长江口及邻近海域枯季水质生态模拟研究

Modeling study on water quality ecology in Changjiang River Estuary and its adjacent areas during dry season

将一个由水动力、粘性泥沙和水质生态模型搭建的综合模拟系统应用于长江口及邻近海域,采用枯季(11—4月)平均水文气象条件设置并驱动模型。将模拟结果与2003年3月观测资料及其他历史资料进行比对发现,模拟结果总体上很好地再现了研究海区枯季温度、盐度、悬浮泥沙、无机氮、无机磷等生态水质要素的水平和垂直分布趋势,说明模型可反映研究海区的水动力、泥沙和水质生态的关键过程。模拟结果中,DIN的分布趋势呈现出"近岸河口高、外海低"的特点,长江口和钱塘江口附近含量最高;DIP的高浓度区不仅仅在长江口及杭州湾,在浙江近海其它区域的浓度值也较高;浮游植物量最大的区域位于舟山以东海域。另外,3个条件响应数值试验表明,枯季长江口及邻近海区无机氮和无机磷均处于富余状态,尤其是无机氮,即使在陆源输入减半的情况下,仍然富余,没有被浮游植物充分利用;研究区域的高质量浓度悬浮泥沙通过限制光照强度控制着浮游植物的生长。

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 River was given by the perennial average from December to April, with an average discharge of 15 200 ma/s and that for the Qiantang River was set as 925 ma/s. At the open boundary, eight tidal components were calculated, that is Q1, O1, P1, K1, N2, M2, S2and 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 Rivers is 0. 547 kg/ma and 0.2 kg/ma , respectively. The biochemical elements for the upper streams of the two rivers were DIN 100 /lmol/L, DIP 0.7 μmol/L, detrital organic matter of N 1.0 /,tool/L, detrital organic matter of P 0.08 μmol/L, phytoplankton N 1.2 〉mol/L, zooplankton N 0.4 /μmol/L, and dissolved oxygen 8.62 mg/L. On the off-sea open boundary, all the scalar fields used 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 was 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 〉mol/L while that of the bottom layer is a little bit higher with the highest value of 80 /μmol/L. For the modeling result, the highest DIN concentration zone matches with the observation data only the value is greater than 50 〉mol/L at the surface. According to the observed data in March 2003, the highest surface DIP concentration is approximately 0.8/,mol/L and that at the bottom approaches 1 μmol/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 DIPconcentration near the surface is 0.75 μmol/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～//oo, 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～//00 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.