Estuaries play a critical role in land–ocean interactions. In the estuary and adjacent coastal environment, energies from river flow and tides converge, producing a special dynamic condition with strong gradients of hydrographic and chemical properties of water. Also, material inputs from land and oceanic sources are mixed by the hydrodynamic processes, and the resulting distributions have important effects in regulating food web structure and ecosystem function. Estuaries also support spawning and/or hatching grounds for many fishery species that have a great economic value. Hence, sustainability of the estuarine habitat and ecosystem (e.g. bio-diversity) is of great concern to human society. Historically, the evolution of human society in many parts of the world has been closely tied to the functioning of estuaries and the adjacent coastal waters through fisheries, trade (navigation) and natural resource extraction (e.g. salt).
Large river estuaries, such as the Changjiang (Yangtze River), have important effects on the ecosystem services of a larger marine environment (Fig. 1). For example, in summer the Changjiang effluent plumes can extend into coastal waters covering an area of up to 10 × 103 km2, and can be tracked as far as Cheju Island and even beyond. In winter, water from the Changjiang flows southward along the east coast of mainland of China and can reach Taiwan Strait and/or further into the South China Sea, when driven by northeasterly monsoon winds (Fig. 2). The critical importance of fresh water flow from Changjiang in affecting the fishing ground of the Zhoushan Archipelagoes has been recognized since the late 1940s (cf. Chu and Yang, 1949). The Changjiang Estuary is an important waterway linking the mainland of China with other economic centers of the world. Similar to other aquatic systems that have been negatively impacted by human influences, the Changjiang Estuary and adjacent coastal environment can be considered to be over-stressed (e.g. reclamation, urbanization, and over-fishing) and experiencing deterioration of ecosystem function, such as evidenced by eutrophication, seasonal hypoxia, and changes in food web structure.