Abstract: A coral cay is an island formed from sediments derived from the reef on which it sits and swept by refracted waves to a focal point on the reef flat where they are deposited. A cay initially may remain intertidal and lack vegetation, but with time it is likely to build up to be above sea level, acquire a vegetation cover, and become partially lithified. Coral cays generally occur on reef flats at or very close to sea level. Low-lying reef islands are described as different types that range from small and unstable unvegetated sandy cays to complex low wooded islands. The four key criteria that form the basis of reef-island classification are as follows: sediment type, island location on the reef flat, island shape and vegetation cover. Coral cays which compose largely unconsolidated sediments, are classic natural systems in a state of dynamic equilibrium. Any change to the cay formation process -weather conditions, sediment budgets, reef morphology or ecology-will produce an immediate response in the cay. The dynamics of coral cays vary in different reef areas and at different time scale. There are seasonal and decadal shoreline changes due to the influence of seasonal and long-term climate oscillations. Even low-frequency but high-energy events could cause rapid erosion or accretion on the coral cays in short time. Many factors could influence cay stability and they varied with different island types. Location, size, shape of cays and its vegetation, sediment budgets and meteorological conditions are often listed under consideration. The status of coral reef ecosystem, sedimentary characteristics of coral cay and the impacts of typhoon and sea level change are most important influence factors for coral cay stability. With the enhancement of cyclones and sea level rise caused by global climate change, the future of coral cay is uncertain. Coral reefs are the most important and distinctive ecosystems in the South China Sea (SCS). Many coral cays are distributed in four regions (Nansha Islands, Xisha Islands, Zhongsha Islands, Dongsha Islands). Especially, there are thirty-one coral cays in the Xisha Islands which have the most number and various types of coral cays in the South China Sea. In the same time, more and more artificial islands are under construction in recent years. It is urgent need to strengthen the study of coral cay dynamic evolution and its stability, the process of biological geomorphology for the coral cay formation and the impacts of global change on the dynamics of coral cay are two main focuses.

Key words: coral cays, dynamic evolution, global change, biodynamical geomorphology