were run to check the modeling response to the change of initial conditions, which led to some helpful conclusions. For example, the high concentration zone of chlorophyll a is in the Changjiang River Estuary with a much higher value if the impact of suspended sediment on the light intensity is not considered. This indicates the great suppression on the growth of phytoplankton by sediment. If only the imput of DIN and detrital organic matter of nitrogen from the two rivers into the area was decreased by 50%,the modeling results of phytoplankton distribution in the surface layer is almost the same as that from standard operation test. But if only the imput of DIP and detrital organic matter of phosphorus from the two rivers into the area was decreased by 50% while keep the DIN value in the standard operation test unchanged, the resulted peak chlorophyll a mass concentration changed little. These three numerical experiments indicate that during dry season, the nutrients of inorganic nitrogen and phosphorous in the Changjiang River Estuary and its adjacent area are surplus especially the former. Even half of the input of the terrigenous inorganic nutrients is still surplus. The factor that controls the growth of phytoplankton is not nutrients but the high mass concentration silt that has great impacts on the light intensity and thus on the growth of phytoplankton.

Key words: Changjiang River Estuary, 3D model, ecosystem, water quality