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).