三峡水库调度背景下长江三口来水与洞庭湖磷素分布的驱动-响应关系

The impacts of large reservoir construction on the ecosystems in the lower reaches of the dam contain a number of important scientific issues. Under the background of Three Gorges Reservoir (TGR) operation, Three Water Inlets (TWI), as the material input interface from Yangtze River to Dongting Lake, are experiencing changes in hydrology, sediment and phosphorus flux. However, as to the chemical response to the changes of water recharge of three water inlets, especially the distribution of phosphorus in Dongting Lake, we are still lack of related direct evidences and to illustrate them. Taking TWI- Dongting Lake as a highly correlated system, some scientific problems are focused in this study, namely water and phosphorus transport characteristics of TWI as well as its difference with local upstream rivers , and the response of phosphorus distribution in Dongting Lake to the hydrological processes under the driven force of TWI. With the above, we try to make breakthrough in the drive (TWI) - response (Dongting Lake) mechanism especially concerning phosphorous. Furthermore, we also attempt to reproduce the historical influence of TWI before the construction of TGR, and compare the restoring situation with current status. The results of the project will enrich the knowledge of the ecological environment evolution for the large lakes connected to the Yangtze River under the background of large-scale water conservancy project operation. With the above, technical support for protection of the Dongting Lake eco-environment and optimal operation of the TGR would also be strengthened.

大型水库建设对大坝下游地区生态环境影响蕴含着重要的科学问题。三峡水库调度运行背景下，长江三口作为长江干流向洞庭湖的物质输入界面，其来水中的水文、泥沙及磷物质通量已然发生变化。然而，上述改变对洞庭湖湖体以磷素分布为表征的水化学效应如何，却缺乏直接的证据和系统的关联来予以阐明。本项目将长江三口-洞庭湖湖体作为有机关联的作用体系，围绕长江三口磷素输送特征及其与区域背景来水的差异性、长江三口驱动下洞庭湖磷素分布对水文物理条件改变的响应机制等科学问题，力求在长江三口-洞庭湖就磷素而言的驱动-响应机制方面取得突破，并进一步反演和对比三峡工程建设前后三口变化带来的影响。项目成果将有利于丰富大型水库调度背景下通江湖泊生态环境演变规律的认识，为洞庭湖水生态安全保障、乃至三峡水库优化调度提供科技支撑。

三峡水库蓄水运行改变了江湖系统原有的水沙关系，影响着以水、沙为输运载体的磷的形态组成、物质通量和输移路径，继而对通江湖泊磷的物质循环过程产生重大影响。本研究以西洞庭湖及其入湖河流为研究对象，综合运用江湖水沙与磷负荷的耦合统计关系分析、入湖负荷与湖区磷的响应关系模型模拟等方法，探讨了江湖水沙关系变化驱动下，西洞庭湖磷的输移转化特征及其对入湖负荷的响应机制。主要结论如下：.（1）长江三口来沙是洞庭湖泥沙的主要来源。三峡水库运行后，长江三口入湖泥沙量急剧减少。研究表明，SS是颗粒态磷在洞庭湖流域内迁移转化的主要载体，因此受长江三口入湖泥沙减少，西洞庭湖水体中颗粒态磷浓度大幅降低，水体中磷形态组成由以颗粒态为主变为以溶解态为主。.（2）与三峡水库建库前相比，三峡水库运行后，长江三口来水总磷浓度大幅降低，长江三口入湖磷负荷对西洞庭湖磷贡献也随之明显降低，西洞庭湖水体中总磷浓度主要受到本地河流磷输入的影响。.（3）丰水期长江三口磷输入是西洞庭湖磷的主要来源，此时长江三口径流量最大，影响范围覆盖大部分湖区，影响程度自北向南逐渐减弱。枯水期西洞庭湖总磷浓度对本地流域负荷输入的响应更为敏感，本地河流入湖负荷贡献占主导，影响范围集中在中南部水域及北部主河道，程度随离河口距离的增加而降低。